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Parasitism is a close relationship between
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
, where one organism, the parasite, lives on or inside another organism, the
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ( ...
, causing it some harm, and is adapted structurally to this way of life. The
entomologist Entomology () is the science, scientific study of insects, a branch of zoology. In the past the term "insect" was less specific, and historically the definition of entomology would also include the study of animals in other arthropod groups, such ...
E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". Parasites include single-celled
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other microorganisms or o ...
ns such as the agents of
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects humans and other animals. Malaria causes Signs and symptoms, symptoms that typically include fever, fatigue (medical), tiredness, vomiting, and headaches. In se ...
,
sleeping sickness African trypanosomiasis, also known as African sleeping sickness or simply sleeping sickness, is an insect-borne parasitic infection of humans and other animals. It is caused by the species ''Trypanosoma brucei''. Humans are infected by two typ ...
, and
amoebic dysentery Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biol ...
; animals such as
hookworm Hookworms are intestinal, blood-feeding, parasitic roundworms The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nemat ...
s,
lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biol ...
,
mosquito Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small Diptera, flies within the family Culicidae (from the Latin ''culex'' meaning "gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish ...
es, and
vampire bat Vampire bats, species of the subfamily Desmodontinae, are leaf-nosed bats found in Central and South America. Their food source is blood of other animals, a dietary trait called hematophagy. Three extant bat species In biology, a specie ...
s;
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
such as
honey fungus ''Armillaria'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biolo ...
and the agents of
ringworm Dermatophytosis, also known as ringworm, is a mycosis, fungal infection of the skin. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the area affected. Symptoms begin four to fourteen days after exposure. Mu ...
; and plants such as
mistletoe Mistletoe is the common name for obligate parasite, obligate parasitic plant, hemiparasitic plants in the Order (biology), order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which the ...
,
dodder ''Cuscuta'' (), commonly known as dodder or amarbel, is a genus of over 201 species of yellow, orange, or red (rarely green) parasitic plants. Formerly treated as the only genus in the family Cuscutaceae, it now is accepted as belonging in the ...
, and the broomrapes. There are six major parasitic
strategies Strategy (from Ancient Greek, Greek στρατηγία ''stratēgia'', "art of troop leader; office of general, command, generalship") is a general plan to achieve one or more long-term or overall goals under conditions of uncertainty. In the se ...
of exploitation of animal hosts, namely
parasitic castration Parasitic castration is the strategy, by a parasite, of blocking reproduction by its host, completely or in part, to its own benefit. This is one of six major strategies within parasitism. Evolutionary strategy The parasitic castration strateg ...
, directly transmitted parasitism (by contact), trophicallytransmitted parasitism (by being eaten), vector-transmitted parasitism,
parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
ism, and micropredation. One major axis of classification concerns invasiveness: an endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface. Like predation, parasitism is a type of consumer–resource interaction, but unlike
predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation (wh ...
, parasites, with the exception of parasitoids, are typically much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialised, and
reproduce Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any life, living system that functions as an individual en ...
at a faster rate than their hosts. Classic examples include interactions between
vertebrate Vertebrates () comprise all animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually ...
hosts and
tapeworms Cestoda is a Class (biology), class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodi ...
, flukes, the malaria-causing ''
Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...
'' species, and
flea Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about long, ar ...
s. Parasites reduce host fitness by general or specialised
pathology Pathology is the study of the causal, causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when us ...
, from parasitic castration to modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their
transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission *** ...
from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions between
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being
saprophytic Saprotrophic nutrition or lysotrophic nutrition is a process of Chemotroph#Chemoheterotroph, chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most o ...
. People have known about parasites such as
roundworm The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
s and tapeworms since
ancient Egypt Ancient Egypt was a civilization in Northeast Africa situated in the Nile Valley. Ancient Egyptian civilization followed prehistoric Egypt and coalesced around 3100Anno Domini, BC (according to conventional Egyptian chronology) with the ...
,
Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...
, and
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (Romulus and Remus, legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg ...
. In early modern times,
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist A microbiologist (from Ancient Greek, Greek ) is a scientist who studies microscopic life forms and processes. This includes study of ...
observed ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The ...
'' in his microscope in 1681, while
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medical doctor, or simply doctor, is a he ...
described internal and external parasites including
sheep liver fluke ''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitic Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the ...
and
tick Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...
s. Modern
parasitology Parasitology is the study of parasites, their host (biology), hosts, and the relationship between them. As a List of biology disciplines, biological discipline, the scope of parasitology is not determined by the organism or environment in questio ...
developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to
satirical Satire is a genre of the visual arts, visual, literature, literary, and performing arts, usually in the form of fiction and less frequently Nonfiction, non-fiction, in which vices, follies, abuses, and shortcomings are held up to ridicule, ...
effect in
Jonathan Swift Jonathan Swift (30 November 1667 – 19 October 1745) was an Anglo-Irish Satire, satirist, author, essayist, political pamphleteer (first for the Whig (British political party), Whigs, then for the Tories (British political party), Tories), poe ...
's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin". In fiction,
Bram Stoker Abraham Stoker (8 November 1847 – 20 April 1912) was an Irish author who is celebrated for his 1897 Gothic Horror, Gothic horror novel ''Dracula''. During his lifetime, he was better known as the personal assistant of actor Sir Henry Irv ...
's 1897
Gothic horror Gothic fiction, sometimes called Gothic horror in the 20th century, is a loose literary aesthetic of fear and haunting. The name is a reference to Gothic architecture of the European Middle Ages, which was characteristic of the settings of ea ...
novel ''
Dracula ''Dracula'' is a novel by Bram Stoker, published in 1897. As an epistolary novel, the narrative is related through letters, diary entries, and newspaper articles. It has no single protagonist, but opens with solicitor Jonathan Harker taking ...
'' and its many later adaptations featured a blood-drinking parasite.
Ridley Scott Sir Ridley Scott (born 30 November 1937) is a British film director and producer. Directing, among others, science fiction films, his work is known for its atmospheric and highly concentrated visual style. Scott has received many accolades th ...
's 1979 film '' Alien'' was one of many works of
science fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imagination, imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, Paral ...
to feature a parasitic alien species.


Etymology

First used in English in 1539, the word ''parasite'' comes from the Medieval French ''parasite'', from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''parasitus'', the latinisation of the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''παράσιτος'' (''parasitos''), "one who eats at the table of another" and that from ''παρά'' (''para''), "beside, by" + ''σῖτος'' (''sitos''), "wheat", hence "food". The related term ''parasitism'' appears in English from 1611.


Evolutionary strategies


Basic concepts

Parasitism is a kind of
symbiosis Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
, a close and persistent long-term biological interaction between a parasite and its host. Unlike
saprotroph Saprotrophic nutrition or lysotrophic nutrition is a process of Chemotroph#Chemoheterotroph, chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most o ...
s, parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Unlike
commensalism Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism (biology), mutualism, in which both o ...
and mutualism, the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Because parasites interact with other species, they can readily act as vectors of pathogens, causing
disease A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medica ...
.
Predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation (wh ...
is by definition not a symbiosis, as the interaction is brief, but the entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". Within that scope are many possible strategies.
Taxonomist In biology, taxonomy () is the science, scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxon, taxa (s ...
s classify parasites in a variety of overlapping schemes, based on their interactions with their hosts and on their life cycles, which are sometimes very complex. An
obligate parasite An obligate parasite or holoparasite is a parasite, parasitic organism that cannot complete its life-cycle without exploiting a suitable host (biology), host. If an obligate parasite cannot obtain a host it will fail to reproduction, reproduce. Thi ...
depends completely on the host to complete its life cycle, while a
facultative parasite A facultative parasite is an organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxo ...
does not. Parasite life cycles involving only one host are called "direct"; those with a definitive host (where the parasite reproduces sexually) and at least one intermediate host are called "indirect". An endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface. Mesoparasites—like some
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s, for example—enter an opening in the host's body and remain partly embedded there. Some parasites can be generalists, feeding on a wide range of hosts, but many parasites, and the majority of protozoans and
helminth Parasitic worms, also known as helminths, are large Parasitism#Basic concepts, macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted helminth, soil-transmitted and intestinal parasit ...
s that parasitise animals, are specialists and extremely host-specific. An early basic, functional division of parasites distinguished microparasites and macroparasites. These each had a
mathematical model A mathematical model is a description of a system using mathematical concepts and language Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languag ...
assigned in order to analyse the population movements of the host–parasite groupings. The microorganisms and viruses that can reproduce and complete their life cycle within the host are known as microparasites. Macroparasites are the multicellular organisms that reproduce and complete their life cycle outside of the host or on the host's body. Much of the thinking on types of parasitism has focussed on terrestrial animal parasites of animals, such as helminths. Those in other environments and with other hosts often have analogous strategies. For example, the snubnosed eel is probably a facultative endoparasite (i.e., it is semiparasitic) that opportunistically burrows into and eats sick and dying fish. Plant-eating insects such as
scale insect Scale insects are small insects of the Order (biology), order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient g ...
s,
aphid Aphids are small sap-sucking insects and members of the Taxonomic rank, superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Erio ...
s, and
caterpillar Caterpillars ( ) are the larva, larval stage of members of the order Lepidoptera (the insect order comprising butterfly, butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawfly ...
s closely resemble ectoparasites, attacking much larger plants; they serve as vectors of bacteria, fungi and viruses which cause
plant diseases Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungus, fung ...
. As female scale insects cannot move, they are obligate parasites, permanently attached to their hosts. The sensory inputs that a parasite employs to identify and approach a potential host are known as "host cues". Such cues can include, for example, vibration, exhaled
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
, skin odours, visual and heat signatures, and moisture. Parasitic plants can use, for example, light, host physiochemistry, and volatiles to recognize potential hosts.


Major strategies

There are six major parasitic
strategies Strategy (from Ancient Greek, Greek στρατηγία ''stratēgia'', "art of troop leader; office of general, command, generalship") is a general plan to achieve one or more long-term or overall goals under conditions of uncertainty. In the se ...
, namely
parasitic castration Parasitic castration is the strategy, by a parasite, of blocking reproduction by its host, completely or in part, to its own benefit. This is one of six major strategies within parasitism. Evolutionary strategy The parasitic castration strateg ...
; directly transmitted parasitism; trophically-transmitted parasitism;
vector Vector most often refers to: *Euclidean vector In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities ...
-transmitted parasitism;
parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
ism; and micropredation. These apply to parasites whose hosts are plants as well as animals. These strategies represent adaptive peaks; intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable. A perspective on the evolutionary options can be gained by considering four key questions: the effect on the fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, the major evolutionary strategies of parasitism emerge, alongside predation.


Parasitic castrators

Parasitic castrators partly or completely destroy their host's ability to reproduce, diverting the energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host's other systems remain intact, allowing it to survive and to sustain the parasite. Parasitic crustaceans such as those in the specialised
barnacle A barnacle is a type of arthropod constituting the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tid ...
genus ''
Sacculina ''Sacculina'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biologi ...
'' specifically cause damage to the gonads of their many species of host
crab Crabs are Decapoda, decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen#Other animals, abdomen) ( el, :wikt:βραχύς, βραχύς , translit=brachys = short, / = tail), usually hid ...
s. In the case of ''Sacculina'', the testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to develop female
secondary sex characteristic Secondary sex characteristics are features that appear during puberty in human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and exceptional cognitive skills due ...
s such as broader abdomens, smaller
claws A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthrop ...
and egg-grasping appendages. Various species of helminth castrate their hosts (such as insects and snails). This may happen directly, whether mechanically by feeding on their gonads, or by secreting a chemical that destroys reproductive cells; or indirectly, whether by secreting a hormone or by diverting nutrients. For example, the trematode '' Zoogonus lasius'', whose sporocysts lack mouths, castrates the intertidal marine snail '' Tritia obsoleta'' chemically, developing in its gonad and killing its reproductive cells.


Directly transmitted

Directly transmitted parasites, not requiring a vector to reach their hosts, include such parasites of terrestrial vertebrates as lice and mites; marine parasites such as
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s and cyamid
amphipod Amphipoda is an order of malacostracan crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, ...
s;
monogenea Monogeneans are a group of ectoparasitic flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', me ...
ns; and many species of nematodes, fungi, protozoans, bacteria, and viruses. Whether endoparasites or ectoparasites, each has a single host-species. Within that species, most individuals are free or almost free of parasites, while a minority carry a large number of parasites; this is known as an aggregated distribution.


Trophically transmitted

Trophically-transmitted parasites are transmitted by being eaten by a host. They include trematodes (all except
schistosomes ''Schistosoma'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biol ...
),
cestodes Cestoda is a Class (biology), class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodi ...
,
acanthocephala Acanthocephala ( Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses ...
ns, pentastomids, many
roundworms The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
, and many protozoa such as ''
Toxoplasma ''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasi ...
''. They have complex life cycles involving hosts of two or more species. In their juvenile stages they infect and often
encyst A microbial cyst is a resting or dormancy, dormant stage of a microorganism, usually a bacteria, bacterium or a protist or rarely an invertebrate animal, that helps the organism to survive in unfavorable environmental conditions. It can be thoug ...
in the intermediate host. When the intermediate-host animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as
intestinal parasite An intestinal parasite infection is a condition in which a parasite infects the gastro-intestinal tract of humans and other animals. Such parasites can live anywhere in the body, but most prefer the intestinal wall. Routes of exposure and infe ...
s. Many trophically transmitted parasites modify the behaviour of their intermediate hosts, increasing their chances of being eaten by a predator. As with directly transmitted parasites, the distribution of trophically transmitted parasites among host individuals is aggregated.
Coinfection Coinfection is the simultaneous infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An i ...
by multiple parasites is common. Autoinfection, where (by exception) the whole of the parasite's
life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...
takes place in a single primary host, can sometimes occur in helminths such as '' Strongyloides stercoralis''.


Vector-transmitted

Vector-transmitted parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually, to carry them from one definitive host to another. These parasites are microorganisms, namely
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other microorganisms or o ...
,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, or
virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...
es, often intracellular
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ ...
s (disease-causers). Their vectors are mostly hematophagic
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s such as fleas, lice, ticks, and mosquitoes. For example, the deer tick ''
Ixodes scapularis ''Ixodes scapularis'' is commonly known as the deer tick or black-legged tick (although some people reserve the latter term for ''Ixodes pacificus'', which is found on the west coast of the US), and in some parts of the US as the bear tick. It wa ...
'' acts as a vector for diseases including
Lyme disease Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium, which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migran ...
,
babesiosis Babesiosis or piroplasmosis is a malaria-like parasitic disease caused by infection with a eukaryotic parasite in the order Piroplasmida, typically a ''Babesia'' or ''Theileria'', in the phylum Apicomplexa. Human babesiosis transmission via tic ...
, and anaplasmosis. Protozoan endoparasites, such as the
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects humans and other animals. Malaria causes Signs and symptoms, symptoms that typically include fever, fatigue (medical), tiredness, vomiting, and headaches. In se ...
l parasites in the genus ''
Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...
'' and sleeping-sickness parasites in the genus ''
Trypanosoma ''Trypanosoma'' is a genus of Kinetoplastida, kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasite, parasitic flagellate protozoa. Trypanosoma is part of the phylum Sarcomastigophora. The name is derived from the ...
'', have infective stages in the host's blood which are transported to new hosts by biting insects.


Parasitoids

Parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
s are insects which sooner or later kill their hosts, placing their relationship close to predation. Most parasitoids are
parasitoid wasp Parasitoid wasps are a large group of hymenopteran Superfamily (zoology), superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, ...
s or other
hymenoptera Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...
ns; others include dipterans such as phorid flies. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts. Idiobiont parasitoids sting their often large prey on capture, either killing them outright or paralysing them immediately. The immobilised prey is then carried to a nest, sometimes alongside other prey if it is not large enough to support a parasitoid throughout its development. An egg is laid on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, feeding on the provisions left for it.
Koinobiont In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
parasitoids, which include
flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwing ...
as well as wasps, lay their eggs inside young hosts, usually larvae. These are allowed to go on growing, so the host and parasitoid develop together for an extended period, ending when the parasitoids emerge as adults, leaving the prey dead, eaten from inside. Some koinobionts regulate their host's development, for example preventing it from
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in the ...
ting or making it
moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
whenever the parasitoid is ready to moult. They may do this by producing hormones that mimic the host's moulting hormones ( ecdysteroids), or by regulating the host's endocrine system. File:Live Tetragnatha montana (RMNH.ARA.14127) parasitized by Acrodactyla quadrisculpta larva (RMNH.INS.593867) - BDJ.1.e992.jpg , Idiobiont
parasitoid wasp Parasitoid wasps are a large group of hymenopteran Superfamily (zoology), superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, ...
s immediately paralyse their hosts for their larvae ( Pimplinae, pictured) to eat. File:CSIRO ScienceImage 2357 Spotted alfalfa aphid being attacked by parasitic wasp.jpg,
Koinobiont In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
parasitoid wasps like this braconid lay their eggs inside their hosts, which continue to grow and moult. File:Female Apocephalus borealis ovipositing into the abdomen of a worker honey bee.png, Phorid fly (centre left) is laying eggs in the abdomen of a worker honey-bee, altering its behaviour.


Micropredators

A micropredator attacks more than one host, reducing each host's fitness by at least a small amount, and is only in contact with any one host intermittently. This behavior makes micropredators suitable as vectors, as they can pass smaller parasites from one host to another. Most micropredators are hematophagic, feeding on blood. They include annelids such as
leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...
es, crustaceans such as
branchiura The family (biology), family Argulidae, whose members are commonly known as carp lice or fish lice, are parasite, parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloid ...
ns and gnathiid isopods, various
diptera Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwing ...
ns such as mosquitoes and tsetse flies, other arthropods such as fleas and ticks, vertebrates such as
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...
s, and mammals such as
vampire bat Vampire bats, species of the subfamily Desmodontinae, are leaf-nosed bats found in Central and South America. Their food source is blood of other animals, a dietary trait called hematophagy. Three extant bat species In biology, a specie ...
s.


Transmission strategies

Parasites use a variety of methods to infect animal hosts, including physical contact, the
fecal–oral route The fecal–oral route (also called the oral–fecal route or orofecal route) describes a particular Transmission (medicine), route of transmission of a disease wherein pathogens in Feces, fecal particles pass from one person to the mouth of anoth ...
, free-living infectious stages, and vectors, suiting their differing hosts, life cycles, and ecological contexts. Examples to illustrate some of the many possible combinations are given in the table.


Variations

Among the many variations on parasitic strategies are hyperparasitism, social parasitism, brood parasitism, kleptoparasitism, sexual parasitism, and adelphoparasitism.


Hyperparasitism

Hyperparasite A hyperparasite, also known as a metaparasite, is a parasite whose Host (biology) , host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera ...
s feed on another parasite, as exemplified by protozoa living in helminth parasites, or facultative or obligate parasitoids whose hosts are either conventional parasites or parasitoids. Levels of parasitism beyond secondary also occur, especially among facultative parasitoids. In
oak gall Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are ap ...
systems, there can be up to five levels of parasitism. Hyperparasites can control their hosts' populations, and are used for this purpose in agriculture and to some extent in
medicine Medicine is the science and Praxis (process), practice of caring for a patient, managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, and Health promotion ...
. The controlling effects can be seen in the way that the CHV1 virus helps to control the damage that
chestnut blight The pathogenic fungus ''Cryphonectria parasitica'' (formerly ''Endothia parasitica'') is a member of the Ascomycota (sac fungi). This necrotrophic fungus is native to East Asia and South East Asia and was introduced into Europe and North America ...
, ''Cryphonectria parasitica'', does to
American chestnut The American chestnut (''Castanea dentata'') is a large, fast-growing deciduous tree of the beech family native to eastern North America. As is true of all species in genus Castanea, the American chestnut produces burred fruit with edible nuts. ...
trees, and in the way that
bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek language, Greek φαγεῖν ('), meaning "to d ...
s can limit bacterial infections. It is likely, though little researched, that most pathogenic microparasites have hyperparasites which may prove widely useful in both agriculture and medicine.


Social parasitism

Social parasites take advantage of interspecific interactions between members of
eusocial Eusociality (from Greek εὖ ''eu'' "good" and social Social organisms, including human(s), live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and whether the exchange is ...
animals such as
ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of 22 ...
s,
termite Termites are small insects that live in colonies and have distinct castes (Eusociality, eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the Taxonomic rank#All ranks, epifamily ...
s, and
bumblebee A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus ...
s. Examples include the large blue butterfly, '' Phengaris arion'', its larvae employing
ant mimicry Ant mimicry or myrmecomorphy is mimicry of ants by other organisms. Ants are abundant all over the world, and potential predators that rely on vision to identify their prey, such as birds and wasps, normally avoid them, because they are either unp ...
to parasitise certain ants, ''
Bombus bohemicus ''Bombus bohemicus'', also known as the gypsy's cuckoo bumblebee, is a species of socially parasitic Psithyrus, cuckoo bumblebee found in most of Europe with the exception of the southern Iberian Peninsula and Iceland. ''B. bohemicus'' practices ...
'', a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and ''
Melipona scutellaris ''Melipona scutellaris'' is a eusocial Eusociality (from Greek εὖ ''eu'' "good" and social Social organisms, including human(s), live collectively in interacting populations. This interaction is considered social whether they are aware o ...
'', a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen. An extreme example of interspecific social parasitism is found in the ant '' Tetramorium inquilinum'', an obligate parasite which lives exclusively on the backs of other ''Tetramorium'' ants. A mechanism for the evolution of social parasitism was first proposed by Carlo Emery in 1909. Now known as " Emery's rule", it states that social parasites tend to be closely related to their hosts, often being in the same genus. Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation.


Brood parasitism

In
brood parasitism Brood parasites are animals that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood Parasitism, parasite manipulates a host (biology), host, either of the same or of another species, to raise its ...
, the hosts act as parents as they raise the young as their own. Brood parasites include birds in different families such as
cowbird Cowbirds are birds belonging to the genus ''Molothrus'' in the Family (biology), family Icteridae. They are of New World origin, and are obligate brood parasites, laying their eggs in the nests of other species. The genus was introduced by Engli ...
s, whydahs,
cuckoo Cuckoos are birds in the Cuculidae family, the sole taxon in the order Cuculiformes . The cuckoo family includes the common cuckoo, common or European cuckoo, Geococcyx, roadrunners, koels, malkohas, couas, coucals and ani (bird), anis. The co ...
s, and black-headed ducks. These do not build nests of their own, but leave their eggs in nests of other
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
. The eggs of some brood parasites
mimic MIMIC, known in capitalized form only, is a former simulation A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of Conceptual model, models; the model represents the ...
those of their hosts, while some cowbird eggs have tough shells, making them hard for the hosts to kill by piercing, both mechanisms implying selection by the hosts against parasitic eggs. The adult female European cuckoo further mimics a predator, the European sparrowhawk, giving her time to lay her eggs in the host's nest unobserved.


Kleptoparasitism

In
kleptoparasitism Kleptoparasitism (etymologically, parasitism by theft) is a form of feeding in which one animal deliberately takes food from another. The strategy is Evolutionarily stable strategy, evolutionarily stable when stealing is less costly than direct f ...
(from Greek κλέπτης (''kleptēs''), "thief"), parasites steal food gathered by the host. The parasitism is often on close relatives, whether within the same species or between species in the same genus or family. For instance, the many lineages of
cuckoo bee The term cuckoo bee is used for a variety of different bee Bees are winged insects closely related to wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; t ...
s lay their eggs in the nest cells of other
bee Bees are winged insects closely related to wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which lo ...
s in the same family. Kleptoparasitism is uncommon generally but conspicuous in birds; some such as
skuas The skuas are a group of predatory seabird Seabirds (also known as marine birds) are birds that are adaptation, adapted to life within the marine (ocean), marine environment. While seabirds vary greatly in lifestyle, behaviour and physi ...
are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch.


Sexual parasitism

A unique approach is seen in some species of
anglerfish The anglerfish are fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, carti ...
, such as '' Ceratias holboelli'', where the males are reduced to tiny sexual parasites, wholly dependent on females of their own species for survival, permanently attached below the female's body, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce the next generation.


Adelphoparasitism

Adelphoparasitism, (from Greek ἀδελφός (''adelphós''), brother), also known as sibling-parasitism, occurs where the host species is closely related to the parasite, often in the same family or genus. In the citrus blackfly parasitoid, '' Encarsia perplexa'', unmated females of which may lay
haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
eggs in the fully developed larvae of their own species, producing male offspring, while the marine worm ''
Bonellia viridis ''Bonellia viridis'', the green spoonworm, is a marine worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and no eyes (though not a ...
'' has a similar reproductive strategy, although the larvae are planktonic.


Illustrations

Examples of the major variant strategies are illustrated. File:Pteromalid hyperparasitoid.jpg, A hyperparasitoid pteromalid wasp on the cocoons of its host, itself a parasitoid braconid wasp File:Maculinea arion Large Blue Upperside SFrance 2009-07-18.jpg, The
large blue The large blue (''Phengaris arion'') is a species of butterfly in the family Lycaenidae. The species was first defined in 1758 and first recorded in Great Britain, Britain in 1795. In 1979 the species became mostly extinct in Britain but has be ...
butterfly is an ant mimic and social parasite. File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg, In
brood parasitism Brood parasites are animals that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood Parasitism, parasite manipulates a host (biology), host, either of the same or of another species, to raise its ...
, the host raises the young of another species, here a
cowbird Cowbirds are birds belonging to the genus ''Molothrus'' in the Family (biology), family Icteridae. They are of New World origin, and are obligate brood parasites, laying their eggs in the nests of other species. The genus was introduced by Engli ...
's egg, that has been laid in its nest. File:Great Skua (cropped).jpg, The
great skua The great skua (''Stercorarius skua''), sometimes known by the name bonxie in Britain, is a large seabird in the skua family Stercorariidae. It is roughly the size of a herring gull. It mainly eats fish caught at the sea surface or taken from ...
is a powerful kleptoparasite, relentlessly pursuing other seabirds until they disgorge their catches of food. File:Северная церапия (cropped).jpg, The male
anglerfish The anglerfish are fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, carti ...
'' Ceratias holboelli'' lives as a tiny sexual parasite permanently attached below the female's body. File:Encarsia perplexa.jpg, '' Encarsia perplexa'' (centre), a parasitoid of citrus blackfly (lower left), is also an adelphoparasite, laying eggs in larvae of its own species


Taxonomic range

Parasitism has an extremely wide taxonomic range, including animals, plants, fungi, protozoans, bacteria, and viruses.


Animals

Parasitism is widespread in the animal kingdom, and has evolved independently from free-living forms hundreds of times. Many types of
helminth Parasitic worms, also known as helminths, are large Parasitism#Basic concepts, macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted helminth, soil-transmitted and intestinal parasit ...
including flukes and
cestodes Cestoda is a Class (biology), class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodi ...
have complete life cycles involving two or more hosts. By far the largest group is the parasitoid wasps in the Hymenoptera. The phyla and classes with the largest numbers of parasitic species are listed in the table. Numbers are conservative minimum estimates. The columns for Endo- and Ecto-parasitism refer to the definitive host, as documented in the Vertebrate and Invertebrate columns.


Plants

A hemiparasite or ''partial parasite'' such as
mistletoe Mistletoe is the common name for obligate parasite, obligate parasitic plant, hemiparasitic plants in the Order (biology), order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which the ...
derives some of its nutrients from another living plant, whereas a holoparasite such as
dodder ''Cuscuta'' (), commonly known as dodder or amarbel, is a genus of over 201 species of yellow, orange, or red (rarely green) parasitic plants. Formerly treated as the only genus in the family Cuscutaceae, it now is accepted as belonging in the ...
derives all of its nutrients from another plant.
Parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All Parasite, parasitic plants develop a specialized organ ...
s make up about one per cent of
angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
and are in almost every
biome A biome () is a biogeographical unit consisting of a biological community (ecology), community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. ...
in the world. which appeared in Spanish as Chapter 2, pp. 7–27 in: J. A. López-Sáez, P. Catalán and L. Sáez ds. ''Parasitic Plants of the Iberian Peninsula and Balearic Islands''. All these plants have modified roots, haustoria, which penetrate the host plants, connecting them to the conductive system—either the
xylem Xylem is one of the two types of transport tissue in vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of embryophyte, land plants ( accepted known species) that have lignin, ...
, the
phloem Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...
, or both. This provides them with the ability to extract water and nutrients from the host. A parasitic plant is classified depending on where it latches onto the host, either the stem or the root, and the amount of nutrients it requires. Since holoparasites have no
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf"). Chlorophyll al ...
and therefore cannot make food for themselves by
photosynthesis Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
, they are always obligate parasites, deriving all their food from their hosts. Some parasitic plants can locate their
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ( ...
plants by detecting
chemicals A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent Chemical element, elements by physical separation m ...
in the air or soil given off by host
shoot In botany, a plant shoot consists of any plant stem A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaf, leaves, flowers and fruits, transports water and dissolved substances between the ...
s or
root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often b ...
s, respectively. About 4,500
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
of parasitic plant in approximately 20
families Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
of
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s are known. Species within the ''
Orobanchaceae Orobanchaceae, the broomrapes, is a family (biology), family of mostly parasitic plants of the order (biology), order Lamiales, with about 90 genus, genera and more than 2000 species. Many of these genera (e.g., ''Pedicularis'', ''Rhinanthus'', ...
'' (broomrapes) are among the most economically destructive of all plants. Species of ''
Striga ''Striga'', commonly known as witchweed, is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. It is currently classified in the family Orobanchaceae, although older classifications place it in the Scrophul ...
'' (witchweeds) are estimated to cost billions of dollars a year in crop yield loss, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. ''Striga'' infects both grasses and grains, including
corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn ( North American and Australian English Australian English (AusE, AusEng, AuE, AuEng, en-AU) is the set of variety (linguistics), varieties ...
,
rice Rice is the seed of the Poaceae, grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera ''Zizania (genus), Zizania'' and ''Porteresia'', bo ...
, and
sorghum ''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many other ...
, undoubtedly some of the most important food crops. '' Orobanche'' also threatens a wide range of other important crops, including
peas The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...
,
chickpeas The chickpea or chick pea (''Cicer arietinum'') is an annual plant, annual legume of the family (biology), family Fabaceae, subfamily Faboideae. Its different types are variously known as gram" or Bengal gram, garbanzo or garbanzo bean, or Egypti ...
,
tomatoes The tomato is the edible Berry (botany), berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to th ...
,
carrot The carrot (''Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the Daucus carota, wild carrot, ''Daucus ...
s, and varieties of
cabbage Cabbage, comprising several cultivars of ''Brassica oleracea'', is a leafy green, red (purple), or white (pale green) biennial plant grown as an Annual plant, annual vegetable crop for its dense-leaved heads. It is descended from the wild cabb ...
. Yield loss from ''Orobanche'' can be total; despite extensive research, no method of control has been entirely successful. Many
plants Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
and
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
exchange carbon and nutrients in mutualistic
mycorrhizal   A mycorrhiza (from Ancient Greek, Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiosis, symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role ...
relationships. Some 400 species of myco-heterotrophic plants, mostly in the tropics, however effectively cheat by taking carbon from a fungus rather than exchanging it for minerals. They have much reduced roots, as they do not need to absorb water from the soil; their stems are slender with few
vascular bundle A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the Plant stem, stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition ...
s, and their leaves are reduced to small scales, as they do not photosynthesize. Their seeds are very small and numerous, so they appear to rely on being infected by a suitable fungus soon after germinating.


Fungi

Parasitic
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
derive some or all of their nutritional requirements from plants, other fungi, or animals. Unlike mycorrhizal fungi which have a mutualistic relationship with their host plants, they are pathogenic. For example, the honey fungi in the genus ''
Armillaria ''Armillaria'' is a genus of fungi that includes the ''Armillaria mellea, A. mellea'' species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as ''A. mellea''. ''Armill ...
'' grow in the roots of a wide variety of trees, and eventually kill them. They then continue to live in the dead wood, feeding
saprophytic Saprotrophic nutrition or lysotrophic nutrition is a process of Chemotroph#Chemoheterotroph, chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most o ...
ally. Fungal infection (
mycosis Fungal infection, also known as mycosis, is Infection, disease caused by pathogenic fungi, fungi. Different types are traditionally divided according to the part of the body affected; superficial, subcutaneous tissue, subcutaneous, and systemic. ...
) is widespread in animals including humans; it kills some 1.6 million people each year.
Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or prot ...
are obligate intracellular parasitic fungi that can also be hyperparasites. They largely affect insects, but some affect vertebrates including humans, where they can cause the intestinal infection microsporidiosis.


Protozoa

Protozoa such as ''
Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...
'', ''
Trypanosoma ''Trypanosoma'' is a genus of Kinetoplastida, kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasite, parasitic flagellate protozoa. Trypanosoma is part of the phylum Sarcomastigophora. The name is derived from the ...
'', and ''
Entamoeba ''Entamoeba'' is a genus of Amoebozoa found as internal parasitism, parasites or commensalism, commensals of animals. In 1875, Fedor Lösch described the first proven case of amoebic dysentery in St. Petersburg, Russia. He referred to the amoe ...
'' are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, and
amoebic dysentery Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biol ...
—and have complex life cycles.


Bacteria

Many bacteria are parasitic, though they are more generally thought of as
pathogens In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ ...
causing disease. Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, ''
Bacillus anthracis ''Bacillus anthracis'' is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent ( obligate) pathogen within the genus '' Bacillus''. Its infection i ...
'', the cause of
anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day and more than two months after the infection is contracted. The sk ...
, is spread by contact with infected
domestic animal This page gives a list of domesticated animals, also including a list of domestication of animals, animals which are or may be currently undergoing the process of domestication and animals that have an extensive relationship with humans beyond simp ...
s; its
spore In biology, a spore is a unit of sexual reproduction, sexual or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of ...
s, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. ''
Borrelia ''Borrelia'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biologica ...
'', the cause of
Lyme disease Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium, which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migran ...
and
relapsing fever Relapsing fever is a vector-borne disease caused by infection with certain bacteria in the genus '' Borrelia'', which is transmitted through the bites of lice or soft-bodied ticks (genus '' Ornithodoros''). Signs and symptoms Most people wh ...
, is transmitted by vectors, ticks of the genus ''
Ixodes ''Ixodes'' is a genus of hard-bodied ticks (family Ixodidae). It includes important disease Vector (epidemiology), vectors of animals and humans (tick-borne disease), and some species (notably ''Ixodes holocyclus'') inject toxins that can ca ...
'', from the diseases' reservoirs in animals such as
deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the ...
. ''
Campylobacter jejuni ''Campylobacter jejuni'' () is a species of pathogenic bacteria, one of the most common causes of food poisoning in Europe and in the US. The vast majority of cases occur as isolated events, not as part of recognized outbreaks. Active surveillanc ...
'', a cause of
gastroenteritis Gastroenteritis, also known as infectious diarrhea and gastro, is an inflammation of the Human gastrointestinal tract, gastrointestinal tract including the stomach and intestine. Symptoms may include diarrhea, vomiting, and abdominal pain. Fever ...
, is spread by the fecal–oral route from animals, or by eating insufficiently cooked
poultry Poultry () are domestication, domesticated birds kept by humans for their egg (food) , eggs, their meat or their feathers. These birds are most typically members of the Superorder (biology), superorder Fowl, Galloanserae (fowl), especially the ...
, or by contaminated water. ''
Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...
'', an agent of
bacterial meningitis Meningitis is Acute (medical), acute or Chronic (medical), chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Ot ...
and respiratory tract infections such as
influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...
and
bronchitis Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. S ...
, is transmitted by droplet contact. ''
Treponema pallidum ''Treponema pallidum'', formerly known as ''Spirochaeta pallida'', is a spirochaete bacterium with various subspecies that cause the diseases syphilis, bejel (also known as endemic syphilis), and yaws. It is transmitted only among humans. It is ...
'', the cause of
syphilis Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, and ...
, is spread by
sexual activity Human sexual activity, human sexual practice or human sexual behaviour is the manner in which human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and except ...
.


Viruses

Virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...
es are obligate intracellular parasites, characterised by extremely limited biological function, to the point where, while they are evidently able to infect all other organisms from bacteria and
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
to animals, plants and fungi, it is unclear whether they can themselves be described as living. They can be either
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
or
DNA virus A DNA virus is a virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganism ...
es consisting of a single or double strand of
genetic material Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the monomers made of three components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. ...
(
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
or DNA, respectively), covered in a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
coat and sometimes a
lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and o ...
envelope. They thus lack all the usual machinery of the
cell Cell most often refers to: * Cell (biology) The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a Cell membrane, membrane, and contains many biomolecules such as proteins, D ...
such as
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are
bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek language, Greek φαγεῖν ('), meaning "to d ...
s, infecting bacteria.


Evolutionary ecology

Parasitism is a major aspect of evolutionary ecology; for example, almost all free-living animals are host to at least one species of parasite. Vertebrates, the best-studied group, are hosts to between 75,000 and 300,000 species of helminths and an uncounted number of parasitic microorganisms. On average, a mammal species hosts four species of nematode, two of trematodes, and two of cestodes. Humans have 342 species of helminth parasites, and 70 species of protozoan parasites. Some three-quarters of the links in
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is Consumer-resource systems, consumer-resource system. Ecologists can broadly lump ...
s include a parasite, important in regulating host numbers. Perhaps 40 percent of described species are parasitic.


Fossil record

Parasitism is hard to demonstrate from the
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, Seashell, shells, exoskeletons, stone imprints of animals or microbes, objects pre ...
, but holes in the
mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
s of several specimens of ''
Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosa ...
'' may have been caused by ''
Trichomonas ''Trichomonas'' is a genus of Anaerobic organism, anaerobic Excavata, excavate parasites of vertebrates. It was first discovered by Alfred François Donné in 1836 when he found these parasites in the pus of a patient suffering from vaginitis, a ...
''-like parasites.


Coevolution

As hosts and parasites evolve together, their relationships often change. When a parasite is in a sole relationship with a host, selection drives the relationship to become more benign, even mutualistic, as the parasite can reproduce for longer if its host lives longer. But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities in
host–parasite coevolution Host–parasite coevolution is a special case of coevolution, where a Host (biology), host and a parasite continually adapt to each other. This can create an evolutionary arms race between them. A more benign possibility is of an evolutionary tra ...
. Evolutionary epidemiology analyses how parasites spread and evolve, whereas Darwinian medicine applies similar evolutionary thinking to non-parasitic diseases like
cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Poss ...
and autoimmune conditions.


Coevolution favouring mutualism

Long-term coevolution sometimes leads to a relatively stable relationship tending to
commensalism Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism (biology), mutualism, in which both o ...
or mutualism, as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals parasitised by worms are often clearly harmed, such infections may also reduce the prevalence and effects of
autoimmune In immunology, autoimmunity is the system of immune responses of an organism against its own healthy Cell biology, cells, Tissue (biology), tissues and other normal body constituents. Any disease resulting from this type of immune response is te ...
disorders in animal hosts, including humans. In a more extreme example, some
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
worms cannot reproduce, or even survive, without infection by ''
Wolbachia ''Wolbachia'' is a genus of intracellular parasite, intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes, and is possibly th ...
'' bacteria.
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolution Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generatio ...
and others have argued, following
Peter Kropotkin Pyotr Alexeyevich Kropotkin (; russian: link=no, Пётр Алексе́евич Кропо́ткин ; 9 December 1842 – 8 February 1921) was a Russian anarchist Anarchism is a political philosophy and Political movement, move ...
's 1902 '' Mutual Aid: A Factor of Evolution'', that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the
symbiogenesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly ...
which formed the
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s from an intracellular relationship between archaea and bacteria, though the sequence of events remains largely undefined.


Competition favoring virulence

Competition between parasites can be expected to favour faster reproducing and therefore more
virulent Virulence is a pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infecti ...
parasites, by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable Phenotypic trait, traits characteristic of a populati ...
. Among competing parasitic insect-killing bacteria of the genera '' Photorhabdus'' and ''
Xenorhabdus ''Xenorhabdus'' is a genus of motile, gram-negative bacteria from the family of the Morganellaceae. All the species of the genus are only known to live in symbiosis with soil entomopathogenic nematodes from the genus ''Steinernema''. Although no ...
'', virulence depended on the relative potency of the antimicrobial
toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1 ...
s ( bacteriocins) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when
caterpillar Caterpillars ( ) are the larva, larval stage of members of the order Lepidoptera (the insect order comprising butterfly, butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawfly ...
s were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.


Cospeciation

A parasite sometimes undergoes cospeciation with its host, resulting in the pattern described in Fahrenholz's rule, that the phylogenies of the host and parasite come to mirror each other. An example is between the simian foamy virus (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial cytochrome c oxidase subunit II from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses cospeciated with Old World primates for at least 30 million years. The presumption of a shared evolutionary history between parasites and hosts can help elucidate how host taxa are related. For instance, there has been a dispute about whether
flamingos Flamingos or flamingoes are a type of Wader, wading bird in the Family (biology), family Phoenicopteridae, which is the only extant family in the order Phoenicopteriformes. There are four flamingo species distributed throughout the Americas ...
are more closely related to
storks Storks are large, long-legged, long-necked wading birds with long, stout Beak, bills. They belong to the family (biology), family called Ciconiidae, and make up the order Ciconiiformes . Ciconiiformes previously included a number of other famil ...
or
ducks Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...
. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication, or the extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of
grebe Grebes () are aquatic diving birds in the order (biology), order Podicipediformes . Grebes are widely distributed freshwater birds, with some species also found in sea, marine habitats during Bird migration, migration and winter. Some flightless ...
s. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then switched hosts to ducks, creating the situation which had confused biologists. Parasites infect
sympatric In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...
hosts (those within their same geographical area) more effectively, as has been shown with digenetic trematodes infecting lake snails. This is in line with the
Red Queen hypothesis The Red Queen hypothesis is a hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can testable, test it. Scientists ...
, which states that interactions between species lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to
allopatric Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
hosts, those from different geographical regions.


Modifying host behaviour

Some parasites modify host behaviour in order to increase their transmission between hosts, often in relation to predator and prey ( parasite increased trophic transmission). For example, in the California coastal salt marsh, the fluke '' Euhaplorchis californiensis'' reduces the ability of its
killifish A killifish is any of various oviparous (egg-laying) cyprinodontiformes, cyprinodontiform fish (including families Aplocheilidae, Cyprinodontidae, Fundulidae, Profundulidae and Valenciidae). All together, there are 1,270 species of killifish, the ...
host to avoid predators. This parasite matures in
egret Egrets ( ) are herons, generally long-legged wading birds, that have white or buff plumage, developing fine plumes (usually milky white) during the breeding season. Egrets are not a biologically distinct group from herons and have the same build ...
s, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan ''
Toxoplasma gondii ''Toxoplasma gondii'' () is an Obligate intracellular parasite, obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-bloode ...
'', a parasite that matures in
cats The cat (''Felis catus'') is a Domestication of the cat, domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it ...
but can be carried by many other
mammals Mammals () are a group of vertebrate animals constituting the class (biology), class Mammalia (), characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk for feeding (nursing) their young, a ...
. Uninfected
rats Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' (pack rats), ''Bandicota'' (bandicoot ...
avoid cat odors, but rats infected with ''T. gondii'' are drawn to this scent, which may increase transmission to feline hosts. The malaria parasite modifies the skin odour of its human hosts, increasing their attractiveness to mosquitoes and hence improving the chance that the parasite will be transmitted. The spider '' Cyclosa argenteoalba'' often have parasitoid wasp larvae attached to them which alter their web-building behavior. Instead of producing their normal sticky spiral shaped webs, they made simplified webs when the parasites were attached. This manipulated behavior lasted longer and was more prominent the longer the parasites were left on the spiders.


Trait loss

Parasites can exploit their hosts to carry out a number of functions that they would otherwise have to carry out for themselves. Parasites which lose those functions then have a selective advantage, as they can divert resources to reproduction. Many insect ectoparasites including bedbugs, batbugs,
lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biol ...
and
flea Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about long, ar ...
s have lost their ability to fly, relying instead on their hosts for transport. Trait loss more generally is widespread among parasites. An extreme example is the myxosporean ''
Henneguya zschokkei ''Henneguya zschokkei'' or ''Henneguya salminicola'' is a species of a myxosporean endoparasite. It afflicts several salmon in the genus ''Oncorhynchus''. It causes milky flesh or tapioca disease. ''H. zschokkei'' is notable for its absence of Mi ...
'', an ectoparasite of fish and the only animal known to have lost the ability to respire aerobically: its cells lack
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
.


Host defences

Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates, the immune system of mammals, insects actively removing parasites, and defensive chemicals in plants. The evolutionary biologist W. D. Hamilton suggested that
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
could have evolved to help to defeat multiple parasites by enabling
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
, the shuffling of genes to create varied combinations. Hamilton showed by mathematical modelling that sexual reproduction would be evolutionarily stable in different situations, and that the theory's predictions matched the actual ecology of sexual reproduction. However, there may be a trade-off between
immunocompetence In immunology, immunocompetence is the ability of the body to produce a normal immune response following exposure to an antigen. Immunocompetence is the opposite of immunodeficiency (also known as ''immuno-incompetence'' or being ''immuno-comprom ...
and breeding male vertebrate hosts'
secondary sex characteristic Secondary sex characteristics are features that appear during puberty in human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and exceptional cognitive skills due ...
s, such as the plumage of
peacock Peafowl is a common name for three bird species in the genera ''Pavo (genus), Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female pea ...
s and the manes of
lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphi ...
s. This is because the male hormone
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate The prostate is both an Male accessory gland ...
encourages the growth of secondary sex characteristics, favouring such males in
sexual selection Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
, at the price of reducing their immune defences.


Vertebrates

The physical barrier of the tough and often dry and waterproof
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
of reptiles, birds and mammals keeps invading microorganisms from entering the body.
Human skin The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue (biology), tissue guarding Skeletal muscle, muscles, bones, ligaments and organ (anatom ...
also secretes
sebum A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called #Sebum, sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greates ...
, which is toxic to most microorganisms. On the other hand, larger parasites such as
trematode Trematoda is a class of flatworms known as flukes. They are obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organi ...
s detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate
saliva Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ex ...
and tears contain
lysozyme Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
, an enzyme that breaks down the
cell walls A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
of invading bacteria. Should the organism pass the mouth, the
stomach The stomach is a muscular, Organ (anatomy), hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the Digestion, dige ...
with its
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is a component of the gas ...
, toxic to most microorganisms, is the next line of defence. Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Once inside the body, parasites must overcome the
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
's serum proteins and
pattern recognition receptor Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of ...
s, intracellular and cellular, that trigger the adaptive immune system's
lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of gnathostomata, most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated immunity, cell-mediated, cytotoxicity, cytotoxic innate imm ...
s such as
T cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell rec ...
s and antibody-producing
B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...
s. These have receptors that recognise parasites.


Insects

Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp '' Polistes canadensis'' nests across multiple combs, rather than building a single comb like much of the rest of its genus, is to avoid infestation by tineid moths. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.


Plants

Plants respond to parasite attack with a series of chemical defences, such as
polyphenol oxidase Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in Browning (food process), fruit browning, is a tetramer that contains four atoms of copper per molecule. PPO may accept monophenol , monophenols and/or ...
, under the control of the jasmonic acid-insensitive (JA) and
salicylic acid Salicylic acid is an organic compound with the formula HOC6H4CO2H. A colorless, bitter-tasting solid, it is a precursor to and a active metabolite, metabolite of aspirin (acetylsalicylic acid). It is a plant hormone, and has been listed by the ...
(SA) signalling pathways. The different biochemical pathways are activated by different attacks, and the two pathways can interact positively or negatively. In general, plants can either initiate a specific or a non-specific response. Specific responses involve recognition of a parasite by the plant's cellular receptors, leading to a strong but localised response: defensive chemicals are produced around the area where the parasite was detected, blocking its spread, and avoiding wasting defensive production where it is not needed. Non-specific defensive responses are systemic, meaning that the responses are not confined to an area of the plant, but spread throughout the plant, making them costly in energy. These are effective against a wide range of parasites. When damaged, such as by
lepidoptera Lepidoptera ( ) is an order (biology), order of insects that includes butterfly, butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 Family (biology), families and 46 Taxonomic r ...
n
caterpillar Caterpillars ( ) are the larva, larval stage of members of the order Lepidoptera (the insect order comprising butterfly, butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawfly ...
s, leaves of plants including
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of Mexico, indigenous ...
and
cotton Cotton is a soft, fluffy Staple (textiles), staple fiber that grows in a wikt:boll, boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose ...
release increased amounts of volatile chemicals such as
terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly Pinophyta, conifers. ...
s that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.


Biology and conservation


Ecology and parasitology

Parasitism and parasite evolution were until the twenty-first century studied by
parasitologist Parasitology is the study of parasites, their host (biology), hosts, and the relationship between them. As a List of biology disciplines, biological discipline, the scope of parasitology is not determined by the organism or environment in questio ...
s, in a science dominated by medicine, rather than by
ecologist Ecology () is the study of the relationships between living organisms, including humans, and their biophysical environment, physical environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosy ...
s or evolutionary biologists. Even though parasite-host interactions were plainly ecological and important in evolution, the history of parasitology caused what the evolutionary ecologist Robert Poulin called a "takeover of parasitism by parasitologists", leading ecologists to ignore the area. This was in his opinion "unfortunate", as parasites are "omnipresent agents of natural selection" and significant forces in evolution and ecology. In his view, the long-standing split between the sciences limited the exchange of ideas, with separate conferences and separate journals. The technical languages of ecology and parasitology sometimes involved different meanings for the same words. There were philosophical differences, too: Poulin notes that, influenced by medicine, "many parasitologists accepted that evolution led to a decrease in parasite virulence, whereas modern evolutionary theory would have predicted a greater range of outcomes". Their complex relationships make parasites difficult to place in food webs: a trematode with multiple hosts for its various life cycle stages would occupy many positions in a
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is Consumer-resource systems, consumer-resource system. Ecologists can broadly lump ...
simultaneously, and would set up loops of energy flow, confusing the analysis. Further, since nearly every animal has (multiple) parasites, parasites would occupy the top levels of every food web. Parasites can play a role in the proliferation of non-native species. For example, invasive green crabs are minimally affected by native trematodes on the Eastern Atlantic coast. This helps them outcompete native crabs such as the rock and Jonah crabs. Ecological parasitology can be important to attempts at control, like during the campaign for eradicating the Guinea worm. Even though the parasite was eradicated in all but four countries, the worm began using frogs as an intermediary host before infecting dogs, making control more difficult than it would have been if the relationships had been better understood.


Rationale for conservation

Although parasites are widely considered to be harmful, the eradication of all parasites would not be beneficial. Parasites account for at least half of life's diversity; they perform important ecological roles; and without parasites, organisms might tend to asexual reproduction, diminishing the diversity of traits brought about by sexual reproduction. Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change. Many parasites require multiple hosts of different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. The presence of parasites thus indicates that an ecosystem is healthy. An ectoparasite, the California condor louse, '' Colpocephalum californici'', became a well-known conservation issue. A major and very costly captive breeding program was run in the United States to rescue the
California condor The California condor (''Gymnogyps californianus'') is a New World vulture and the largest North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to t ...
. It was host to a louse, which lived only on it. Any lice found were "deliberately killed" during the program, to keep the condors in the best possible health. The result was that one species, the condor, was saved and returned to the wild, while another species, the parasite, became extinct. Although parasites are often omitted in depictions of
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is Consumer-resource systems, consumer-resource system. Ecologists can broadly lump ...
s, they usually occupy the top position. Parasites can function like
keystone species A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine (zoologist), Robert T. Paine. Keystone species play ...
, reducing the dominance of superior competitors and allowing competing species to co-exist.


Quantitative ecology

A single parasite species usually has an aggregated distribution across host animals, which means that most hosts carry few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders
parametric statistics Parametric statistics is a branch of statistics which assumes that sample data comes from a population that can be adequately modeled by a probability distribution In probability theory and statistics, a probability distribution is the mathem ...
as commonly used by biologists invalid. Log-transformation of data before the application of parametric test, or the use of
non-parametric statistics Nonparametric statistics is the branch of statistics that is not based solely on Statistical parameter, parametrized families of probability distributions (common examples of parameters are the mean and variance). Nonparametric statistics is based ...
is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.


History


Ancient

Human parasite Human parasites include various protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organ ...
s including roundworms, the
Guinea worm ''Dracunculus medinensis'', or Guinea worm, is a nematode that causes dracunculiasis, also known as guinea worm disease. The disease is caused by the female which, at up to in length, is among the longest nematodes infecting humans. In contra ...
, threadworms and tapeworms are mentioned in Egyptian papyrus records from 3000 BC onwards; the Ebers Papyrus describes
hookworm Hookworms are intestinal, blood-feeding, parasitic roundworms The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nemat ...
. In
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean Sea, Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, classical antiquity ( AD 600), th ...
, parasites including the bladder worm are described in the
Hippocratic Corpus The Hippocratic Corpus (Latin: ''Corpus Hippocraticum''), or Hippocratic Collection, is a collection of around 60 early Ancient Greek medical works strongly associated with the physician Hippocrates and his teachings. The Hippocratic Text corpus ...
, while the comic playwright
Aristophanes Aristophanes (; grc, Ἀριστοφάνης, ; c. 446 – c. 386 BC), son of Philippus, of the deme Kydathenaion ( la, Cydathenaeum), was a comedy, comic playwright or comedy-writer of Classical Athens, ancient Athens and a poet of Ancient G ...
called tapeworms "hailstones". The Roman physicians Celsus and
Galen Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicization, Anglicized as Galen () or Galen of Pergamon, was a Ancient Greeks, Greek physician, surgeon and Philosophy, philosopher i ...
documented the roundworms ''
Ascaris lumbricoides ''Ascaris lumbricoides'' is a large parasitic worm Parasitic worms, also known as helminths, are large Parasitism#Basic concepts, macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transm ...
'' and ''
Enterobius vermicularis The pinworm (species ''Enterobius vermicularis''), also known as threadworm (in the United Kingdom, Australia and New Zealand) or seatworm, is a parasitic worm. It is a nematode (roundworm) and a common intestinal parasite or helminth, espec ...
''.


Medieval

In his '' Canon of Medicine'', completed in 1025, the Persian physician
Avicenna Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persians, Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the ...
recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms. In his 1397 book ''Traité de l'état, science et pratique de l'art de la Bergerie'' (Account of the state, science and practice of the art of shepherding), wrote the first description of a trematode endoparasite, the sheep liver fluke ''
Fasciola hepatica ''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitic trematode (fluke or flatworm, a type of helminth) of the class Trematoda, phylum Platyhelminthes. It infects the livers of various mammals, ...
''.


Early modern

In the early modern period,
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medical doctor, or simply doctor, is a he ...
's 1668 book ''Esperienze Intorno alla Generazione degl'Insetti'' (''Experiences of the Generation of Insects''), explicitly described ecto- and endoparasites, illustrating
ticks Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...
, the larvae of nasal flies of deer, and
sheep liver fluke ''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitic Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the ...
. Redi noted that parasites develop from eggs, contradicting the theory of
spontaneous generation Spontaneous generation is a Superseded scientific theories, superseded scientific theory that held that living creatures could arise from nonliving matter and that such processes were commonplace and regular. It was hypothesized that certain form ...
. In his 1684 book ''Osservazioni intorno agli animali viventi che si trovano negli animali viventi'' (''Observations on Living Animals found in Living Animals''), Redi described and illustrated over 100 parasites including the large roundworm in humans that causes
ascariasis Ascariasis is a disease caused by the parasitic roundworm The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes ...
. Redi was the first to name the cysts of ''
Echinococcus granulosus ''Echinococcus granulosus,'' also called the hydatid worm, hyper tape-worm or dog tapeworm, is a cyclophyllid cestode that dwells in the small intestine of canids as an adult, but which has important intermediate hosts such as livestock and huma ...
'' seen in dogs and sheep as parasitic; a century later, in 1760,
Peter Simon Pallas Peter Simon Pallas Fellow of the Royal Society, FRS FRSE (22 September 1741 – 8 September 1811) was a Prussian zoologist and botanist who worked in Russia between 1767 and 1810. Life and work Peter Simon Pallas was born in Berlin, the son ...
correctly suggested that these were the larvae of tapeworms. In 1681,
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist A microbiologist (from Ancient Greek, Greek ) is a scientist who studies microscopic life forms and processes. This includes study of ...
observed and illustrated the protozoan parasite ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. The ...
'', and linked it to "his own loose stools". This was the first protozoan parasite of humans to be seen under a microscope. A few years later, in 1687, the Italian biologists Giovanni Cosimo Bonomo and Diacinto Cestoni described
scabies Scabies (; also sometimes known as the seven-year itch) is a contagious skin infestation by the mite ''Sarcoptes scabiei''. The most common symptoms are severe itchiness and a papular, pimple-like rash. Occasionally, tiny burrows may appear on ...
as caused by the parasitic mite ''
Sarcoptes scabiei ''Sarcoptes scabiei'' or the itch mite is a parasite, parasitic mite that burrows into skin and causes scabies. The mite is found in all parts of the world. Humans are not the only mammals that can become infected. Other mammals, such as wild an ...
'', marking it as the first disease of humans with a known microscopic causative agent.


Parasitology

Modern
parasitology Parasitology is the study of parasites, their host (biology), hosts, and the relationship between them. As a List of biology disciplines, biological discipline, the scope of parasitology is not determined by the organism or environment in questio ...
developed in the 19th century with accurate observations and experiments by many researchers and clinicians; the term was first used in 1870. In 1828, James Annersley described
amoebiasis Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba ''Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, Weight loss, loss of weight, U ...
, protozoal infections of the intestines and the liver, though the pathogen, ''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mill ...
'', was not discovered until 1873 by Friedrich Lösch.
James Paget Sir James Paget, 1st Baronet FRS HFRSE (11 January 1814 – 30 December 1899) (, rhymes with "gadget") was an English surgeon and pathologist Pathology is the study of the causal, causes and effects of disease or injury. The word ''pat ...
discovered the intestinal nematode ''
Trichinella spiralis ''Trichinella spiralis'' is a viviparous nematode parasite, occurring in rodents, pigs, bears, hyenas and humans, and is responsible for the disease trichinosis. It is sometimes referred to as the "pork worm" due to it being typically encou ...
'' in humans in 1835. James McConnell described the human liver fluke, ''
Clonorchis sinensis ''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class trematode, Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, ...
'', in 1875.
Algernon Thomas Sir Algernon Phillips Withiel Thomas (3 June 1857 – 28 December 1937) was a New Zealand university professor, geologist, biologist and educationalist. He was born in Birkenhead, Cheshire, England in 1857 and died in Auckland, New Zealand in 19 ...
and
Rudolf Leuckart Karl Georg Friedrich Rudolf Leuckart (7 October 1822 – 22 February 1898) was a German zoologist born in Helmstedt. He was a nephew to naturalist Friedrich Sigismund Leuckart (1794–1843). Academic career He earned his degree from the Univ ...
independently made the first discovery of the life cycle of a trematode, the sheep liver fluke, by experiment in 1881–1883. In 1877
Patrick Manson Sir Patrick Manson (3 October 1844 – 9 April 1922) was a Scottish physician who made important discoveries in parasitology, and was a founder of the field of tropical medicine. He graduated from University of Aberdeen with degrees in Master o ...
discovered the life cycle of the filarial worms that cause
elephantiasis Elephantiasis is the enlargement and hardening of limbs or body parts due to tissue swelling. It is characterised by edema Edema, also spelled oedema, and also known as fluid retention, dropsy, hydropsy and swelling, is the build-up of fluid i ...
transmitted by mosquitoes. Manson further predicted that the
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects humans and other animals. Malaria causes Signs and symptoms, symptoms that typically include fever, fatigue (medical), tiredness, vomiting, and headaches. In se ...
parasite, ''
Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...
'', had a mosquito vector, and persuaded
Ronald Ross Sir Ronald Ross (13 May 1857 – 16 September 1932) was a British medical doctor who received the Nobel Prize for Physiology or Medicine in 1902 for his work on the transmission of malaria, becoming the first British Nobel laureate, and the f ...
to investigate. Ross confirmed that the prediction was correct in 1897–1898. At the same time, Giovanni Battista Grassi and others described the malaria parasite's life cycle stages in ''
Anopheles ''Anopheles'' () is a genus of mosquito first described and named by Johann Wilhelm Meigen, J. W. Meigen in 1818. About 460 species are recognised; while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus ' ...
'' mosquitoes. Ross was controversially awarded the 1902 Nobel prize for his work, while Grassi was not. In 1903, David Bruce identified the protozoan parasite and the
tsetse fly Tsetse ( , or ) (sometimes spelled tzetze; also known as tik-tik flies), are large, biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus ''Glossina'', which are placed in their own family, Glos ...
vector of
African trypanosomiasis African trypanosomiasis, also known as African sleeping sickness or simply sleeping sickness, is an insect-borne parasitic infection of humans and other animals. It is caused by the species ''Trypanosoma brucei''. Humans are infected by two typ ...
.


Vaccine

Given the importance of malaria, with some 220 million people infected annually, many attempts have been made to interrupt its transmission. Various methods of
malaria prophylaxis Malaria prophylaxis is the preventive treatment of malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects humans and other animals. Malaria causes Signs and symptoms, symptoms that typically include fever, ...
have been tried including the use of
antimalarial drug Antimalarial medications or simply antimalarials are a type of antiparasitic chemical agent, often natural product, naturally derived, that can be used to treat or to prevent malaria, in the latter case, most often aiming at two susceptible target ...
s to kill off the parasites in the blood, the eradication of its mosquito vectors with organochlorine and other insecticides, and the development of a
malaria vaccine A malaria vaccine is a vaccine that is used to prevent malaria. The only approved use of a vaccine outside the European Union, EU, as of 2022, is RTS,S, known by the brand name ''Mosquirix''. In October 2021, the WHO for the first time recommend ...
. All of these have proven problematic, with
drug resistance Drug resistance is the reduction in effectiveness of a medication such as an antimicrobial or an antineoplastic in treating a disease or condition. The term is used in the context of resistance that pathogens or cancers have "acquired", that is, ...
, insecticide resistance among mosquitoes, and repeated failure of vaccines as the parasite mutates. The first and as of 2015 the only licensed vaccine for any parasitic disease of humans is
RTS,S RTS,S/AS01 (trade name Mosquirix) is a Subunit vaccine, recombinant protein-based malaria vaccine. In October 2021, the vaccine was endorsed by the World Health Organization (WHO) for "broad use" in children, making it the first malaria vaccine ...
for ''
Plasmodium falciparum ''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans, and the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mosqu ...
'' malaria.


Resistance

Poulin observes that the widespread
prophylactic Preventive healthcare, or prophylaxis, consists of measures taken for the purposes of disease prevention.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, prolonging life, and promoting physical and mental hea ...
use of anthelmintic drugs in domestic sheep and cattle constitutes a worldwide un
controlled experiment A scientific control is an experiment or observation designed to minimize the effects of variables other than the independent variable (i.e. confounding variables). This increases the reliability of the results, often through a comparison betwe ...
in the life-history evolution of their parasites. The outcomes depend on whether the drugs decrease the chance of a helminth larva reaching adulthood. If so, natural selection can be expected to favour the production of eggs at an earlier age. If on the other hand the drugs mainly affects adult
parasitic worm Parasitic worms, also known as helminths, are large Parasitism#Basic concepts, macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted helminth, soil-transmitted and intestinal parasit ...
s, selection could cause delayed maturity and increased
virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host (biology), host. The Pathogen#Pathogenicity, pathogen ...
. Such changes appear to be underway: the nematode '' Teladorsagia circumcincta'' is changing its adult size and reproductive rate in response to drugs.


Cultural significance


Classical times

In the
classical era Classical antiquity (also the classical era, classical period or classical age) is the period of cultural history between the 8th century BC and the 5th century AD centred on the Mediterranean Sea, comprising the interlocking civilizations of ...
, the concept of the parasite was not strictly pejorative: the ''parasitus'' was an accepted role in Roman society, in which a person could live off the hospitality of others, in return for "flattery, simple services, and a willingness to endure humiliation".


Society

Parasitism has a derogatory sense in popular usage. According to the immunologist John Playfair, Playfair is comparing the popular usage to a biologist's view of parasitism, which he calls (heading the same page) "an ancient and respectable view of life". The
satirical Satire is a genre of the visual arts, visual, literature, literary, and performing arts, usually in the form of fiction and less frequently Nonfiction, non-fiction, in which vices, follies, abuses, and shortcomings are held up to ridicule, ...
cleric
Jonathan Swift Jonathan Swift (30 November 1667 – 19 October 1745) was an Anglo-Irish Satire, satirist, author, essayist, political pamphleteer (first for the Whig (British political party), Whigs, then for the Tories (British political party), Tories), poe ...
alludes to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes": A 2022 study examined the naming of some 3000 parasite species discovered in the previous two decades. Of those named after scientists, over 80% were named for men, whereas about a third of authors of papers on parasites were women. The study found that the percentage of parasite species named for relatives or friends of the author has risen sharply in the same period.


Fiction

In
Bram Stoker Abraham Stoker (8 November 1847 – 20 April 1912) was an Irish author who is celebrated for his 1897 Gothic Horror, Gothic horror novel ''Dracula''. During his lifetime, he was better known as the personal assistant of actor Sir Henry Irv ...
's 1897
Gothic horror Gothic fiction, sometimes called Gothic horror in the 20th century, is a loose literary aesthetic of fear and haunting. The name is a reference to Gothic architecture of the European Middle Ages, which was characteristic of the settings of ea ...
novel ''
Dracula ''Dracula'' is a novel by Bram Stoker, published in 1897. As an epistolary novel, the narrative is related through letters, diary entries, and newspaper articles. It has no single protagonist, but opens with solicitor Jonathan Harker taking ...
'', and its many film adaptations, the eponymous
Count Dracula Count Dracula () is the title character of Bram Stoker's 1897 gothic horror novel ''Dracula''. He is considered to be both the prototypical and the archetypal vampire in subsequent works of fiction. Aspects of the character are believed by some ...
is a blood-drinking parasite (a vampire). The critic Laura Otis argues that as a "thief, seducer, creator, and mimic, Dracula is the ultimate parasite. The whole point of vampirism is sucking other people's blood—living at other people's expense." Disgusting and terrifying parasitic alien species are widespread in
science fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imagination, imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, Paral ...
, as for instance in
Ridley Scott Sir Ridley Scott (born 30 November 1937) is a British film director and producer. Directing, among others, science fiction films, his work is known for its atmospheric and highly concentrated visual style. Scott has received many accolades th ...
's 1979 film '' Alien''. In one scene, a Xenomorph bursts out of the chest of a dead man, with blood squirting out under high pressure assisted by explosive squibs. Animal organs were used to reinforce the shock effect. The scene was filmed in a single take, and the startled reaction of the actors was genuine.


See also

*
Antiparasitic Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, parasitic nutrition#Ectoparasitism, ectoparasites, Microsporum, parasitic fungi, and protozoa, among o ...
* Carcinogenic parasite * Effects of parasitic worms on the immune system *
List of parasites of humans Endoparasites Protozoan organisms Helminths (worms) Helminth organisms (also called helminths or intestinal worms) include: Tapeworms Flukes Roundworms Other organisms Ectoparasites References

{{Portal bar, ...


Notes


References


Sources

*


Further reading

* * *


External links


Aberystwyth University: Parasitology
class outline with links to full text articles on parasitism and parasitology.
Division of Parasitic Diseases
Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the National public health institutes, national public health agency of the United States. It is a Federal agencies of the United States, United States federal agency, under the United S ...

KSU: Parasitology Research
parasitology articles and links
Parasitology Resources on the World Wide Web: A Powerful Tool for Infectious Disease Practitioners
(Oxford University Press)
Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance
Wikibooks {{Authority control Parasitology Ecology Disease ecology