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Parasitism is a
close relationship
close relationship
between
species In biology, a species is the basic unit of biological classification, 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 individu ...

species
, 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 (19 ...
, causing it some harm, and is
adapted In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a state reached by the popula ...
structurally to this way of life. The
entomologist upright=1.2, A Phyllium sp., mimicking a leaf Entomology () is the scientific Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions abo ...

entomologist
E. O. Wilson Edward Osborne Wilson (born June 10, 1929), usually cited as E. O. Wilson, is an American biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has ...
has characterised parasites as "predators that eat prey in units of less than one". Parasites include single-celled
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that ...

protozoa
ns such as the agents of
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
, sleeping sickness, and
amoebic dysentery Amoebiasis or amoebic dysentery Dysentery () is a type of gastroenteritis that results in bloody diarrhea. Other symptoms may include fever, abdominal pain, and a feeling of incomplete defecation. Complications may include dehydration In ...
; animals such as
hookworm Hookworms are intestinal, blood-feeding, parasitic roundworms that cause types of infection known as helminthiases. Hookworm infection is found in many parts of the world, and is common in areas with poor access to adequate water, sanitation, ...
s,
lice Louse (plural: lice) is the common name for members of the clade Phthiraptera, which contains nearly 5,000 species of wingless Parasitism, parasitic insect. Phthiraptera has variously been recognized as an order (biology), order, infraorder, ...

lice
,
mosquito Mosquitoes are members of a group of almost 3,600 species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defin ...

mosquito
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 Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly ...
s;
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
such as
honey fungus ''Armillaria'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, virus ...

honey fungus
and the agents of
ringworm Dermatophytosis, also known as ringworm, is a fungal infection Mycosis is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplication, and t ...

ringworm
; and plants such as
mistletoe Mistletoe is the common name for obligate{{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe 300px, Aerobic and anaerobic bacte ...

mistletoe
,
dodder ''Cuscuta'' () (dodder) is a genus of over 201 species of yellow, orange, or red (rarely green) parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirement from another living plant. They make up about ...
, 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 sens ...
of exploitation of animal hosts, namely
parasitic castrationParasitic castration is the strategy, by a parasite Parasitism is a symbiotic relationship between species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as wel ...
, directly transmitted parasitism (by contact), trophically transmitted parasitism (by being eaten),
vector Vector may refer to: Biology *Vector (epidemiology) In epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and risk factor, determinants of health and disease conditions in defined pop ...
-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. 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 (which ...

predators
, 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 (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

reproduce
at a faster rate than their hosts. Classic examples include interactions between
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
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 bod ...

tapeworms
, 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 ...

Plasmodium
'' species, and
fleas Flea, the common name for the order (biology), order Siphonaptera, includes 2,500 species of small flightless insects that survive as external parasitism, parasites of mammals and birds. Fleas live by consuming blood, or hematophagy, from the ...
. Parasites reduce host fitness by general or specialised
pathology Pathology is the study of the causesCauses, or causality, is the relationship between one event and another. It may also refer to: * Causes (band), an indie band based in the Netherlands * Causes (company), an online company See also * Cau ...
, 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: Science and technology * Power transmissionPower transmission is the movement of energy from its place of generation to a location where it is applied to perform useful Mechanical work, work. Power (physics), Power is d ...
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 biological classification, 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 individu ...

species
, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being
saprophytic Mycelial cord made up of a collection of hyphae; an essential part in the process of saprotrophic nutrition, it is used for the intake of organic matter through its cell wall. The network of hyphae is referred to as a mycelium, which is fundamental ...
. People have known about parasites such as
roundworms The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes being known as eelworms. They are a diverse animal phylum inhabiting a bro ...

roundworms
and tapeworms since
ancient Egypt Ancient Egypt was a civilization  A civilization (or civilisation) is a that is characterized by , , a form of government, and systems of communication (such as ). Civilizations are intimately associated with additional char ...

ancient Egypt
,
Greece Greece ( el, Ελλάδα, Elláda, ), officially the Hellenic Republic, is a country located in Southeastern Europe Southeast Europe or Southeastern Europe () is a geographical subregion A subregion is a part of a larger region In geogr ...
, and
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption = The te ...
. In
Early Modern The early modern period of modern history Human history, or world history, is the narrative of 's past. It is understood through , , , and , and since the , from and s. Humanity's written history was preceded by its , beginning with ...
times,
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium ...
observed ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like ...

Giardia lamblia
'' in his microscope in 1681, while
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italian langua ...

Francesco Redi
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, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or ins ...
and
tick Ticks (suborder Ixodida) are parasitic arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of in ...

tick
s. Modern
parasitology Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question but by their way of life. This means it f ...
developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to
satirical Satire is a genre Genre () is any form or type of communication in any mode (written, spoken, digital, artistic, etc.) with socially-agreed-upon conventions developed over time. In popular usage, it normally describes a Category of being, ...
effect in
Jonathan Swift Jonathan Swift (30 November 1667 – 19 October 1745) was an Anglo-Irish Anglo-Irish () is a term which was more commonly used in the 19th and early 20th centuries to identify an ethnic group An ethnic group or ethnicity is a grouping of ...
'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, best known today for his 1897 Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ethnonym of a group of East Germanic tribes * ...

Bram Stoker
's 1897
Gothic horror Gothic fiction, sometimes called Gothic horror in the 20th century, is a genre of literature and film that covers horror Horror may refer to: Arts, entertainment, and media Genres *Horror fiction, a genre of fiction **Japanese horror, Jap ...
novel ''
Dracula ''Dracula'' is a novel by Bram Stoker Abraham Stoker (8 November 1847 – 20 April 1912) was an Irish author, best known today for his 1897 Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ...

Dracula
'' and its many later adaptations featured a blood-drinking parasite.
Ridley Scott Sir Ridley Scott (born 30 November 1937) is an English film director and producer. He has directed, among others, the science fiction horror film ''Alien Alien primarily refers to: * Alien (law) In law, an alien is any person A person ...
's 1979 film ''
Alien Alien primarily refers to: * Alien (law) In law, an alien is any person A person (plural people or persons) is a being that has certain capacities or attributes such as reason, morality, consciousness or self-consciousness, and being a part ...
'' 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, Parall ...

science fiction
to feature a parasitic alien species.


Etymology

First used in English in 1539, the word ''parasite'' comes from the
Medieval French Old French (, , ; Modern French: ) was the language spoken in Northern France from the 8th century to the 14th century. Rather than a unified language A language is a structured system of communication used by humans, including speech (sp ...
''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 spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
''parasitus'', the
latinisation Latinisation or Latinization can refer to: * Latinisation of names, the practice of rendering a non-Latin name in a Latin style * Latinisation in the Soviet Union, the campaign in the USSR during the 1920s and 1930s to replace traditional writing sy ...
of the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
''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 ...

symbiosis
, a close and persistent long-term biological interaction between a parasite and its host. Unlike
saprotroph Image:Hyphae.JPG, Mycelial cord made up of a collection of hyphae; an essential part in the process of saprotrophic nutrition, it is used for the intake of organic matter through its cell wall. The network of hyphae is referred to as a mycelium, whi ...
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 In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their phy ...

commensalism
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 Vector may refer to: Biology *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism; a disease vector *Vector (molecular biology), a DNA molecule used as a vehicle to artificially carr ...
of pathogens, causing
disease A disease is a particular abnormal condition that negatively affects the structure A structure is an arrangement and organization of interrelated elements in a material object or system A system is a group of Interaction, interactin ...
.
Predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

Predation
is by definition not a symbiosis, as the interaction is brief, but the entomologist
E. O. Wilson Edward Osborne Wilson (born June 10, 1929), usually cited as E. O. Wilson, is an American biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has ...
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 scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxon, taxa (singular ...
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 parasitic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. ...
depends completely on the host to complete its life cycle, while a
facultative parasiteA facultative parasite is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Out ...
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), ...

copepod
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 macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract The gastrointestinal tract, (GI ...
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 A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environm ...
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 insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as L ...

scale insect
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 ...

aphid
s, and
caterpillar Caterpillars ( ) are the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Anim ...

caterpillar
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 disease A disease is a particular abnormal condition that negatively affects the structure A structure is an arrangement and organization of interrelated elements in ...
. 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 A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
, 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 sens ...
, namely
parasitic castrationParasitic castration is the strategy, by a parasite Parasitism is a symbiotic relationship between species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as wel ...
; directly transmitted parasitism; trophically-transmitted parasitism;
vector Vector may refer to: Biology *Vector (epidemiology) In epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and risk factor, determinants of health and disease conditions in defined pop ...
-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 ; 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 castratorParasitic 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. For example, '' Hemioniscus balani'', a parasitic castrator ...
s 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 An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference sh ...

barnacle
genus ''
Sacculina ''Sacculina'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a ...
'' specifically cause damage to the gonads of their many species of host
crab Crabs are decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda ...

crab
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 characteristics Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexual dimorphism, sexually dimorphic phenotypic traits that distinguis ...
such as broader abdomens, smaller
claws A domestic cat's retractable claw in protracted position A claw is a curved, pointed appendage found at the end of a toe or finger in most amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bo ...
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), ...

copepod
s and cyamid
amphipod Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far de ...

amphipod
s;
monogenea Monogeneans are a group of ectoparasitic flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

monogenea
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 An aggregated distribution, commonly found among predator 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 beh ...
.


Trophically transmitted

Trophically-transmitted parasites are transmitted by being eaten by a host. They include trematodes (all except ), ,
acanthocephala Acanthocephala (Greek language, Greek , ', thorn + , ', head) is a phylum (biology), phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an wiktionary:evert, eversibl ...

acanthocephala
ns, pentastomids, many round worms, and many protozoa such as ''Toxoplasma''. They have complex life-cycles involving hosts of two or more species. In their juvenile stages they infect and often encyst 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 parasites. Many trophically-transmitted parasites Parasite increased trophic transmission , 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 by multiple parasites is common. Strongyloides stercoralis#Autoinfection , Autoinfection, where (by exception) the whole of the parasite's biological life cycle , life-cycle takes place in a single primary host, can sometimes occur in helminths such as ''Strongyloides stercoralis''.


Vector-transmitted

Vector (epidemiology), 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 protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that ...

protozoa
, bacteria, or viruses, often intracellular pathogens (disease-causers). Their vectors are mostly hematophagy, hematophagic arthropods such as fleas, lice, ticks, and mosquitoes. For example, the deer tick ''Ixodes scapularis'' acts as a vector for diseases including Lyme disease, babesiosis, and Human granulocytic anaplasmosis, anaplasmosis. Protozoan endoparasites, such as the
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
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 ...

Plasmodium
'' and sleeping-sickness parasites in the genus ''Trypanosoma'', have infective stages in the host's blood which are transported to new hosts by biting insects.


Parasitoids

Parasitoids are insects which sooner or later kill their hosts, placing their relationship close to predation. Most parasitoids are parasitoid wasps or other hymenopterans; others include fly, dipterans such as phoridae , 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 oviposition , egg is laid on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, mass provisioning , feeding on the provisions left for it. Koinobiont parasitoids, which include Diptera , flies 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 pupating or making it ecdysis , moult 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 wasps 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 parasitoid wasps like this Braconidae , braconid ovipositor , 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, Phoridae , Phorid fly (centre left) is oviposition , laying eggs in the abdomen of a worker honey-bee, Behavior-altering parasites and parasitoids , 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 hematophagy, hematophagic, feeding on blood. They include annelids such as leeches, crustaceans such as branchiurans and Gnathiidae , gnathiid isopods, various dipterans such as mosquitoes and tsetse flies, other arthropods such as fleas and ticks, vertebrates such as lampreys, 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 Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly ...
s.


Transmission strategies

Parasites use a variety of methods to infect animal hosts, including physical contact, the fecal–oral route, 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

Hyperparasites 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 systems, there can be up to five levels of parasitism. Hyperparasites can control their hosts' populations, and are used for this purpose biological pest control, in agriculture and to some extent in medicine. The controlling effects can be seen in the way that the Hypovirus#CHV1–Chestnut blight hypovirulence, CHV1 virus helps to control the damage that chestnut blight, ''Cryphonectria parasitica'', does to Castanea dentata, American chestnut trees, and in the way that bacteriophages 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 eusociality, eusocial animals such as ants, termites, and bumblebees. Examples include the large blue butterfly, ''Phengaris arion'', its larvae employing ant mimicry to parasitise certain ants, ''Bombus bohemicus'', a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and ''Melipona scutellaris'', a eusociality, 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, the hosts act as parents as they raise the young as their own. Brood parasites include birds in different families such as cowbirds, Viduidae, whydahs, cuckoos, 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 biological classification, 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 individu ...

species
. The eggs of some brood parasites mimicry, mimic 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 (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 bees lay their eggs in the nest cells of other bees in the same family. Kleptoparasitism is uncommon generally but conspicuous in birds; some such as skuas 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, such as ''Ceratias holboelli'', where the males are reduced to tiny Sexual parasitism, 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 wikt:ἀδελφός, 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 Ploidy#haploid, haploid eggs in the fully developed larvae of their own species, producing male offspring, while the marine worm ''Bonellia viridis'' 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 Pteromalidae, 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 butterfly is an ant mimicry, ant mimic and social parasite. File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg, In Brood parasite, brood parasitism, the host raises the young of another species, here a cowbird's egg, that has been laid in its nest. File:Great Skua (cropped).jpg, The great skua is a powerful kleptoparasite, relentlessly pursuing other seabirds until they disgorge their catches of food. File:Северная церапия (cropped).jpg, The male anglerfish ''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 macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract The gastrointestinal tract, (GI ...
including trematoda, flukes and Cestoidea, cestodes have complete life cycles involving two or more hosts. By far the largest group is the parasitoid wasps in the Hymenoptera. The phylum, phyla and class (biology), 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{{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe 300px, Aerobic and anaerobic bacte ...

mistletoe
derives some of its nutrients from another living plant, whereas a obligate parasite, holoparasite such as
dodder ''Cuscuta'' () (dodder) is a genus of over 201 species of yellow, orange, or red (rarely green) parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirement from another living plant. They make up about ...
derives all of its nutrients from another plant. Parasitic plants make up about one per cent of angiosperms and are in almost every biome in the world. which appeared in Spanish as Chapter 2, pp. 7–27 in: J. A. López-Sáez, P. Catalán and L. Sáez [eds.], ''Parasitic Plants of the Iberian Peninsula and Balearic Islands''. All these plants have modified roots, haustorium, haustoria, which penetrate the host plants, connecting them to the conductive system – either the xylem, the phloem, 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 and therefore cannot make food for themselves by photosynthesis, they are always obligate parasites, deriving all their food from their hosts. Some parasitic plants can locate their host (biology), host plants by detecting Chemical substance, chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500
species In biology, a species is the basic unit of biological classification, 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 individu ...

species
of parasitic plant in approximately 20 Family (biology), families of flowering plants are known. Species within ''Orobanchaceae'' (broomrapes) are some of the most economically destructive of all plants. Species of ''Striga'' (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 Zea mays, corn, Oryza sativa, rice and sorghum, undoubtedly some of the most important food crops. ''Orobanche'' also threatens a wide range of other important crops, including peas, Cicer arietinum, chickpeas, Solanum lycopersicum, tomatoes, carrots, and varieties of cabbage. Yield loss from ''Orobanche'' can be total; despite extensive research, no method of control has been entirely successful. Many plants and
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
exchange carbon and nutrients in mutualistic mycorrhizal relationships. Some 400 species of myco-heterotrophy, myco-heterotrophic plants, mostly in the tropics, however effectively Cheating (biology), 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 bundles, 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 The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
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'' grow in the roots of a wide variety of trees, and eventually kill them. They then continue to live in the dead wood, feeding saprophytically. Fungal infection (mycosis) is widespread in animals including humans; it kills some 1.6 million people each year. Microsporidia 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 ...

Plasmodium
'', ''Trypanosoma'', and ''Entamoeba'', are endoparasitic. They cause serious diseases in vertebrates including humans – in these examples, malaria, sleeping sickness, and
amoebic dysentery Amoebiasis or amoebic dysentery Dysentery () is a type of gastroenteritis that results in bloody diarrhea. Other symptoms may include fever, abdominal pain, and a feeling of incomplete defecation. Complications may include dehydration In ...
– and have complex life cycles.


Bacteria

Many bacteria are parasitic, though they are more generally thought of as Pathogenic bacteria, pathogens causing disease. Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, ''Bacillus anthracis'', the cause of anthrax, is spread by contact with infected domestic animals; its spores, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. ''Borrelia'', the cause of Lyme disease and relapsing fever, is transmitted by vectors, ticks of the genus ''Ixodes'', from the diseases' reservoirs in animals such as deer. ''Campylobacter jejuni'', a cause of gastroenteritis, is spread by the fecal–oral route from animals, or by eating insufficiently cooked poultry, or by contaminated water. ''Haemophilus influenzae'', an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis, is transmitted by droplet contact. ''Treponema pallidum'', the cause of syphilis, is sexually transmitted disease, spread by sexual activity.


Viruses

Viruses 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 to animals, plants and fungi, it is unclear whether they can themselves be described as living. They can be either RNA viruses, RNA or DNA viruses consisting of a single or double strand of genetic material (RNA or DNA respectively), covered in a protein coat and sometimes a lipid envelope. They thus lack all the usual machinery of the cell (biology), cell such as enzymes, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are bacteriophages, 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 webs 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, but holes in the mandibles of several specimens of ''Tyrannosaurus'' may have been caused by ''Trichomonas''-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. Evolutionary epidemiology analyses how parasites spread and evolve, whereas Darwinian medicine applies similar evolutionary thinking to non-parasitic diseases like cancer and Autoimmune disease, 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 In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their phy ...

commensalism
or Mutualism (biology), 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 helminth, worms are often clearly harmed, such infections may also reduce the prevalence and effects of Autoimmunity, autoimmune disorders in animal hosts, including humans. In a more extreme example, some nematode worms cannot reproduce, or even survive, without infection by ''Wolbachia'' bacteria. Lynn Margulis and others have argued, following Peter Kropotkin'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 which formed the eukaryotes 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 Virulence, virulent parasites, by natural selection. Among competing parasitic insect-killing bacteria of the genera ''Photorhabdus'' and ''Xenorhabdus'', virulence depended on the relative potency of the antimicrobial toxins (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 larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Anim ...

caterpillar
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 Phoenicopteriformes, flamingos are more closely related to Ciconiiformes, storks or Anseriformes, ducks. 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 grebes. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then host switch, switched hosts to ducks, creating the situation which had confused biologists. Parasites infect sympatry, sympatric hosts (those within their same geographical area) more effectively, as has been shown with Digenea, digenetic trematodes infecting lake snails. This is in line with the Red Queen's Hypothesis, Red Queen hypothesis, 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 speciation, allopatric hosts, those from different geographical regions.


Modifying host behaviour

Some parasites Behavior-altering 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 host to avoid predators. This parasite matures in egrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan ''Toxoplasma gondii'', a parasite that matures in Felis silvestris catus, cats but can be carried by many other Mammalia, mammals. Uninfected Rattus rattus, rats 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 Bed bug (insect), bedbugs, Polyctenidae, batbugs,
lice Louse (plural: lice) is the common name for members of the clade Phthiraptera, which contains nearly 5,000 species of wingless Parasitism, parasitic insect. Phthiraptera has variously been recognized as an order (biology), order, infraorder, ...

lice
and fleas have lost their insect flight, 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'', an ectoparasite of fish and the only animal known to have lost the ability to respire aerobically: its cells lack mitochondria.


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 could have evolved to help to defeat multiple parasites by enabling genetic recombination, 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 and breeding male vertebrate hosts' secondary sex characteristics, such as the plumage of peacocks and the manes of lions. This is because the male hormone testosterone encourages the growth of secondary sex characteristics, favouring such males in sexual selection, at the price of reducing their immune defences.


Vertebrates

The physical barrier of the tough and often dry and waterproof skin of reptiles, birds and mammals keeps invading microorganisms from entering the body. Human skin also secretes sebum, which is toxic to most microorganisms. On the other hand, larger parasites such as trematodes detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate saliva and tears contain lysozyme, an enzyme that breaks down the Bacterial cell structure#Cell wall, cell walls of invading bacteria. Should the organism pass the mouth, the stomach with its hydrochloric acid, 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's serum proteins and pattern recognition receptors, intracellular and cellular, that trigger the adaptive immune system's lymphocytes such as T cells and antibody-producing B cells. 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 honeycomb, combs, rather than building a single comb like much of the rest of its genus, is to avoid infestation by Tineidae, 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, under the control of the jasmonic acid, jasmonic acid-insensitive (JA) and salicylic acid (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. Nonspecific 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 lepidopteran
caterpillar Caterpillars ( ) are the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Anim ...

caterpillar
s, leaves of plants including maize and cotton release increased amounts of volatile chemicals such as terpenes 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 parasitology, parasitologists, in a science dominated by medicine, rather than by ecology, ecologists or evolutionary biology, 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 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 Eradication of dracunculiasis, 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 Californian condor. 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 webs, they usually occupy the top position. Parasites can function like keystone species, reducing the dominance of superior competitors and allowing competition (biology), 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 as commonly used by biologists invalid. Data transformation (statistics)#Logarithmic transformation, Log-transformation of data before the application of parametric test, or the use of non-parametric statistics 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 parasites including roundworms, the Guinea worm, Pinworm (parasite), 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 that cause types of infection known as helminthiases. Hookworm infection is found in many parts of the world, and is common in areas with poor access to adequate water, sanitation, ...
. In ancient Greece, parasites including the bladder worm are described in the Hippocratic Corpus, while the comic playwright Aristophanes called tapeworms "hailstones". The Roman physicians Celsus and Galen documented the roundworms ''Ascaris lumbricoides'' and ''Enterobius vermicularis''.


Medieval

In his ''The Canon of Medicine, Canon of Medicine'', completed in 1025, the Persian physician Avicenna 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''.


Early Modern

In the
Early Modern The early modern period of modern history Human history, or world history, is the narrative of 's past. It is understood through , , , and , and since the , from and s. Humanity's written history was preceded by its , beginning with ...
period,
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italian langua ...

Francesco Redi
's 1668 book ''Esperienze Intorno alla Generazione degl'Insetti'' (''Experiences of the Generation of Insects''), explicitly described ecto- and endoparasites, illustrating ticks, the larvae of Cephenemyiinae, nasal flies of deer, and Fasciola hepatica, sheep liver fluke. Redi noted that parasites develop from eggs, contradicting the theory of spontaneous generation. 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 Ascaris lumbricoides, large roundworm in humans that causes ascariasis. Redi was the first to name the cysts of ''Echinococcus granulosus'' seen in dogs and sheep as parasitic; a century later, in 1760, Peter Simon Pallas 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 Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium ...
observed and illustrated the protozoan parasite ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like ...

Giardia lamblia
'', 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 as caused by the parasitic mite ''Sarcoptes scabiei'', marking it as the first disease of humans with a known microscopic causative agent.


Parasitology

Modern
parasitology Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question but by their way of life. This means it f ...
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, protozoal infections of the intestines and the liver, though the pathogen, ''Entamoeba histolytica'', was not discovered until 1873 by Friedrich Lösch. James Paget discovered the intestinal nematode ''Trichinella spiralis'' in humans in 1835. James McConnell described the human liver fluke, ''Clonorchis sinensis'', in 1875. Algernon Thomas and Rudolf Leuckart 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 discovered the life cycle of the filarioidea, filarial worms that cause lymphatic filariasis, elephantiasis transmitted by mosquitoes. Manson further predicted that the
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
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 ...

Plasmodium
'', had a mosquito vector, and persuaded Ronald Ross 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'' mosquitoes. Ross was Nobel Prize controversies#Physiology or medicine, controversially awarded the 1902 Nobel prize for his work, while Grassi was not. In 1903, David Bruce (microbiologist), David Bruce identified the protozoan parasite and the tsetse fly vector of African trypanosomiasis.


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 have been tried including the use of antimalarial drugs to kill off the parasites in the blood, the eradication of its mosquito vectors with insecticide, organochlorine and other insecticides, and the development of a malaria vaccine. All of these have proven problematic, with drug resistance, 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 for ''Plasmodium falciparum'' malaria.


Resistance

Poulin observes that the widespread prophylactic use of anthelmintic, anthelmintic drugs in domestic sheep and cattle constitutes a worldwide uncontrolled experiment 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 worms, selection could cause delayed maturity and increased virulence. Such changes appear to be underway: the nematode ''Teladorsagia circumcincta'' is changing its adult size and fecundity, reproductive rate in response to drugs.


Cultural significance


Classical times

In the classical era, the concept of the parasite was not strictly pejorative: the ''parasitus'' was an patronage in ancient Rome, 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 Parasitism (social offense), 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 Genre () is any form or type of communication in any mode (written, spoken, digital, artistic, etc.) with socially-agreed-upon conventions developed over time. In popular usage, it normally describes a Category of being, ...
cleric
Jonathan Swift Jonathan Swift (30 November 1667 – 19 October 1745) was an Anglo-Irish Anglo-Irish () is a term which was more commonly used in the 19th and early 20th centuries to identify an ethnic group An ethnic group or ethnicity is a grouping of ...
refers to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes":


Fiction

In
Bram Stoker Abraham Stoker (8 November 1847 – 20 April 1912) was an Irish author, best known today for his 1897 Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ethnonym of a group of East Germanic tribes * ...

Bram Stoker
's 1897
Gothic horror Gothic fiction, sometimes called Gothic horror in the 20th century, is a genre of literature and film that covers horror Horror may refer to: Arts, entertainment, and media Genres *Horror fiction, a genre of fiction **Japanese horror, Jap ...
novel ''
Dracula ''Dracula'' is a novel by Bram Stoker Abraham Stoker (8 November 1847 – 20 April 1912) was an Irish author, best known today for his 1897 Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ...

Dracula
'', and Dracula in popular culture, its many film adaptations, the eponymous Count Dracula is a blood-drinking parasite. 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 Parasites in fiction, 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, Parall ...

science fiction
, as for instance in
Ridley Scott Sir Ridley Scott (born 30 November 1937) is an English film director and producer. He has directed, among others, the science fiction horror film ''Alien Alien primarily refers to: * Alien (law) In law, an alien is any person A person ...
's 1979 film ''
Alien Alien primarily refers to: * Alien (law) In law, an alien is any person A person (plural people or persons) is a being that has certain capacities or attributes such as reason, morality, consciousness or self-consciousness, and being a part ...
''. In one scene, a Alien (creature in Alien franchise), Xenomorph bursts out of the chest of a dead man, with blood squirting out under high pressure assisted by bullet hit squib, explosive squibs. Organ (anatomy), 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.


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
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 Parasitism, Parasitology Ecology Disease ecology