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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, causing it some harm, and is
adapted In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
structurally to this way of life. The
entomologist Entomology () is the 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 as arach ...
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 single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
ns such as the agents of malaria,
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 ty ...
, and
amoebic dysentery 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, loss of weight, colonic ulce ...
; animals such as hookworms,
lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...
, mosquitoes, 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 feed solely on blood: the c ...
s; fungi such as
honey fungus ''Armillaria'' is a genus of fungi that includes the '' 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''. ''Armillarias'' are long-l ...
and the agents of
ringworm Dermatophytosis, also known as ringworm, is a 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. Multiple ar ...
; and plants such as mistletoe,
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 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 sense of the " ar ...
of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism (by contact), trophicallytransmitted parasitism (by being eaten), vector-transmitted parasitism, parasitoidism, 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, 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 at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the malaria-causing '' Plasmodium'' species, and fleas. Parasites reduce host fitness by general or specialised pathology, 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 chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...
. People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In early modern times,
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...
observed '' Giardia lamblia'' in his microscope in 1681, while
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian physician, naturalist, biologist, and poet. He is referred to as the "founder of experimental biology", and as the "father of modern parasitology". He was the first person to ch ...
described internal and external parasites including sheep liver fluke 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 developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift's 1733 poem "On Poetry: A Rhapsody", comparing poets to
hyperparasitical A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two othe ...
"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 novel ''Dracula''. During his lifetime, he was better known as the personal assistant of actor Sir Henry Irving and busines ...
'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'' and its many later adaptations featured a blood-drinking parasite. Ridley Scott's 1979 film ''
Alien Alien primarily refers to: * Alien (law), a person in a country who is not a national of that country ** Enemy alien, the above in times of war * Extraterrestrial life, life which does not originate from Earth ** Specifically, intelligent extrater ...
'' 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 imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel univers ...
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 most of the northern half of France from approximately the 8th to the 14th centuries. Rather than a unified language, Old French was a linkage of Romance dialects, mutually intelligib ...
''parasite'', from the Latin ''parasitus'', the latinisation of the Greek ''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 Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasi ...
, a close and persistent long-term biological interaction between a parasite and its host. Unlike saprotrophs, 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, in which both organisms benefit fr ...
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. Predation 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. Taxonomists 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 depends completely on the host to complete its life cycle, while a
facultative parasite A facultative parasite is an organism that may resort to parasitic activity, but does not absolutely rely on any host for completion of its life cycle. Examples of facultative parasitism occur among many species of fungi, such as family members ...
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. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...
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 helminths 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 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 The snubnosed eel, ''Simenchelys parasitica'', also known as the pug-nosed eel, slime eel, or snub-nose parasitic eel, is a species of deep-sea eel and the only member of its genus. Some authors classify it as the sole member of the subfamily Sim ...
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 Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient grouping than the ...
s,
aphid Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...
s, and
caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Symph ...
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 fungi, oom ...
. 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, 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 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 sense of the " ar ...
, namely parasitic castration; directly transmitted parasitism; trophically-transmitted parasitism; vector-transmitted parasitism; parasitoidism; 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 castrator 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 ...
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 constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive ...
genus ''
Sacculina ''Sacculina'' is a genus of barnacles that is a parasitic castrator of crabs. They belong to a group called '' Rhizocephala''. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because t ...
'' specifically cause damage to the gonads of their many species of host
crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
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 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 tarsus ...
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. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...
s and
cyamid A whale louse is a commensal crustacean of the family Cyamidae. Despite the name, it is not a true louse (which are insects), but rather is related to the skeleton shrimp, most species of which are found in shallower waters. Whale lice are exter ...
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 descri ...
s; monogeneans; 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 predators and parasites, is a highly uneven ( skewed) statistical distribution pattern in which they collect or aggregate in regions, which may be widely separated, where their prey or hosts are at ...
.


Trophically transmitted

Trophically-transmitted parasites are transmitted by being eaten by a host. They include
trematodes Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive hos ...
(all except
schistosomes ''Schistosoma'' is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed ''schistosomiasis'', which is considered by the World Health Organi ...
),
cestodes Cestoda is a 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 bodies consist of ma ...
,
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 to p ...
ns, pentastomids, many
roundworms The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
, and many protozoa such as ''
Toxoplasma ''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but felids, such as ...
''. 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 dormant stage of a microorganism, usually a bacterium or a protist or rarely an invertebrate animal, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of ...
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 inf ...
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 of a host by multiple pathogen species. In virology, coinfection includes simultaneous infection of a single cell by two or more virus particles. An example is the coinfection of liver cells with he ...
by multiple parasites is common.
Autoinfection Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
, 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 ''Strongyloides stercoralis'' is a human pathogenic parasitic roundworm causing the disease strongyloidiasis. Its common name in the US is threadworm. In the UK and Australia, however, the term ''threadworm'' can also refer to nematodes of the ...
''.


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 single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, bacteria, or viruses, often intracellular pathogens (disease-causers). Their vectors are mostly
hematophagic Hematophagy (sometimes spelled haematophagy or hematophagia) is the practice by certain animals of feeding on blood (from the Greek words αἷμα ' "blood" and φαγεῖν ' "to eat"). Since blood is a fluid tissue rich in nutritious pr ...
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, o ...
s such as fleas, lice, ticks, and mosquitoes. For example, the deer tick '' Ixodes scapularis'' acts as a vector for diseases including Lyme disease,
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 Anaplasmosis is a tick-borne disease affecting ruminants, dogs, and horses, and is caused by ''Anaplasma'' bacteria. Anaplasmosis is an infectious but not contagious disease. Anaplasmosis can be transmitted through mechanical and biological vector ...
. Protozoan endoparasites, such as the malarial parasites in the genus '' Plasmodium'' and sleeping-sickness parasites in the genus ''
Trypanosoma ''Trypanosoma'' is a genus of kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasitic flagellate protozoa. Trypanosoma is part of the phylum Sarcomastigophora. The name is derived from the Greek ''trypano-'' (bore ...
'', 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 dipterans such as phorid flies. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts.
Idiobiont In evolutionary ecology, a parasitoid is an organism that lives in close association with its host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionary strategies within parasitis ...
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 parasitoids, which include
flies Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
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 moult whenever the parasitoid is ready to moult. They may do this by producing hormones that mimic the host's moulting hormones (
ecdysteroid Ecdysteroids are arthropod steroid hormones that are mainly responsible for molting, development and, to a lesser extent, reproduction; examples of ecdysteroids include ecdysone, ecdysterone, turkesterone and 2-deoxyecdysone. These compounds are ...
s), 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 In evolutionary ecology, a parasitoid is an organism that lives in close association with its host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionary strategies within parasitis ...
parasitoid wasps immediately paralyse their hosts for their larvae (
Pimplinae Pimplinae are a worldwide subfamily of the parasitic wasp family Ichneumonidae.Gavin Broad (1966Identification key to the subfamilies of Ichneumonidae/ref> Pimplinae are parasitoids of Endopterygota, often the pupae of Lepidoptera. Various spec ...
, pictured) to eat. File:CSIRO ScienceImage 2357 Spotted alfalfa aphid being attacked by parasitic wasp.jpg, Koinobiont parasitoid wasps like this
braconid The Braconidae are a family of parasitoid wasps. After the closely related Ichneumonidae, braconids make up the second-largest family in the order Hymenoptera, with about 17,000 recognized species and many thousands more undescribed. One analysis ...
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 The Phoridae are a family of small, hump-backed flies resembling fruit flies. Phorid flies can often be identified by their escape habit of running rapidly across a surface rather than taking to the wing. This behaviour is a source of one of the ...
(centre left) is laying eggs in the abdomen of a worker
honey-bee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current cosmop ...
, 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 Hematophagy (sometimes spelled haematophagy or hematophagia) is the practice by certain animals of feeding on blood (from the Greek words αἷμα ' "blood" and φαγεῖν ' "to eat"). Since blood is a fluid tissue rich in nutritious pr ...
, feeding on blood. They include annelids such as
leech Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodie ...
es, crustaceans such as
branchiura The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family, ...
ns and gnathiid isopods, various dipterans 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 jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like ...
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 feed solely on blood: the c ...
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 route of transmission of a disease wherein pathogens in fecal particles pass from one person to the mouth of another person. Main causes of feca ...
, 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, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two ot ...
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. Oak apples range in size from in diameter and are caused by chemicals injected by the larva of certain kinds of gall w ...
systems, there can be up to five levels of parasitism. Hyperparasites can control their hosts' populations, and are used for this purpose
in agriculture IN, In or in may refer to: Places * India (country code IN) * Indiana, United States (postal code IN) * Ingolstadt, Germany (license plate code IN) * In, Russia, a town in the Jewish Autonomous Oblast Businesses and organizations * Indepe ...
and to some extent in medicine. 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 φαγεῖν ('), meaning "to devour". Bacteri ...
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 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 22,0 ...
s,
termite Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...
s, and
bumblebee A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related gener ...
s. Examples include the large blue butterfly, ''
Phengaris arion 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 Britain in 1795. In 1979 the species became mostly extinct in Britain but has been successfully ...
'', 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 un ...
to parasitise certain ants, ''
Bombus bohemicus ''Bombus bohemicus'', also known as the gypsy's cuckoo bumblebee, is a species of socially parasitic cuckoo bumblebee found in most of Europe with the exception of the southern Iberian Peninsula and Iceland. ''B. bohemicus'' practices inquili ...
'', 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 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 ''Tetramorium inquilinum'' is an ectoparasitic ant found in Europe. It was discovered by Swiss myrmecologist Heinrich Kutter. The species is unusual for lacking a worker caste, the queens and males living entirely off the care of the pavement ...
'', 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 In 1909, the entomologist Carlo Emery noted that social parasites among insects (e.g., kleptoparasites) tend to be parasites of species or genera to which they are closely related.Emery, C. "Über den Ursprung der dulotischen, parasitischen und my ...
", 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 capuchin The wedge-capped capuchin or Guianan weeper capuchin (''Cebus olivaceus'') is a capuchin monkey from South America. It is found in northern Brazil, Guyana and Venezuela. ''Cebus olivaceus'' is known to dwell in tall, primary forest and travel o ...
s, 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
cowbird Cowbirds are birds belonging to the genus ''Molothrus'' in the 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 English naturalist Will ...
s, whydahs,
cuckoo Cuckoos are birds in the Cuculidae family, the sole taxon in the order Cuculiformes . The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals and anis. The coucals and anis are sometimes sepa ...
s, and
black-headed duck The black-headed duck (''Heteronetta atricapilla'') is a South American duck in subfamily Oxyurinae of family Anatidae.HBW and BirdLife International (2021) Handbook of the Birds of the World and BirdLife International digital checklist of the ...
s. 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 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 The common cuckoo (''Cuculus canorus'') is a member of the cuckoo order of birds, Cuculiformes, which includes the roadrunners, the anis and the coucals. This species is a widespread summer migrant to Europe and Asia, and winters in Africa. It ...
further mimics a predator, the
European sparrowhawk The Eurasian sparrowhawk (''Accipiter nisus''), also known as the northern sparrowhawk or simply the sparrowhawk, is a small bird of prey in the family Accipitridae. Adult male Eurasian sparrowhawks have bluish grey upperparts and orange-barred ...
, 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 bee The term cuckoo bee is used for a variety of different bee lineages which have evolved the kleptoparasitic behaviour of laying their eggs in the nests of other bees, reminiscent of the behavior of cuckoo birds. The name is perhaps best applied to ...
s lay their eggs in the nest cells of other
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfami ...
s in the same family. Kleptoparasitism is uncommon generally but conspicuous in birds; some such as
skuas The skuas are a group of predatory seabirds with seven species forming the genus ''Stercorarius'', the only genus in the family Stercorariidae. The three smaller skuas, the long-tailed skua, the Arctic skua, and the pomarine skua are called jae ...
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 of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence c ...
, such as ''
Ceratias holboelli ''Ceratias'' is a small genus of seadevils. Species There are currently three recognized species in this genus: * '' Ceratias holboelli'' Krøyer, 1845 (Krøyer's deep sea angler fish) * '' Ceratias tentaculatus'' Norman Norman or Normans may ...
'', 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 ''Encarsia perplexa'' is a tiny parasitic wasp, a parasitoid of the citrus blackfly, ''Aleurocanthus woglumi'', which is a global pest of citrus trees. It was originally misidentified as ''Encarsia opulenta'', but was recorded as a new specie ...
'', unmated females of which may lay haploid 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 (class Polychaeta , phylum Annelida) noted for displaying exceptional sexual dimorphism and for the biocidal properties of a pigment in its skin.Murina, G. (2008). Bonellia viridis R ...
'' 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 A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two othe ...
pteromalid wasp on the cocoons of its host, itself a parasitoid
braconid wasp The Braconidae are a family of parasitoid wasps. After the closely related Ichneumonidae, braconids make up the second-largest family in the order Hymenoptera, with about 17,000 recognized species and many thousands more undescribed. One analysis ...
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 Britain in 1795. In 1979 the species became mostly extinct in Britain but has been successfully ...
butterfly is an
ant mimic 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 un ...
and social parasite. File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg, In brood parasitism, the host raises the young of another species, here a
cowbird Cowbirds are birds belonging to the genus ''Molothrus'' in the 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 English naturalist Will ...
'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 fro ...
is a powerful
kleptoparasite Kleptoparasitism (etymologically, parasitism by theft) is a form of feeding in which one animal deliberately takes food from another. The strategy is evolutionarily stable when stealing is less costly than direct feeding, which can mean when foo ...
, relentlessly pursuing other seabirds until they disgorge their catches of food. File:Северная церапия (cropped).jpg, The male
anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence c ...
''
Ceratias holboelli ''Ceratias'' is a small genus of seadevils. Species There are currently three recognized species in this genus: * '' Ceratias holboelli'' Krøyer, 1845 (Krøyer's deep sea angler fish) * '' Ceratias tentaculatus'' Norman Norman or Normans may ...
'' lives as a tiny sexual parasite permanently attached below the female's body. File:Encarsia perplexa.jpg, ''
Encarsia perplexa ''Encarsia perplexa'' is a tiny parasitic wasp, a parasitoid of the citrus blackfly, ''Aleurocanthus woglumi'', which is a global pest of citrus trees. It was originally misidentified as ''Encarsia opulenta'', but was recorded as a new specie ...
'' (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 including flukes and
cestodes Cestoda is a 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 bodies consist of ma ...
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 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 parasitic plants develop a specialized organ called th ...
or ''partial parasite'' such as mistletoe derives some of its nutrients from another living plant, whereas a
holoparasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, ...
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 parasitic plants develop a specialized organ called th ...
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 words ('container, vessel') and ('seed'), and refers to those plants th ...
and are in almost every
biome A biome () is a biogeographical unit consisting of a biological 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. Biome is a broader ...
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 In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates ...
, 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 plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
, 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 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 Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
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 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 elements by physical separation methods, i.e., with ...
in the air or soil given off by host
shoot In botany, a plant shoot consists of any plant stem together with its appendages, leaves and lateral buds, flowering stems, and flower buds. The new growth from seed germination that grows upward is a shoot where leaves will develop. In the spr ...
s or roots, 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 of flowering plants are known. Species within the ''
Orobanchaceae Orobanchaceae, the broomrapes, is a family of mostly parasitic plants of the order Lamiales, with about 90 genera and more than 2000 species. Many of these genera (e.g., ''Pedicularis'', ''Rhinanthus'', '' Striga'') were formerly included in ...
'' (broomrapes) are among 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
corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...
, rice, and sorghum, undoubtedly some of the most important food crops. ''
Orobanche ''Orobanche'', commonly known as broomrape, is a genus of over 200 species of small parasitic herbaceous plants, mostly native to the temperate Northern Hemisphere. It is the type genus of the broomrape family Orobanchaceae. Description Broom ...
'' also threatens a wide range of other important crops, including peas,
chickpeas The chickpea or chick pea (''Cicer arietinum'') is an annual legume of the family Fabaceae, subfamily Faboideae. Its different types are variously known as gram" or Bengal gram, garbanzo or garbanzo bean, or Egyptian pea. Chickpea seeds are ...
, 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 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 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'', ''
Trypanosoma ''Trypanosoma'' is a genus of kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasitic flagellate protozoa. Trypanosoma is part of the phylum Sarcomastigophora. The name is derived from the Greek ''trypano-'' (bore ...
'', and ''Entamoeba'' are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, and amoebiasis, amoebic dysentery—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 Orthornavirae, 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
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 φαγεῖν ('), meaning "to devour". Bacteri ...
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 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 (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, in which both organisms benefit fr ...
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 of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Symph ...
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 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 Cimex, bedbugs, Polyctenidae, batbugs,
lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...
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 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. 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 lepidopteran
caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Symph ...
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 parasitologists, in a science dominated by medicine, rather than by ecologists 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 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 California 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), 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. 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 period,
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian physician, naturalist, biologist, and poet. He is referred to as the "founder of experimental biology", and as the "father of modern parasitology". He was the first person to ch ...
'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 microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...
observed and illustrated the protozoan parasite '' 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 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 parasite, '' 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 cleric Jonathan Swift 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 novel ''Dracula''. During his lifetime, he was better known as the personal assistant of actor Sir Henry Irving and busines ...
'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'', and Dracula in popular culture, its many film adaptations, the eponymous Count Dracula 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 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 imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel univers ...
, as for instance in Ridley Scott's 1979 film ''
Alien Alien primarily refers to: * Alien (law), a person in a country who is not a national of that country ** Enemy alien, the above in times of war * Extraterrestrial life, life which does not originate from Earth ** Specifically, intelligent extrater ...
''. 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.


See also

*Antiparasitic *Carcinogenic parasite *Effects of parasitic worms on the immune system *List of parasites of humans


Notes


References


Sources

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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