There are six major parasiticstrategies of exploitation of animal hosts, namelyparasitic castration, directly transmitted parasitism (by contact),trophically-transmitted 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 ofconsumer–resource interaction,[3] but unlikepredators, parasites, with the exception of parasitoids, are 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 highlyspecialised, each parasite species living on one given animal species, andreproduce at a faster rate than their hosts. Classic examples include interactions betweenvertebrate hosts andtapeworms,flukes, and those between the malaria-causingPlasmodium species, andfleas.
Parasites reduce hostfitness by general or specialisedpathology, that ranges fromparasitic castration tomodification 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 theirtransmission from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions betweenspecies, grading via parasitoidism into predation, through evolution intomutualism, and in some fungi, shading into beingsaprophytic.
First used in English in 1539, the wordparasite comes from theMedieval Frenchparasite, from theLatinised formparasitus, from Ancient Greek παράσιτος[5] (parasitos)'one who eats at the table of another' in turn fromπαρά[6] (para)'beside, by' and σῖτος (sitos)'wheat, food'.[7] The related termparasitism appears in English from 1611.[8]
Parasitism is a kind ofsymbiosis, a close and persistent long-term biological interaction between a parasite and its host. Unlikesaprotrophs, parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Unlikecommensalism andmutualism, 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 asvectors of pathogens, causingdisease.[9][10][11]Predation is by definition not a symbiosis, as the interaction is brief, but the entomologistE. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".[2]
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 theirlife cycles, which can be complex. Anobligate parasite depends completely on the host to complete its life cycle, while afacultative parasite 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".[12][13] An endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface.[14] Mesoparasites—like somecopepods, for example—enter an opening in the host's body and remain partly embedded there.[15] Some parasites can be generalists, feeding on a wide range of hosts, but many parasites, and the majority of protozoans andhelminths that parasitise animals, are specialists and extremely host-specific.[14] An early basic, functional division of parasites distinguished microparasites and macroparasites. These each had amathematical model assigned in order to analyse the population movements of the host–parasite groupings.[16] 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.[16][17]
Much of the thinking on types of parasitism has focused on terrestrial animal parasites of animals, such as helminths. Those in other environments and with other hosts often have analogous strategies. For example, thesnubnosed eel is probably a facultative endoparasite (i.e., it is semiparasitic) that opportunistically burrows into and eats sick and dying fish.[18]Plant-eating insects such asscale insects,aphids, andcaterpillars closely resemble ectoparasites, attacking much larger plants; they serve as vectors of bacteria, fungi and viruses which causeplant diseases. As female scale insects cannot move, they are obligate parasites, permanently attached to their hosts.[16]
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,[19] exhaledcarbon dioxide, skin odours, visual and heat signatures, and moisture.[20] Parasitic plants can use, for example, light, host physiochemistry, and volatiles to recognize potential hosts.[21]
There are six major parasiticstrategies, namelyparasitic 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.[16][22] These strategies representadaptive peaks; intermediate strategies are possible, but organisms in many different groups have consistentlyconverged on these six, which are evolutionarily stable.[22]
A perspective on the evolutionary options can be gained by considering four key questions: the effect on thefitness 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.[23]
Evolutionary strategies in parasitism and predation[23] (Intensity-dependent: green, roman; Intensity-independent: purple, italics)
Parasitic castrators partly or completely destroy their host's ability to reproduce, diverting the energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host's other systems remain intact, allowing it to survive and to sustain the parasite.[22][24] Parasitic crustaceans such as those in the specialisedbarnacle genusSacculina specifically cause damage to the gonads of their many species[25] of hostcrabs. In the case ofSacculina, the testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to develop femalesecondary sex characteristics such as broader abdomens, smallerclaws 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, thetrematodeZoogonus lasius, whosesporocysts lack mouths, castrates the intertidal marine snailTritia obsoleta chemically, developing in its gonad and killing its reproductive cells.[24][26]
Directly transmitted parasites, not requiring a vector to reach their hosts, include such parasites of terrestrial vertebrates as lice and mites; marine parasites such ascopepods andcyamid amphipods;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 anaggregated distribution.[22]
Trophically-transmitted parasites are transmitted by being eaten by a host. They include trematodes (all exceptschistosomes),cestodes,acanthocephalans,pentastomids, manyroundworms, and many protozoa such asToxoplasma.[22] They have complex life cycles involving hosts of two or more species. In their juvenile stages they infect and oftenencyst 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 asintestinal parasites. Many trophically transmitted parasitesmodify 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.[22]Coinfection by multiple parasites is common.[27]Autoinfection, where (by exception) the whole of the parasite'slife cycle takes place in a single primary host, can sometimes occur in helminths such asStrongyloides stercoralis.[28]
Vector-transmitted parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually,[14] to carry them from one definitive host to another.[22] These parasites are microorganisms, namelyprotozoa,bacteria, orviruses, often intracellularpathogens (disease-causers).[22] Their vectors are mostlyhematophagic arthropods such as fleas, lice, ticks, and mosquitoes.[22][29] For example, the deer tickIxodes scapularis acts as a vector for diseases includingLyme disease,babesiosis, andanaplasmosis.[30] Protozoan endoparasites, such as themalarial parasites in the genusPlasmodium and sleeping-sickness parasites in the genusTrypanosoma, have infective stages in the host's blood which are transported to new hosts by biting insects.[31]
Parasitoids are insects which sooner or later kill their hosts, placing their relationship close to predation.[32] Most parasitoids areparasitoid wasps or otherhymenopterans; others includedipterans such asphorid flies. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts.[33]
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. Anegg 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.[33]
Koinobiont parasitoids, which includeflies 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 frompupating or making itmoult 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.[33]
Mosquitoes are micropredators and important vectors of disease
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.[22][23][34] Most micropredators arehematophagic, feeding on blood. They include annelids such asleeches, crustaceans such asbranchiurans andgnathiid isopods, variousdipterans such as mosquitoes andtsetse flies, other arthropods such as fleas and ticks, vertebrates such aslampreys, and mammals such asvampire bats.[22]
Parasites use a variety of methods to infect animal hosts, including physical contact, thefecal–oral route, free-living infectious stages, and vectors, suiting their differing hosts, life cycles, and ecological contexts.[35] Examples to illustrate some of the many possible combinations are given in the table.
Examples of transmission methods in different ecological contexts[35]
Among the many variations on parasitic strategies are hyperparasitism,[37] social parasitism,[38] brood parasitism,[39] kleptoparasitism,[40] sexual parasitism,[41] and adelphoparasitism.[42]
Hyperparasites feed on another parasite, as exemplified by protozoa living in helminth parasites,[37] or facultative or obligate parasitoids whose hosts are either conventional parasites or parasitoids.[22][33] Levels of parasitism beyond secondary also occur, especially among facultative parasitoids. Inoak gall systems, there can be up to four levels of parasitism.[43]
Hyperparasites can control their hosts' populations, and are used for this purposein agriculture and to some extent inmedicine. The controlling effects can be seen in the way that theCHV1 virus helps to control the damage thatchestnut blight,Cryphonectria parasitica, does toAmerican chestnut trees, and in the way thatbacteriophages 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.[44]
Social parasites take advantage of interspecific interactions between members ofeusocial animals such asants,termites, andbumblebees. Examples include the large blue butterfly,Phengaris arion, its larvae employingant mimicry to parasitise certain ants,[38]Bombus bohemicus, a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, andMelipona scutellaris, a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen.[45] An extreme example of interspecific social parasitism is found in the antTetramorium inquilinum, an obligate parasite which lives exclusively on the backs of otherTetramorium ants.[46] A mechanism for the evolution of social parasitism was first proposed by Carlo Emery in 1909.[47] 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.[48][49][50]
Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. Inwedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation.[51]
Inbrood parasitism, the hosts suffer increased parental investment and energy expenditure to feed parasitic young, which are commonly larger than host young. The growth rate of host nestlings is slowed, reducing the host's fitness. Brood parasites include birds in different families such ascowbirds,whydahs,cuckoos, andblack-headed ducks. These do not build nests of their own, but leave their eggs in nests of otherspecies. In the familyCuculidae, over 40% of cuckoo species are obligate brood parasites, while others are either facultative brood parasites or provide parental care.[52] The eggs of some brood parasitesmimic 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.[39][53][54] The adult femaleEuropean cuckoo further mimics a predator, theEuropean sparrowhawk, giving her time to lay her eggs in the host's nest unobserved.[55] Host species often combat parasitic egg mimicry through eggpolymorphism, having two or more egg phenotypes within a single population of a species. Multiple phenotypes in host eggs decrease the probability of a parasitic species accurately "matching" their eggs to host eggs.[56]
Inkleptoparasitism (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 ofcuckoo bees lay their eggs in the nest cells of otherbees in the same family.[40] Kleptoparasitism is uncommon generally but conspicuous in birds; some such asskuas are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch.[57]
A unique approach is seen in some species ofanglerfish, such asCeratias holboelli, where the males are reduced to tinysexual 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.[41]
Adelphoparasitism, (from Greekἀδελφός (adelphós), brother[58]), also known as sibling-parasitism, occurs where the host species is closely related to the parasite, often in the same family or genus.[42] In the citrus blackfly parasitoid,Encarsia perplexa, unmated females may layhaploid eggs in the fully developed larvae of their own species, producing male offspring,[59] while the marine wormBonellia viridis has a similar reproductive strategy, although the larvae are planktonic.[60]
Parasitism is widespread in the animal kingdom,[65] and has evolved independently from free-living forms hundreds of times.[22] Many types ofhelminth includingflukes andcestodes have complete life cycles involving two or more hosts. By far the largest group is the parasitoid wasps in the Hymenoptera.[22] Thephyla andclasses 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.[62]
Ahemiparasite orpartial parasite such asmistletoe derives some of its nutrients from another living plant, whereas aholoparasite such asCuscuta derives all of its nutrients from another plant.[66]Parasitic plants make up about one per cent ofangiosperms and are in almost everybiome in the world.[67][68][69] All these plants have modified roots,haustoria, which penetrate the host plants, connecting them to the conductive system—either thexylem, thephloem, 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 nochlorophyll and therefore cannot make food for themselves byphotosynthesis, they are always obligate parasites, deriving all their food from their hosts.[68] Some parasitic plants can locate theirhost plants by detectingchemicals in the air or soil given off by hostshoots orroots, respectively. About 4,500species of parasitic plant in approximately 20families offlowering plants are known.[68][70]
Species within theOrobanchaceae (broomrapes) are among the most economically destructive of all plants. Species ofStriga (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, includingcorn,rice, andsorghum, which are among the world's most important food crops.Orobanche also threatens a wide range of other important crops, includingpeas,chickpeas,tomatoes,carrots, and varieties ofcabbage. Yield loss fromOrobanche can be total; despite extensive research, no method of control has been entirely successful.[71]
Manyplants andfungi exchange carbon and nutrients in mutualisticmycorrhizal relationships. Some 400 species ofmyco-heterotrophic plants, mostly in the tropics, however effectivelycheat 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 fewvascular bundles, and their leaves are reduced to small scales, as they do not photosynthesize. Their seeds are small and numerous, so they appear to rely on being infected by a suitable fungus soon after germinating.[72]
The honey fungus,Armillaria mellea, is a parasite of trees, and asaprophyte feeding on the trees it has killed.
Parasiticfungi derive some or all of their nutritional requirements from plants, other fungi, or animals.
Plant pathogenic fungi are classified into three categories depending on their mode of nutrition: biotrophs,hemibiotrophs and necrotrophs. Biotrophic fungi derive nutrients from living plant cells, and during the course of infection they colonise their plant host in such a way as to keep it alive for a maximally long time.[73] One well-known example of a biotrophic pathogen isUstilago maydis, causative agent of the corn smut disease. Necrotrophic pathogens on the other hand, kill host cells and feedsaprophytically, an example being the root-colonising honey fungi in the genusArmillaria.[74] Hemibiotrophic pathogens begin their colonising their hosts as biotrophs, and subsequently killing off host cells and feeding as necrotrophs, a phenomenon termed thebiotrophy-necrotrophy switch.[75]
Pathogenic fungi are well-known causative agents of diseases on animals as well as humans. Fungal infections (mycosis) are estimated to kill 1.6 million people each year.[76] One example of a potent fungal animal pathogen areMicrosporidia - obligate intracellular parasitic fungi that largely affect insects, but may also affect vertebrates including humans, causing the intestinal infectionmicrosporidiosis.[77]
Protozoa such asPlasmodium,Trypanosoma, andEntamoeba[78] are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, andamoebic dysentery—and have complex life cycles.[31]
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 andarchaea to animals, plants and fungi, it is unclear whether they can themselves be described as living. They can be eitherRNA orDNA viruses consisting of a single or double strand ofgenetic material (RNA orDNA, respectively), covered in aprotein coat and sometimes alipid envelope. They thus lack all the usual machinery of thecell such asenzymes, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses arebacteriophages, infecting bacteria.[81][82][83][84]
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.[85] Humans have 342 species of helminth parasites, and 70 species of protozoan parasites.[86] Some three-quarters of the links infood webs include a parasite, important in regulating host numbers. Perhaps 40 per cent of described species are parasitic.[85]
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.[91] But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities inhost–parasite coevolution.[92]
Long-term partnerships can lead to a relatively stable relationship tending tocommensalism ormutualism, 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 byworms are often clearly harmed, such infections may also reduce the prevalence and effects ofautoimmune disorders in animal hosts, including humans.[91] In a more extreme example, somenematode worms cannot reproduce, or even survive, without infection byWolbachia bacteria.[94]
Lynn Margulis and others have argued, followingPeter Kropotkin's 1902Mutual 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 thesymbiogenesis which formed theeukaryotes from an intracellular relationship betweenarchaea and bacteria, though the sequence of events remains largely undefined.[95][96]
Competition between parasites can be expected to favour faster reproducing and therefore morevirulent parasites, bynatural selection.[92][97]
Biologists long suspectedcospeciation offlamingos andducks with their parasiticlice, which were similar in the two families. Cospeciation did occur, but it led to flamingos andgrebes, with a laterhost switch of flamingo lice to ducks.
Among competing parasitic insect-killing bacteria of the generaPhotorhabdus andXenorhabdus, virulence depended on the relative potency of the antimicrobialtoxins (bacteriocins) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But whencaterpillars 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.[92]
A parasite sometimes undergoescospeciation with its host, resulting in the pattern described inFahrenholz's rule, that the phylogenies of the host and parasite come to mirror each other.[98]
An example is between thesimian foamy virus (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrialcytochrome 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.[99]
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 whetherflamingos are more closely related tostorks orducks. 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 ofgrebes. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice thenswitched hosts to ducks, creating the situation which had confused biologists.[100]
Parasites infectsympatric hosts (those within their same geographical area) more effectively, as has been shown withdigenetic trematodes infecting lake snails.[101] This is in line with theRed 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 toallopatric hosts, those from different geographical regions.[101]
Some parasitesmodify host behaviour in order to increase their transmission between hosts, often in relation to predator and prey (parasite increased trophic transmission). For example, in theCalifornia coastal salt marsh, the flukeEuhaplorchis californiensis reduces the ability of itskillifish host to avoid predators.[102] This parasite matures inegrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoanToxoplasma gondii, a parasite that matures incats but can be carried by many othermammals. Uninfectedrats avoid cat odors, but rats infected withT. gondii are drawn to this scent, which may increase transmission to feline hosts.[103] The malaria parasite modifies the skin odour of its human hosts, increasing their attractiveness to mosquitoes and hence improving the chance for the parasite to be transmitted.[36] The spiderCyclosa 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.[104]
Trait loss: bedbugCimex lectularius is flightless, like many insect ectoparasites.
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 includingbedbugs,batbugs,lice andfleas have lost theirability to fly, relying instead on their hosts for transport.[105] Trait loss more generally is widespread among parasites.[106] An extreme example is themyxosporeanHenneguya zschokkei, an ectoparasite of fish and the only animal known to have lost the ability to respire aerobically: its cells lackmitochondria.[107]
Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates,[108] the immune system of mammals,[109] insects actively removing parasites,[110] and defensive chemicals in plants.[111]
The evolutionary biologistW. D. Hamilton suggested thatsexual reproduction could have evolved to help to defeat multiple parasites by enablinggenetic 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.[112][113] However, there may be a trade-off betweenimmunocompetence and breeding male vertebrate hosts'secondary sex characteristics, such as the plumage ofpeacocks and the manes oflions. This is because the male hormonetestosterone encourages the growth of secondary sex characteristics, favouring such males insexual selection, at the price of reducing their immune defences.[114]
The dryskin of vertebrates such as theshort-horned lizard prevents the entry of many parasites.
The physical barrier of the tough and often dry and waterproofskin of reptiles, birds and mammals keeps invading microorganisms from entering the body.Human skin also secretessebum, which is toxic to most microorganisms.[108] On the other hand, larger parasites such astrematodes detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebratesaliva and tears containlysozyme, an enzyme that breaks down thecell walls of invading bacteria.[108] Should the organism pass the mouth, thestomach with itshydrochloric acid, toxic to most microorganisms, is the next line of defence.[108] 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 theimmune system'sserum proteins andpattern recognition receptors, intracellular and cellular, that trigger the adaptive immune system'slymphocytes such asT cells and antibody-producingB cells. These have receptors that recognise parasites.[109]
Leaf spot onoak. The spread of the parasitic fungus is limited by defensive chemicals produced by the tree, resulting in circular patches of damaged tissue.
Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the waspPolistes canadensis nests across multiplecombs, rather than building a single comb like much of the rest of its genus, is to avoid infestation bytineid 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.[110]
Plants respond to parasite attack with a series of chemical defences, such aspolyphenol oxidase, under the control of thejasmonic acid-insensitive (JA) andsalicylic acid (SA) signalling pathways.[111][115] 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.[115][116] 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.[116] 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.[116] When damaged, such as bylepidopterancaterpillars, leaves of plants includingmaize andcotton release increased amounts of volatile chemicals such asterpenes that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.[117]
Parasitism and parasite evolution were until the twenty-first century studied byparasitologists, in a science dominated by medicine, rather than byecologists orevolutionary 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.[118] 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".[118]
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 afood 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.[86]
Parasites can play a role in the proliferation of non-native species. For example, invasivegreen crabs are minimally affected by nativetrematodes on the Eastern Atlantic coast. This helps them outcompete native crabs such as theAtlantic Rock andJonah crabs.[119]
Ecological parasitology can be important to attempts at control, like during thecampaign 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.[120]
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.[121] Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change.[122] 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.[123]
An ectoparasite, the California condor louse,Colpocephalum californici, became a well-known conservation issue. A large and costly captive breeding program was run in the United States to rescue theCalifornia 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.[124]
Although parasites are often omitted in depictions offood webs, they usually occupy the top position. Parasites can function likekeystone species, reducing the dominance of superior competitors and allowingcompeting species to co-exist.[86][125][126]
Parasites aredistributed unevenly among their hosts, most hosts having no parasites, and a few hosts harbouring most of the parasite population. This distribution makes sampling difficult and requires careful use of statistics.
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 rendersparametric statistics as commonly used by biologists invalid.Log-transformation of data before the application of parametric test, or the use ofnon-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.[127]
A plate fromFrancesco Redi'sOsservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on living animals found inside living animals), 1684
In hisCanon of Medicine, completed in 1025, the Persian physicianAvicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.[128]
In his 1397 bookTraité de l'état, science et pratique de l'art de la Bergerie (Account of the state, science and practice of the art of shepherding),Jehan de Brie [fr] wrote the first description of a trematode endoparasite, the sheep liver flukeFasciola hepatica.[129][130]
In theearly modern period,Francesco Redi's 1668 bookEsperienze Intorno alla Generazione degl'Insetti (Experiences of the Generation of Insects), explicitly described ecto- and endoparasites, illustratingticks, the larvae ofnasal flies of deer, andsheep liver fluke.[131] Redi noted that parasites develop from eggs, contradicting the theory ofspontaneous generation.[132] In his 1684 bookOsservazioni 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 thelarge roundworm in humans that causesascariasis.[131] Redi was the first to name the cysts ofEchinococcus 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.[128]
In 1681,Antonie van Leeuwenhoek observed and illustrated the protozoan parasiteGiardia lamblia, and linked it to "his own loose stools". This was the first protozoan parasite of humans to be seen under a microscope.[128] A few years later, in 1687, the Italian biologistsGiovanni Cosimo Bonomo andDiacinto Cestoni describedscabies as caused by the parasitic miteSarcoptes scabiei, marking it as the first disease of humans with a known microscopic causative agent.[133]
Ronald Ross won the 1902Nobel Prize for showing that themalaria parasite is transmitted by mosquitoes. This 1897 notebook page records his first observations of the parasite in mosquitoes.
Given the importance of malaria, with some 220 million people infected annually, many attempts have been made to interrupt its transmission. Various methods ofmalaria prophylaxis have been tried including the use ofantimalarial drugs to kill off the parasites in the blood, the eradication of its mosquito vectors withorganochlorine and other insecticides, and the development of amalaria vaccine. All of these have proven problematic, withdrug resistance,insecticide resistance among mosquitoes, and repeated failure of vaccines as the parasite mutates.[136] The first and as of 2015 the only licensed vaccine for any parasitic disease of humans isRTS,S forPlasmodium falciparum malaria.[137]
Poulin observes that the widespreadprophylactic use ofanthelmintic 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 adultparasitic worms, selection could cause delayed maturity and increasedvirulence. Such changes appear to be underway: the nematodeTeladorsagia circumcincta is changing its adult size andreproductive rate in response to drugs.[141]
In theclassical era, the concept of the parasite was not strictly pejorative: theparasitus was anaccepted 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".[142][143]
Parasitism hasa derogatory sense in popular usage. According to the immunologist John Playfair,[144]
In everyday speech, the term 'parasite' is loaded with derogatory meaning. A parasite is a sponger, a lazy profiteer, a drain on society.[144]
Thesatirical clericJonathan Swift alludes to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes":[145]
The vermin only teaze and pinch Their foes superior by an inch. So nat'ralists observe, a flea Hath smaller fleas that on him prey;
And these have smaller fleas to bite 'em. And so proceedsad infinitum. Thus every poet, in his kind, Is bit by him that comes behind:
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.[146]
Theentomopathogenic fungusCordyceps is represented culturally as a deadly threat to the human race. The video game seriesThe Last of Us (2013–present) and itstelevision adaptation presentCordyceps as a parasite of humans, causing azombie apocalypse.[153] Its human hosts initially become violent "infected" beings, before turning into blind zombie "clickers", complete with fruiting bodies growing out from their faces.[153]
^Trophically-transmitted parasites are transmitted to their definitive host, a predator, when their intermediate host is eaten. These parasites often modify the behaviour of their intermediate hosts, causing them to behave in a way that makes them likely to be eaten, such as by climbing to a conspicuous point: this gets the parasites transmitted at the cost of the intermediate host's life.
^Thewolf is a social predator, hunting in packs; thecougar is a solitary predator, hunting alone. Neither strategy is conventionally considered parasitic.[23]
^abWilson, Edward O. (2014).The Meaning of Human Existence. W. W. Norton & Company. p. 112.ISBN978-0-87140-480-0.Parasites, in a phrase, are predators that eat prey in units of less than one. Tolerable parasites are those that have evolved to ensure their own survival and reproduction but at the same time with minimum pain and cost to the host.
^παράσιτος, Liddell, Henry George; Scott, Robert,A Greek–English Lexicon, on Perseus Digital Library
^παρά, Henry George Liddell, Robert Scott,A Greek–English Lexicon, on Perseus Digital Library
^σῖτος, Liddell, Henry George; Scott, Robert,A Greek–English Lexicon, on Perseus Digital Library
^σιτισμός, Liddell, Henry George; Scott, Robert,A Greek–English Lexicon, on Perseus Digital Library
^Overview of Parasitology. Australian Society of Parasitology and Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology. July 2010.ISBN978-1-86499-991-4.Parasitism is a form of symbiosis, an intimate relationship between two different species. There is a biochemical interaction between host and parasite; i.e. they recognize each other, ultimately at the molecular level, and host tissues are stimulated to react in some way. This explains why parasitism may lead to disease, but not always.
^abcOverview of Parasitology. Australian Society of Parasitology and Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology. July 2010.ISBN978-1-86499-991-4.
^Randle, C. P.; Cannon, B. C.; Faust, A. L.; et al. (2018). "Host Cues Mediate Growth and Establishment of Oak Mistletoe (Phoradendron leucarpum, Viscaceae), an Aerial Parasitic Plant".Castanea.83 (2):249–262.doi:10.2179/18-173.S2CID92178009.
^Stevens, Alison N. P. (2010)."Predation, Herbivory, and Parasitism".Nature Education Knowledge.3 (10): 36. Retrieved12 February 2018.Predation, herbivory, and parasitism exist along a continuum of severity in terms of the extent to which they negatively affect an organism's fitness. ... In most situations, parasites do not kill their hosts. An exception, however, occurs with parasitoids, which blur the line between parasitism and predation.
^abDissanaike, A. S. (1957). "On Protozoa hyperparasitic in Helminth, with some observations onNosema helminthorum Moniez, 1887".Journal of Helminthology.31 (1–2):47–64.doi:10.1017/s0022149x00033290.PMID13429025.S2CID35487084.
^abPietsch, Theodore W. (25 August 2005). "Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes)".Ichthyological Research.52 (3):207–236.Bibcode:2005IchtR..52..207P.doi:10.1007/s10228-005-0286-2.S2CID24768783.
^Bourke, Andrew F. G.; Franks, Nigel R. (July 1991). "Alternative adaptations, sympatric speciation and the evolution of parasitic, inquiline ants".Biological Journal of the Linnean Society.43 (3):157–178.doi:10.1111/j.1095-8312.1991.tb00591.x.ISSN0024-4066.
^O'Brien, Timothy G. (1988). "Parasitic nursing behavior in the wedge-capped capuchin monkey (Cebus olivaceus)".American Journal of Primatology.16 (4):341–344.doi:10.1002/ajp.1350160406.PMID32079372.S2CID86176932.
^Payne, R. B. (15 September 2005).The Cuckoos (1st ed.). Oxford: Oxford University Press.ISBN9780198502135.{{cite book}}: CS1 maint: date and year (link)
^Rothstein, S. I. (1990). "A model system for coevolution: avian brood parasitism".Annual Review of Ecology and Systematics.21:481–508.doi:10.1146/annurev.ecolsys.21.1.481.
^Furness, R. W. (1978). "Kleptoparasitism by great skuas (Catharacta skua Brünn.) and Arctic skuas (Stercorarius parasiticus L.) at a Shetland seabird colony".Animal Behaviour.26:1167–1177.doi:10.1016/0003-3472(78)90107-0.S2CID53155057.
^Nickrent, Daniel L. (2002)."Parasitic Plants of the World"(PDF).Archived(PDF) from the original on 6 March 2016. Retrieved10 April 2018. 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.
^Nickrent, D. L.; Musselman, L. J. (2004). "Introduction to Parasitic Flowering Plants".The Plant Health Instructor.doi:10.1094/PHI-I-2004-0330-01.
^Ponomarenko, A.G. (1976) A new insect from the Cretaceous of Transbaikalia, a possible parasite of pterosaurians.Paleontological Journal 10(3):339-343 (English) /Paleontologicheskii Zhurnal 1976(3):102-106 (Russian)
^abcCheng, Thomas C. (1973).General Parasitology. Academic Press. pp. 120–134.ISBN978-0-12-170750-7.The 19th century might be thought of as the genesis of modern parasitology.
^Humphrey-Smith, Ian, ed. (1993).Sept siècles de parasitologie en France [The French School of Parasitology] (in French). Société Française de Parasitologie. pp. 26–29.
^abIoli, A.; Petithory, J. C.; Theodorides, J. (1997). "Francesco Redi and the birth of experimental parasitology".Hist Sci Med.31 (1):61–66.PMID11625103.
^Hoddle, M. S.; Van Driesche, R. G.; Sanderson, J. P. (1998). "Biology and Use of the Whitefly Parasitoid Encarsia Formosa".Annual Review of Entomology.43:645–669.doi:10.1146/annurev.ento.43.1.645.PMID15012401.
^Damon, Cynthia (1997)."5".The Mask of the Parasite: A Pathology of Roman Patronage. University of Michigan Press. p. 148.ISBN978-0-472-10760-5.A satirist seeking to portray client misery naturally focuses on the relationship with the greatest dependency, that in which a client gets his food from his patron, and for this the prefabricated persona of the parasite proved itself extremely useful.
^abPlayfair, John (2007).Living with Germs: In health and disease. Oxford University Press. p. 19.ISBN978-0-19-157934-9. 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".
^Swift, Jonathan (1733).On Poetry: A Rapsody. And sold by J. Huggonson, next to Kent's Coffee-house, near Serjeant's-inn, in Chancery-lane; [and] at the bookseller's and pamphletshops.
![Idiobiont parasitoid wasps immediately paralyse their hosts for their larvae (Pimplinae, pictured) to eat.[22]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLive_Tetragnatha_montana_(RMNH.ARA.14127)_parasitized_by_Acrodactyla_quadrisculpta_larva_(RMNH.INS.593867)_-_BDJ.1.e992.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
