Editing Parasitism (section)

== Evolutionary strategies ==

{{see also|Parasitology}}
{{anchor|Endoparasite}}{{anchor|Ectoparasite}}{{anchor|Semiparasite}}

=== Basic concepts ===

{{further|Parasitic life cycle}}

[[File:Taenia solium tapeworm scolex with its four suckers and two rows of hooks 5262 lores.jpg|thumb|Head (scolex) of tapeworm ''[[Taenia solium]]'', an [[intestinal parasite]], has hooks and [[sucker (zoology)|suckers]] to attach to its [[host (biology)|host]]]]

Parasitism is a kind of [[symbiosis]], a close and persistent long-term biological interaction between a parasite and its host. Unlike [[saprotroph]]s, parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Unlike [[commensalism]] and [[mutualism (biology)|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 [[Disease vector|vectors]] of pathogens, causing [[parasitic disease|disease]].<ref>{{cite book |title=Overview of Parasitology |url=http://parasite.org.au/para-site/introduction/introduction-essay.html |publisher=Australian Society of Parasitology and Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology |date=July 2010 |isbn=978-1-86499-991-4 |quote=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.}}</ref><ref name="SuzukiSasaki2019">{{cite journal |last1=Suzuki |first1=Sayaki U. |last2=Sasaki |first2=Akira |title=Ecological and Evolutionary Stabilities of Biotrophism, Necrotrophism, and Saprotrophism |journal=The American Naturalist |volume=194 |issue=1 |year=2019 |pages=90–103 |issn=0003-0147 |doi=10.1086/703485|pmid=31251653 |bibcode=2019ANat..194...90S |s2cid=133349792 |url=http://pure.iiasa.ac.at/id/eprint/15927/1/703485.pdf |archive-url=https://web.archive.org/web/20200306065231/http://pure.iiasa.ac.at/id/eprint/15927/1/703485.pdf |archive-date=6 March 2020 |url-status=live }}</ref><ref name="pmid37458178">{{cite journal |last1=Rozsa |first1=L. | last2=Garay |first2=J. |title= Definitions of parasitism, considering its potentially opposing effects at different levels of hierarchical organization| journal=Parasitology|volume=150 |issue=9 |pages= 761–768|year=2023 |pmid=37458178 |pmc=10478066 |doi=10.1017/S0031182023000598 }}</ref>  [[Predation]] is by definition not a symbiosis<!--def: a long-term relnship-->, 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".<ref name=Wilson2014>{{cite book |last=Wilson |first=Edward O. |author-link=E. O. Wilson |title=The Meaning of Human Existence |year=2014 |publisher=W. W. Norton & Company |isbn=978-0-87140-480-0 |page=112 |quote=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.}}</ref>

Within that scope are many possible strategies. [[Taxonomist]]s classify parasites in a variety of overlapping schemes, based on their interactions with their hosts and on their [[Biological life cycle|life cycles]], which can be complex. An [[obligate parasite]] depends completely on the host to complete its life cycle, while a [[facultative 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".<ref>{{cite web |url=http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm |title=A Classification of Animal-Parasitic Nematodes |work=plpnemweb.ucdavis.edu |access-date=25 February 2016 |archive-date=6 October 2017 |archive-url=https://web.archive.org/web/20171006152348/http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm |url-status=dead }}</ref><ref>{{cite journal |last=Garcia |first=L. S. |title=Classification of Human Parasites, Vectors, and Similar Organisms |journal=Clinical Infectious Diseases |year=1999 |volume=29 |issue=4 |pages=734–746 |pmid=10589879 |doi=10.1086/520425|doi-access=free}}</ref> An endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface.<ref name=Para-Site>{{cite book |title=Overview of Parasitology |url=http://parasite.org.au/para-site/introduction/introduction-essay.html |publisher=Australian Society of Parasitology and Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology |date=July 2010 |isbn=978-1-86499-991-4}}</ref> Mesoparasites—like some [[copepod]]s, for example—enter an opening in the host's body and remain partly embedded there.<ref>{{cite journal |last1=Vecchione |first1=Anna |last2=Aznar |first2=Francisco Javier |title=The mesoparasitic copepod Pennella balaenopterae and its significance as a visible indicator of health status in dolphins (Delphinidae): a review |journal=Journal of Marine Animals and Their Ecology |date=2008 |volume=7 |issue=1 |pages=4–11 |url=http://www.oers.ca/journal/volume7/issue1/briefCommunication.pdf |access-date=11 April 2018 |archive-date=10 April 2018 |archive-url=https://web.archive.org/web/20180410051646/http://www.oers.ca/journal/volume7/issue1/briefCommunication.pdf |url-status=dead }}</ref> Some parasites can be generalists, feeding on a wide range of hosts, but many parasites, and the majority of protozoans and [[helminth]]s that parasitise animals, are specialists and extremely host-specific.<ref name=Para-Site/> 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.<ref name="Rollinson"/> 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.<ref name="Rollinson"/><ref>{{Cite web |url=https://search.credoreference.com/content/entry/bkeeem/parasitism/0 |title=Parasitism {{!}} The Encyclopedia of Ecology and Environmental Management |publisher=Blackwell Science |access-date=8 April 2018}}</ref>

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, the [[snubnosed eel]] is probably a facultative endoparasite (i.e., it is semiparasitic) that opportunistically burrows into and eats sick and dying fish.<ref name=Caira1997>{{cite journal |journal=Environmental Biology of Fishes |volume=49 |pages=139–144 |year=1997 |url=https://www.researchgate.net/publication/226076003 |title=Pugnose eels, ''Simenchelys parasiticus'' (Synaphobranchidae) from the heart of a shortfin mako, ''Isurus oxyrinchus'' (Lamnidae) |last1=Caira |first1=J. N. |last2=Benz |first2=G. W. |last3=Borucinska |first3=J. |last4=Kohler |first4=N. E.|issue=1 |doi=10.1023/a:1007398609346 |bibcode=1997EnvBF..49..139C |s2cid=37865366 }}</ref> [[phytophagous|Plant-eating]] insects such as [[scale insect]]s, [[aphid]]s, and [[caterpillar]]s closely resemble ectoparasites, attacking much larger plants; they serve as vectors of bacteria, fungi and viruses which cause [[plant pathology|plant diseases]]. As female scale insects cannot move, they are obligate parasites, permanently attached to their hosts.<ref name="Rollinson"/>

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,<ref>{{cite journal |last=Lawrence |first=P. O. |year=1981 |title=Host vibration—a cue to host location by the parasite, ''Biosteres longicaudatus'' |journal=Oecologia |volume=48 |issue=2 |pages=249–251 |pmid=28309807 |doi=10.1007/BF00347971 |bibcode=1981Oecol..48..249L |s2cid=6182657 }}</ref> exhaled [[carbon dioxide]], skin odours, visual and heat signatures, and moisture.<ref>{{cite journal |last=Cardé |first=R. T. |year=2015 |title=Multi-cue integration: how female mosquitoes locate a human host|journal=Current Biology |volume=25 |issue=18 |pages=R793–R795 |doi=10.1016/j.cub.2015.07.057 |pmid=26394099 |doi-access=free |bibcode=2015CBio...25.R793C }} {{open access}}</ref> Parasitic plants can use, for example, light, host physiochemistry, and volatiles to recognize potential hosts.<ref>{{cite journal |last1=Randle |first1=C. P. |last2=Cannon |first2=B. C. |last3=Faust |first3=A. L. |display-authors=etal |year=2018 |title=Host Cues Mediate Growth and Establishment of Oak Mistletoe (Phoradendron leucarpum, Viscaceae), an Aerial Parasitic Plant |journal=Castanea |volume=83 |issue=2 |pages=249–262 |doi=10.2179/18-173 |s2cid=92178009 }}</ref>

=== Major strategies ===

There are six major parasitic [[Behavioral ecology#Evolutionarily stable strategy|strategies]], namely [[parasitic castrator|parasitic castration]]; directly transmitted parasitism; [[wikt:trophic|trophically]]-transmitted parasitism; [[vector (epidemiology)|vector]]-transmitted parasitism; [[parasitoid]]ism; and micropredation. These apply to parasites whose hosts are plants as well as animals.<ref name=Rollinson>{{cite journal |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |editor1=Rollinson, D. |editor2=Hay, S. I. |title=The Many Roads to Parasitism: A Tale of Convergence |journal=Advances in Parasitology |url=https://books.google.com/books?id=9y4AlXka7t0C&pg=PA28 |year=2011 |volume=74 |publisher=Academic Press |isbn=978-0-12-385897-9 |pages=27–28 |doi=10.1016/B978-0-12-385897-9.00001-X |pmid=21295676 }}</ref><ref name=PoulinRandhawa2015>{{cite journal |last1=Poulin |first1=Robert |author1-link=Robert Poulin (zoologist) |last2=Randhawa |first2=Haseeb S. |title=Evolution of parasitism along convergent lines: from ecology to genomics |journal=Parasitology |date=February 2015 |volume=142 |issue=Suppl 1 |pages=S6–S15 |doi=10.1017/S0031182013001674 |pmc=4413784 |pmid=24229807}} {{open access}}</ref> These strategies represent [[adaptation|adaptive peaks]]; intermediate strategies are possible, but organisms in many different groups have consistently [[convergent evolution|converged]] on these six, which are evolutionarily stable.<ref name=PoulinRandhawa2015/>

A perspective on the evolutionary options can be gained by considering four key questions: the effect on the [[Fitness (biology)|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.<ref name=LaffertyKuris2002>{{cite journal |last1=Lafferty |first1=K. D. |last2=Kuris |first2=A. M. |date=2002 |url=http://homes.msi.ucsb.edu/~lafferty/Publications/Parasites-evolution_files/Laff%26Kur02TREE.pdf |title=Trophic strategies, animal diversity and body size |journal=Trends in Ecology and Evolution |volume=17 |issue=11 |pages=507–513 |doi=10.1016/s0169-5347(02)02615-0 |archive-url=https://web.archive.org/web/20191003124738/http://homes.msi.ucsb.edu/~lafferty/Publications/Parasites-evolution_files/Laff%26Kur02TREE.pdf |archive-date=3 October 2019}}</ref>

{|class="wikitable"
|+ Evolutionary strategies in parasitism and predation<ref name=LaffertyKuris2002/><br/>({{font color|green|Intensity-dependent: green, roman}};<br/>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;''{{font color|purple |Intensity-independent: purple, italics}}'')
! Host [[Fitness (biology)|fitness]] !! Single host, stays alive !! Single host, dies !! Multiple hosts
|-
|'''Able to<br/>reproduce<br/>(fitness > 0)''' ||{{font color|green|Conventional parasite}}<br/>&nbsp;&nbsp;&nbsp;''{{font color|purple |Pathogen}}'' ||{{font color|green|Trophically-transmitted parasite}}{{efn|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.}}<br/>&nbsp;&nbsp;&nbsp;''{{font color|purple |Trophically-transmitted pathogen}}'' ||{{font color|green|Micropredator}}<br/>&nbsp;&nbsp;&nbsp;''{{font color|purple |Micropredator}}''
|-
|'''Unable to<br/>reproduce<br/>(fitness = 0)''' ||{{font color|green|----- }}<br/>&nbsp;&nbsp;&nbsp;''{{font color|purple |Parasitic castrator}}'' ||{{font color|green|Trophically-transmitted parasitic castrator}}<br/>&nbsp;&nbsp;&nbsp;''{{font color|purple |Parasitoid}}'' ||{{font color|green|Social predator}}{{efn|The [[wolf]] is a social predator, hunting in packs; the [[cougar]] is a solitary predator, hunting alone. Neither strategy is conventionally considered parasitic.<ref name=LaffertyKuris2002/>}}<br/>&nbsp;&nbsp;&nbsp;''{{font color|purple |Solitary predator}}''
|}

==== Parasitic castrators ====

{{main|Parasitic castration}}

[[File:Sacculina carcini.jpg|thumb|The parasitic castrator ''[[Sacculina carcini]]'' (highlighted) attached to [[Liocarcinus holsatus|its crab host]]]]

[[Parasitic castrator]]s partly or completely destroy their host's ability to reproduce, diverting the energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host's other systems remain intact, allowing it to survive and to sustain the parasite.<ref name=PoulinRandhawa2015/>{{sfn|Poulin|2007|p=111}} Parasitic crustaceans such as those in the specialised [[barnacle]] genus ''[[Sacculina]]'' specifically cause damage to the gonads of their many species<ref name="Elumalai Viswanathan Pravinkumar Raffi">{{cite journal |last1=Elumalai |first1=V. |last2=Viswanathan |first2=C. |last3=Pravinkumar |first3=M. |last4=Raffi |first4=S. M. |title=Infestation of parasitic barnacle Sacculina spp. in commercial marine crabs |journal=Journal of Parasitic Diseases |volume=38 |issue=3 |date=2013 |doi=10.1007/s12639-013-0247-z |pmid=25035598 |pages=337–339|pmc=4087306 }}</ref> of host [[crab]]s. In the case of ''Sacculina'', the testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to develop female [[secondary sex characteristic]]s such as broader abdomens, smaller [[chela (organ)|claws]] 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 [[Trematoda|trematode]] ''[[Zoogonus lasius]]'', whose [[Trematode life cycle stages|sporocysts]] lack mouths, castrates the intertidal marine snail ''[[Tritia obsoleta]]<!--formerly Ilyanassa-->'' chemically, developing in its gonad and killing its reproductive cells.{{sfn|Poulin|2007|p=111}}<ref name=Cheng13>{{cite book |last=Cheng |first=Thomas C. |title=General Parasitology |url=https://books.google.com/books?id=d4GQlYzode8C&pg=PA13 |year=2012 |publisher=Elsevier Science |isbn=978-0-323-14010-2 |pages=13–15}}</ref>

==== Directly transmitted ====

<!--[[File:Schistosoma mansoni2.jpg|thumb|''[[Schistosoma mansoni]]'' an obligate directly-transmitted [[endoparasite]], causes [[schistosomiasis]] (snail fever)]]-->
[[File:Male human head louse.jpg|thumb|[[Pediculus humanus capitis|Human head-lice]] are directly transmitted [[Obligate parasite|obligate]] ectoparasites]]

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]]s and [[Cyamidae|cyamid]] amphipods; [[monogenea]]ns; and many species of nematodes, fungi, protozoans, bacteria, and viruses. Whether endoparasites or ectoparasites, each has a single host-species. Within that species, most individuals are free or almost free of parasites, while a minority carry a large number of parasites; this is known as an [[aggregated distribution]].<ref name=PoulinRandhawa2015/>

{{anchor|Autoinfection}}

==== Trophically transmitted ====

[[File:Clonorchis sinensis 2.png|thumb|''[[Clonorchis sinensis]]'', the Chinese liver fluke, is trophically transmitted]]

[[wikt:trophic|Trophically]]-transmitted parasites are transmitted by being eaten by a host. They include trematodes (all except [[schistosomes]]), [[cestodes]], [[acanthocephala]]ns, [[Pentastomida|pentastomids]], many [[roundworms]], and many protozoa such as ''[[Toxoplasma]]''.<ref name=PoulinRandhawa2015/> They have complex life cycles involving hosts of two or more species. In their juvenile stages they infect and often [[encyst]] in the intermediate host. When the intermediate-host animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as [[intestinal parasite]]s. Many trophically transmitted parasites [[Parasite increased trophic transmission|modify the behaviour]] of their intermediate hosts, increasing their chances of being eaten by a predator. As with directly transmitted parasites, the distribution of trophically transmitted parasites among host individuals is aggregated.<ref name=PoulinRandhawa2015/> [[Coinfection]] by multiple parasites is common.<ref>{{cite journal |last1=Cox |first1=F. E. |url=https://researchonline.lshtm.ac.uk/17745/1/Concom.pdf |archive-url=https://web.archive.org/web/20171202083654/http://researchonline.lshtm.ac.uk/17745/1/Concom.pdf |archive-date=2 December 2017 |url-status=live |title=Concomitant infections, parasites and immune responses |series=122 |journal=Parasitology |volume=Supplement |pages=S23–38 |year=2001 |pmid=11442193 |doi =10.1017/s003118200001698x |s2cid=150432 }}</ref> [[Strongyloides stercoralis#Autoinfection|Autoinfection]], where (by exception) the whole of the parasite's [[biological life cycle|life cycle]] takes place in a single primary host, can sometimes occur in helminths such as ''[[Strongyloides stercoralis]]''.<ref name=ASP>{{cite web |title=Helminth Parasites |url=http://parasite.org.au/para-site/contents/helminth-intoduction.html |publisher=Australian Society of Parasitology |access-date=9 October 2017}}</ref>

==== Vector-transmitted ====

{{further|Disease vector}}

[[File:Trypanosoma sp. PHIL 613 lores.jpg|thumb|The [[vector (epidemiology)|vector-transmitted]] protozoan endoparasite ''[[Trypanosoma]]'' among human [[red blood cell]]s]]

[[Disease vector|Vector-transmitted]] parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually,<ref name=Para-Site/> to carry them from one definitive host to another.<ref name=PoulinRandhawa2015/> These parasites are microorganisms, namely [[protozoa]], [[bacteria]], or [[virus]]es, often intracellular [[pathogen]]s (disease-causers).<ref name=PoulinRandhawa2015/> Their vectors are mostly [[hematophagy|hematophagic]] arthropods such as fleas, lice, ticks, and mosquitoes.<ref name=PoulinRandhawa2015/><ref>{{cite web |url=http://www.peoi.org/Courses/Coursesen/phfoundation/contents/frame3a.html |title=Pathogenic Parasitic Infections |publisher=PEOI |access-date=18 July 2013}}</ref> For example, the deer tick ''[[Ixodes scapularis]]'' acts as a vector for diseases including [[Lyme disease]], [[babesiosis]], and [[Human granulocytic anaplasmosis|anaplasmosis]].<ref name=pmid11450660>{{cite journal |last=Steere |first=A. C. |title=Lyme disease |journal=[[New England Journal of Medicine]] |volume=345 |issue=2 |pages=115–125 |date=July 2001 |pmid=11450660 |doi=10.1056/NEJM200107123450207 }}</ref> Protozoan endoparasites, such as the [[malaria]]l parasites in the genus ''[[Plasmodium]]'' and sleeping-sickness parasites in the genus ''[[Trypanosoma]]'', have infective stages in the host's blood which are transported to new hosts by biting insects.<ref name="PollittMacGregor2011">{{cite journal |last1=Pollitt |first1=Laura C. |last2=MacGregor |first2=Paula |last3=Matthews |first3=Keith |last4=Reece |first4=Sarah E. |title=Malaria and trypanosome transmission: different parasites, same rules? |journal=Trends in Parasitology |volume=27 |issue=5 |year=2011 |pages=197–203 |doi=10.1016/j.pt.2011.01.004|pmid=21345732 |pmc=3087881 }}</ref>

==== Parasitoids ====

{{main|Parasitoid|Parasitoid wasp}}

[[Parasitoid]]s are insects which sooner or later kill their hosts, placing their relationship close to predation.<ref>{{cite journal |last1=Stevens |first1=Alison N. P. |title=Predation, Herbivory, and Parasitism |journal=Nature Education Knowledge |date=2010 |volume=3 |issue=10 |page=36 |url=https://www.nature.com/scitable/knowledge/library/predation-herbivory-and-parasitism-13261134 |access-date=12 February 2018 |quote=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.}}</ref> Most parasitoids are [[parasitoid wasp]]s or other [[hymenoptera]]ns; others include [[dipterans]] such as [[phoridae|phorid flies]]. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts.<ref name=GullanCranston2010>{{cite book |last1=Gullan |first1=P. J. |last2=Cranston |first2=P. S. |date=2010 |title=The Insects: An Outline of Entomology |url=https://archive.org/details/insectsoutlineen00pjgu |url-access=limited |publisher=Wiley |edition=4th |isbn=978-1-118-84615-5 |pages=[https://archive.org/details/insectsoutlineen00pjgu/page/n332 308], 365–367, 375, 440–441}}</ref>

[[Idiobiont]] parasitoids sting their often-large prey on capture, either killing them outright or paralysing them immediately. The immobilised prey is then carried to a nest, sometimes alongside other prey if it is not large enough to support a parasitoid throughout its development. An [[oviposition|egg is laid]] on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, [[mass provisioning|feeding on the provisions]] left for it.<ref name=GullanCranston2010/>

[[Koinobiont]] parasitoids, which include [[Diptera|flies]] as well as wasps, lay their eggs inside young hosts, usually larvae. These are allowed to go on growing, so the host and parasitoid develop together for an extended period, ending when the parasitoids emerge as adults, leaving the prey dead, eaten from inside. Some koinobionts regulate their host's development, for example preventing it from [[pupa]]ting or making it [[ecdysis|moult]] whenever the parasitoid is ready to moult. They may do this by producing hormones that mimic the host's moulting hormones ([[ecdysteroid]]s), or by regulating the host's endocrine system.<ref name=GullanCranston2010/>

<gallery mode="packed">
File:Live Tetragnatha montana (RMNH.ARA.14127) parasitized by Acrodactyla quadrisculpta larva (RMNH.INS.593867) - BDJ.1.e992.jpg |Idiobiont [[parasitoid wasp]]s immediately paralyse their hosts for their larvae ([[Pimplinae]], pictured) to eat.<ref name=PoulinRandhawa2015/>
File:CSIRO ScienceImage 2357 Spotted alfalfa aphid being attacked by parasitic wasp.jpg|[[Koinobiont]] parasitoid wasps like this [[Braconidae|braconid]] lay their eggs via an [[ovipositor]] inside their hosts, which continue to grow and moult.
File:Female Apocephalus borealis ovipositing into the abdomen of a worker honey bee.png|[[Phoridae|Phorid fly]] (centre left) is [[oviposition|laying eggs]] in the abdomen of a worker [[honey-bee]], [[Behavior-altering parasites and parasitoids|altering its behaviour]].
</gallery>

{{anchor|Micropredator}}

==== Micropredators ====

[[File:Anopheles minimus.jpg|thumb|[[Mosquito]]es<!-- such as ''[[Anopheles minimus]]''--> 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.<ref name=PoulinRandhawa2015/><ref name=LaffertyKuris2002/><ref name="Wilson2017">{{cite journal |last1=Wilson |first1=Anthony J. |display-authors=etal |title =What is a vector? |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=372 |issue=1719 |date=March 2017 |doi=10.1098/rstb.2016.0085 |pmid=28289253 |page=20160085 |pmc=5352812}}</ref> Most micropredators are [[hematophagy|hematophagic]], feeding on blood. They include annelids such as [[leech]]es, crustaceans such as [[branchiura]]ns and [[Gnathiidae|gnathiid]] isopods, various [[diptera]]ns such as mosquitoes and [[tsetse fly|tsetse flies]], other arthropods such as fleas and ticks, vertebrates such as [[lamprey]]s, and mammals such as [[vampire bat]]s.<ref name=PoulinRandhawa2015/>

=== Transmission strategies ===

[[File:Entamoeba histolytica life cycle-en.svg|thumb|upright=1.7<!--format for legibility of multi-image diagram-->|[[Biological life cycle|Life cycle]] of ''[[Entamoeba histolytica]]'', an anaerobic parasitic protozoan transmitted by the [[fecal–oral route]]]]

Parasites use a variety of methods to infect animal hosts, including physical contact, the [[fecal–oral route]], free-living infectious stages, and vectors, suiting their differing hosts, life cycles, and ecological contexts.<ref name=Godfrey2013>{{cite journal |last=Godfrey |first=Stephanie S. |title=Networks and the ecology of parasite transmission: A framework for wildlife parasitology |journal=Wildlife |date=December 2013 |volume=2 |pages=235–245 |doi=10.1016/j.ijppaw.2013.09.001 |pmid=24533342 |pmc=3862525|bibcode=2013IJPPW...2..235G }}</ref> Examples to illustrate some of the many possible combinations are given in the table.

{|class="wikitable sortable"
|+ Examples of transmission methods in different ecological contexts<ref name=Godfrey2013/>
! Parasite !! Host !! Transmission method !! Ecological context
|-
|''[[Gyrodactylus turnbulli]]''<br/>(a [[monogenean]]) ||''[[Poecilia reticulata]]''<br/>(guppy) ||physical contact ||social behaviour
|-
|[[Nematode]]s<br/>e.g. ''[[Strongyloides]]'' ||''[[Macaca fuscata]]''<br/>(Japanese macaque) ||fecal–oral ||
social behaviour<br/>(grooming)
|-
|''[[Heligmosomoides polygyrus]]''<br/>(a nematode) ||''[[Apodemus flavicollis]]''<br/>(yellow-necked mouse) ||fecal–oral ||sex-biased transmission<br/>(mainly to males)
|-
|''[[Amblyomma]]''<br/>(a tick) ||''[[Sphenodon punctatus]]''<br/>(tuatara) ||free-living infectious stages ||social behaviour
|-
|''[[Plasmodium]]''<br/>([[malaria]] parasite) ||[[Bird]]s, [[mammal]]s<br/>(inc. humans) ||''[[Anopheles]]'' mosquito vector,<br/>attracted by odour of<br/>infected human host<ref name="de Boer2017">{{cite journal |last1=de Boer |first1=Jetske G. |last2=Robinson |first2=Ailie |last3=Powers |first3=Stephen J. |last4=Burgers |first4=Saskia L. G. E. |last5=Caulfield |first5=John C. |last6=Birkett |first6=Michael A. |last7=Smallegange |first7=Renate C. |last8=van Genderen |first8=Perry J. J. |last9=Bousema |first9=Teun |last10=Sauerwein |first10=Robert W. |last11=Pickett |first11=John A. |last12=Takken |first12=Willem |last13=Logan |first13=James G. |title=Odours of Plasmodium falciparum-infected participants influence mosquito–host interactions |journal=Scientific Reports |volume=7 |issue=1 |pages=9283 |date=August 2017 |doi=10.1038/s41598-017-08978-9|pmid=28839251 |pmc=5570919 |bibcode=2017NatSR...7.9283D }}</ref> ||— 
|}

=== Variations ===

Among the many variations on parasitic strategies are hyperparasitism,<ref name=Dissanaike1957/> social parasitism,<ref name=Thomas2010/> brood parasitism,<ref name=Payne1997/> kleptoparasitism,<ref name=SlaterRosenblatt2005/> sexual parasitism,<ref name=Pietsch2005/> and adelphoparasitism.<ref name=Rochat2001/>

==== Hyperparasitism ====

{{main|Hyperparasite}}

[[Hyperparasite]]s feed on another parasite, as exemplified by protozoa living in helminth parasites,<ref name=Dissanaike1957>{{cite journal |last1=Dissanaike |first1=A. S. |title=On Protozoa hyperparasitic in Helminth, with some observations on ''Nosema helminthorum'' Moniez, 1887 |journal=Journal of Helminthology |date=1957 |volume=31 |issue=1–2 |pages=47–64 |pmid=13429025 |doi=10.1017/s0022149x00033290|s2cid=35487084 }}</ref> or facultative or obligate parasitoids whose hosts are either conventional parasites or parasitoids.<ref name=PoulinRandhawa2015/><ref name=GullanCranston2010/> Levels of parasitism beyond secondary also occur, especially among facultative parasitoids. In [[oak gall]] systems, there can be up to four levels of parasitism.<ref>{{cite journal |last=Askew |first=R. R. |url=https://www.researchgate.net/publication/238289888 |title=On the Biology of the Inhabitants of Oak Galls of Cynipidae (Hymenoptera) in Britain |journal=Transactions of the Society for British Entomology |date=1961 |volume=14 |pages=237–268}}</ref>

Hyperparasites can control their hosts' populations, and are used for this purpose [[biological pest control|in agriculture]] and to some extent in [[medicine]]. The controlling effects can be seen in the way that the [[Hypovirus#CHV1–Chestnut blight hypovirulence|CHV1 virus]] helps to control the damage that [[chestnut blight]], ''Cryphonectria parasitica'', does to [[American chestnut]] trees, and in the way that [[bacteriophage]]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.<ref name="ParrattLaine2016">{{cite journal |last1=Parratt |first1=Steven R. |last2=Laine |first2=Anna-Liisa |title=The role of hyperparasitism in microbial pathogen ecology and evolution |journal=The ISME Journal |volume=10 |issue=8 |date=January 2016|doi=10.1038/ismej.2015.247 |pmid=26784356 |pages=1815–1822|pmc=5029149 |bibcode=2016ISMEJ..10.1815P }}</ref>

==== Social parasitism ====

{{further|Ant mimicry|Cuckoo bee|Emery's rule}}

Social parasites take advantage of interspecific interactions between members of [[eusocial]] animals such as [[ant]]s, [[termite]]s, and [[bumblebee]]s. Examples include the large blue butterfly, ''[[Phengaris arion]]'', its larvae employing [[ant mimicry]]<!--(myrmecomorphy)--> to parasitise certain ants,<ref name="Thomas2010">{{cite journal |last1=Thomas |first1=J. A. |last2=Schönrogge |first2=K. |author3=Bonelli, S. |author4=Barbero, F. |author5=Balletto, E. |title=Corruption of ant acoustical signals by mimetic social parasites: ''Maculinea'' butterflies achieve elevated status in host societies by mimicking the acoustics of queen ants |journal=Commun Integr Biol |volume=3 |issue=2 |pages=169–171 |year=2010 |pmid=20585513 |pmc=2889977 |doi=10.4161/cib.3.2.10603}}</ref> ''[[Bombus bohemicus]]'', a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and ''[[Melipona scutellaris]]'', a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen.<ref>{{Cite journal |first1=Annette |last1=Van Oystaeyen |first2=Denise |last2=Araujo Alves |first3=Ricardo |last3=Caliari Oliveira |first4=Daniela |last4=Lima do Nascimento |first5=Fábio |last5=Santos do Nascimento |first6=Johan |last6=Billen |first7=Tom |last7=Wenseleers |title=Sneaky queens in ''Melipona'' bees selectively detect and infiltrate queenless colonies |journal=Animal Behaviour |date=September 2013 |volume=86 |issue=3 |pages=603–609 |doi=10.1016/j.anbehav.2013.07.001 |citeseerx=10.1.1.309.6081 |s2cid=12921696 }}</ref> An extreme example of interspecific social parasitism is found in the ant ''[[Tetramorium inquilinum]]'', an obligate parasite which lives exclusively on the backs of other ''Tetramorium'' ants.<ref>{{cite web |url=https://www.antkeepers.com/facts/ant-colony/social-parasites/ |title=Social Parasites in the Ant Colony |publisher=Antkeepers |access-date=4 April 2016}}</ref> A mechanism for the evolution of social parasitism was first proposed by Carlo Emery in 1909.<ref>{{Cite journal |last=Emery |first=Carlo |date=1909 |title=Über den Ursprung der dulotischen, parasitischen un myrmekophilen Ameisen |journal=Biologischen Centralblatt |volume=29 |pages=352–362}}</ref> 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.<ref name="RD">{{cite web |url=http://www.nature.com/scitable/knowledge/library/social-parasitism-in-ants-13256421 |title=Social Parasitism in Ants |last=Deslippe |first=Richard |publisher=Nature Education Knowledge |year=2010 |access-date=29 October 2010}}</ref><ref name="Emery_1909">{{cite journal |last=Emery |first=C. |url=https://archive.org/details/ants_11715/page/n1 |title=Über den Ursprung der dulotischen, parasitischen und myrmekophilen Ameisen |journal=Biologisches Centralblatt |date=1909 |volume=29 |pages=352–362}}</ref><ref>{{Cite journal |last1=Bourke |first1=Andrew F. G. |last2=Franks |first2=Nigel R. |date=July 1991 |title=Alternative adaptations, sympatric speciation and the evolution of parasitic, inquiline ants |journal=Biological Journal of the Linnean Society |volume=43 |issue=3 |pages=157–178 |doi=10.1111/j.1095-8312.1991.tb00591.x |issn=0024-4066}}</ref>

Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In [[wedge-capped capuchin]]s, higher ranking females sometimes take milk from low ranking females without any reciprocation.<ref name=obrien5>{{cite journal |last=O'Brien |first=Timothy G. |title=Parasitic nursing behavior in the wedge-capped capuchin monkey (''Cebus olivaceus'') |journal=American Journal of Primatology |year=1988 |volume=16 |issue=4 |pages=341–344 |doi =10.1002/ajp.1350160406 |pmid=32079372 |s2cid=86176932 }}</ref>

==== Brood parasitism ====

{{further|Brood parasitism}}

In [[brood 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 as [[cowbird]]s, [[Viduidae|whydahs]], [[cuckoo]]s, and [[black-headed duck]]s. These do not build nests of their own, but leave their eggs in nests of other [[species]]. In the family ''[[Cuckoo|Cuculidae]]'', over 40% of cuckoo species are obligate brood parasites, while others are either facultative brood parasites or provide parental care.<ref>{{Cite book |last=Payne |first=R. B. |title=The Cuckoos |date=September 15, 2005 |publisher=Oxford University Press |year=2005 |isbn=9780198502135 |edition=1st |location=Oxford |language=en}}</ref> The eggs of some brood parasites [[mimicry|mimic]] those of their hosts, while some cowbird eggs have tough shells, making them hard for the hosts to kill by piercing, both mechanisms implying selection by the hosts against parasitic eggs.<ref name=Payne1997>{{cite book |last=Payne |first=R. B. |date=1997 |chapter=Avian brood parasitism |editor1=Clayton, D. H. |editor2=Moore, J. |title=Host–parasite evolution: General principles and avian models |pages=[https://archive.org/details/hostparasiteevol1997unse/page/338 338–369] |publisher=Oxford University Press |isbn=978-0-19-854892-8 |chapter-url=https://archive.org/details/hostparasiteevol1997unse/page/338 }}</ref><ref>{{cite journal |last1=Rothstein |first1=S. I. |year=1990 |title=A model system for coevolution: avian brood parasitism |journal=Annual Review of Ecology and Systematics |volume=21 |pages=481–508 |doi=10.1146/annurev.ecolsys.21.1.481}}</ref><ref name="DeMarsico2013">{{cite journal |last1=De Marsico |first1=M. C. |last2=Gloag |first2=R. |last3=Ursino |first3=C. A. |last4=Reboreda |first4=J. C. |title=A novel method of rejection of brood parasitic eggs reduces parasitism intensity in a cowbird host |journal=Biology Letters |volume=9 |issue=3 |date=March 2013 |doi=10.1098/rsbl.2013.0076 |pmid=23485877 |pages=20130076|pmc=3645041 }}</ref> The adult female [[European cuckoo]] further mimics a predator, the [[European sparrowhawk]], giving her time to lay her eggs in the host's nest unobserved.<ref name=Welbergen2011>{{cite journal |last1=Welbergen |first1=J. |last2=Davies |first2=N. B. |year=2011 |title=A parasite in wolf's clothing: hawk mimicry reduces mobbing of cuckoos by hosts |journal=[[Behavioral Ecology (journal)|Behavioral Ecology]] |volume=22 |issue=3 |pages=574–579 |doi=10.1093/beheco/arr008|doi-access=free }}</ref> Host species often combat parasitic egg mimicry through egg [[Polymorphism (biology)|polymorphism]], 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.<ref>{{Cite journal |last=Yang, C., X. Si, W. Liang, and A. P. Møller |title=Spatial variation in egg polymorphism among cuckoo hosts across 4 continents |url=https://doi.org/10.1093/cz/zoaa011 |journal=Current Zoology |date=2020 |volume=66 |issue=5 |pages=477–483 |doi=10.1093/cz/zoaa011 |pmid=33293928 |pmc=7705517 |via=Oxford Academic}}</ref>

==== Kleptoparasitism ====

{{further|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]]s lay their eggs in the nest cells of other [[bee]]s in the same family.<ref name="SlaterRosenblatt2005">{{cite book |last1=Slater |first1=Peter J. B. |last2=Rosenblatt |first2=Jay S. |author3=Snowdon, Charles T. |author4=Roper, Timothy J. |author5=Brockmann, H. Jane |author-link5=H. Jane Brockmann |author6=Naguib, Marc |title=Advances in the Study of Behavior |url=https://books.google.com/books?id=WQl2s_xwfagC&pg=PA365 |date=30 January 2005 |publisher=Academic Press |isbn=978-0-08-049015-1 |page=365}}</ref> Kleptoparasitism is uncommon generally but conspicuous in birds; some such as [[skuas]] are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch.<ref name="Furness1978">{{cite journal |last=Furness |first=R. W. |title=Kleptoparasitism by great skuas (''Catharacta skua'' Brünn.) and Arctic skuas (''Stercorarius parasiticus'' L.) at a Shetland seabird colony |journal=Animal Behaviour |volume=26 |year=1978 |doi=10.1016/0003-3472(78)90107-0 |pages=1167–1177|s2cid=53155057 }}</ref>

==== Sexual parasitism ====

{{main|Sexual parasitism}}

A unique approach is seen in some species of [[anglerfish]], such as ''[[Ceratias holboelli]]'', where the males are reduced to tiny [[sexual parasite]]s, 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.<ref name=Pietsch2005>{{cite journal |last=Pietsch |first=Theodore W. |title=Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes) |journal=Ichthyological Research |date=25 August 2005 |volume=52 |issue=3 |pages=207–236 |doi=10.1007/s10228-005-0286-2|bibcode=2005IchtR..52..207P |s2cid=24768783 }}</ref>

==== Adelphoparasitism ====

Adelphoparasitism, (from Greek [[wikt:ἀδελφός|ἀδελφός]] (''adelphós''), brother<ref>{{cite book |last1=Maggenti |first1=Armand R. |last2=Maggenti |first2=Mary Ann |last3=Gardner |first3=Scott Lyell |title=Online Dictionary of Invertebrate Zoology |url=http://slovarji.info/dictionaries/en_dictionary_of_invertebras.pdf#page=29 |archive-url=https://web.archive.org/web/20180418031907/http://slovarji.info/dictionaries/en_dictionary_of_invertebras.pdf#page=29 |url-status=dead |archive-date=18 April 2018 |publisher=University of Nebraska |page=22 |date=2005}}</ref>), also known as sibling-parasitism, occurs where the host species is closely related to the parasite, often in the same family or genus.<ref name=Rochat2001>{{cite journal |last1=Rochat |first1=Jacques |last2=Gutierrez |first2=Andrew Paul |title=Weather-mediated regulation of olive scale by two parasitoids |journal=Journal of Animal Ecology |date=May 2001 |volume=70 |issue=3 |pages=476–490 |doi=10.1046/j.1365-2656.2001.00505.x|bibcode=2001JAnEc..70..476R |s2cid=73607283 |doi-access=free }}</ref> In the citrus blackfly parasitoid, ''[[Encarsia perplexa]]'', unmated females may lay [[Ploidy#haploid|haploid]] eggs in the fully developed larvae of their own species, producing male offspring,<ref>{{cite web |title=Featured Creatures. ''Encarsia perplexa'' |url=http://entnemdept.ufl.edu/creatures/beneficial/encarsia_perplexa.htm |publisher=University of Florida |access-date=6 January 2018}}</ref> while the marine worm ''[[Bonellia viridis]]'' has a similar reproductive strategy, although the larvae are planktonic.<ref name="BerecSchembri2005">{{cite journal |last1=Berec |first1=Ludek |last2=Schembri |first2=Patrick J. |last3=Boukal |first3=David S. |url=https://core.ac.uk/download/pdf/46603451.pdf |archive-url=https://web.archive.org/web/20191003131349/https://core.ac.uk/download/pdf/46603451.pdf |archive-date=3 October 2019 |url-status=live |title=Sex determination in ''Bonellia viridis'' (Echiura: Bonelliidae): population dynamics and evolution |journal=Oikos |volume=108 |issue=3 |year=2005 |pages=473–484 |doi=10.1111/j.0030-1299.2005.13350.x|bibcode=2005Oikos.108..473B }}</ref>

==== Illustrations ====

Examples of the major variant strategies are illustrated.

<gallery class="center" mode="nolines" widths="180px">
File:Pteromalid hyperparasitoid.jpg|A hyperparasitoid [[Pteromalidae|pteromalid wasp]] on the cocoons of its host, itself a parasitoid [[braconid wasp]]
File:Maculinea arion Large Blue Upperside SFrance 2009-07-18.jpg|The [[large blue]] butterfly is an [[ant mimicry|ant mimic]] and social parasite.
File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg|In [[Brood parasite|brood parasitism]], the host raises the young of another species: here a [[cowbird]]'s egg in an [[Eastern phoebe]]'s nest.
File:Great Skua (cropped).jpg|The [[great skua]] is a powerful [[Kleptoparasitism|kleptoparasite]], relentlessly pursuing other seabirds until they disgorge their catches of food.
File:Северная церапия (cropped).jpg|The male of the [[anglerfish]] species ''[[Ceratias holboelli]]'' lives as a tiny [[sexual parasite]] permanently attached below the female's body.
File:Encarsia perplexa.jpg|''[[Encarsia perplexa]]'' (centre), a parasitoid of [[citrus blackfly]] (lower left), is also an adelphoparasite, laying eggs in larvae of its own species
</gallery>