Editing Parasitism (section)

=== 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 |url-access=subscription }}</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/>