Editing Parasitism (section)

=== 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>