Parasitism 43937 225579293 2008-07-14T11:59:30Z 24.81.194.107 grammar {{Otheruses4|a relationship between organisms|other uses|Parasite (disambiguation)}} '''Parasitism''' is a type of [[Symbiosis|symbiotic]] relationship between [[organism]]s of different [[species]] in which one, the parasite, benefits from a prolonged, close association with the other, the [[host (biology)|host]], which is harmed. In general, parasites are much smaller than their hosts, show a high degree of [[specialization (biology)|specialization]] for their mode of life and [[reproduction|reproduce]] more quickly and in greater numbers than their hosts. Classic examples of parasitism include the interactions between [[vertebrate]] hosts and such diverse animals as the [[Cestoda|tapeworms]], [[Trematoda|flukes]], ''[[Plasmodium]]'' species and [[Siphonaptera|fleas]]. The harm and benefit in parasitic interactions concern the [[Fitness (biology)|biological fitness]] of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized [[pathology]] (such as castration), impairment of [[secondary sex characteristic]]s, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for food, habitat and dispersal. Although the concept of parasitism applies unambiguously to many cases in nature, it is best considered part of a continuum of types of [[Biological interactions|interactions]] between [[species]], rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may be short-lived. For example, because of the episodic nature of its feeding habits, the [[mosquito]] is not considered parasitic. In [[medicine]], only [[eukaryote|eukaryotic]] organisms are considered parasites, to the exclusion of [[bacteria]] and [[viruses]]. Some branches of [[biology]], however, do regard members of these groups to be parasitic.{{Fact|date=February 2008}} [[Image:Parasitismus.jpg|thumb|right|200px|[[Mite]]s parasitising a [[harvestman]]]] ==Types of parasitism== [[Image:Fleabite.JPG|right|thumb|200px|[[Flea]] bites on a human.]] Parasites are classified based on a variety of aspects of their interactions with their hosts and on their life cycles. Those that live inside the host are called '''endoparasites''' (e.g., [[hookworm]]s) and those that live on its surface are called '''ectoparasites''' (e.g., some [[mite]]s). An '''epiparasite''' is one that feeds on another parasite. This relationship is also sometimes referred to as "hyperparasitism". [[Image:Female Catolaccus grandis wasp.jpg|thumb|200px|left|A female ''[[Pteromalidae|Catolaccus grandis]]'' wasp homes in on a [[boll weevil]] larva.]] '''[[Parasitoids]]''' are organisms that cause the host to die as a result of parasitism. Thus, the interaction between the parasitoid and the host is fundamentally different than true parasites and their host, and shares some characteristics with [[predation]] '''Social parasites''' take advantage of interactions between members of social organisms such as [[ant]]s or [[termite]]s. In [[kleptoparasitism]], parasites appropriate food gathered by the host. An example is the [[brood parasitism]] practiced by many species of [[cuckoo]]. Many cuckoos use other bird [[species]] as "babysitters", depositing their eggs in the nest of the host species, which raise the cuckoo young as one of their own. Parasitism can take the form of isolated '''[[Cheating (biology)|cheating]]''' or '''exploitation''' among more generalized [[mutualism|mutualistic ]] interactions. For example, broad classes of [[plants]] and [[fungi]] exchange carbon and nutrients in common mutualistic [[mycorrhizal]] relationships; however, a few plants species (known as [[myco-heterotrophy|myco-heterotrophs]]) "cheat" by taking carbon from a fungus rather than donating it. For '''parasitic conjoined twins''', see [[Parasitic twin]]. ==Evolutionary aspects== Biotrophic parasitism is an extremely common mode of life that has arisen independently many times in the course of [[evolution]]. Depending on the definition used, as many as half of all [[animal]]s have at least one parasitic phase in their life cycles,<ref>Price, P.W. 1980. Evolutionary Biology of Parasites. Princeton University Press, Princeton</ref> and it is also frequent in [[plant]]s and [[fungi]]. Moreover, almost all free-living animals are host to one or more parasite [[taxon|taxa]].<ref>Price, P.W. 1980. Evolutionary Biology of Parasites. Princeton University Press, Princeton</ref> Parasites evolve in response to defense mechanisms of their hosts. Examples of host defenses include the [[toxin]]s produced by [[plant]]s to deter parasitic [[fungus|fungi]] and [[bacterium|bacteria]], the complex [[vertebrate]] [[immune system]], which can target parasites through contact with bodily fluids, and behavioural defenses. An example of the latter is the avoidance by [[sheep]] of open pastures during spring, when [[nematode|roundworm]] eggs accumulated over the previous year hatch en masse. As a result of these and other host defenses, some parasites evolve adaptations that are specific to a particular host [[taxon]] and specialize to the point where they infect only a single species. Such narrow '''host specificity''' can be costly over evolutionary time, however, if the host species becomes [[extinction|extinct]]. Thus, many parasites are capable of infecting a variety of host species that are more or less closely related, with varying success. Host defenses also evolve in response to attacks by parasites. Theoretically, parasites may have an advantage in this [[evolutionary arms race]] because of their more rapid [[generation time]]. Hosts reproduce less quickly than parasites, and therefore have fewer chances to [[adaptation|adapt]] than their parasites do over a given span of time. In some cases, a parasite species may [[Co-evolution|coevolve]] with its host [[taxon|taxa]]. In theory, long-term coevolution should lead to a relatively stable relationship tending to [[commensalism]] or [[mutualism]], in that it is in the evolutionary interest of the parasite that its host thrives. For example, although animals infected with [[parasitic worm]]s are often clearly harmed, and therefore parasitized, such infections may also reduce the prevalence and effects of [[Autoimmunity|autoimmune]] disorders in animal hosts, including humans.<ref>{{cite journal |author=Rook, G.A.W. |title=The hygiene hypothesis and the increasing prevalence of chronic inflammatory disorders|journal=Transactions of the Royal Society of Tropical Medicine and Hygiene |volume=101 |pages=1072–4 |year=2007 | doi = 10.1016/j.trstmh.2007.05.014 <!--Retrieved from CrossRef by DOI bot-->}}</ref> The presumption of a shared evolutionary history between parasites and hosts can sometimes elucidate how host taxa are related. For instance, there has been dispute about whether [[Phoenicopteriformes|flamingos]] are more closely related to the [[Ciconiiformes|storks]] and their allies or to [[Anseriformes|ducks, geese]] and their relatives. The fact that flamingos share parasites with ducks and geese is evidence these groups may be more closely related to each other than either is to storks. Parasitism is part of one explanation for the evolution of [[secondary sex characteristic]]s seen in breeding males throughout the [[animal]] world, such as the plumage of male [[peacock]]s and manes of male [[lion]]s. According to this theory, female hosts select males for breeding based on such characteristics because they indicate resistance to parasites and other [[disease]]. ==Ecology== Because they are small and often hidden from view, parasites are often ignored in [[ecology]]. However, parasites are ubiquitous and play important roles in [[ecosystem]]s and, considered in their own right, pose unique problems in ecology. More recently, therefore, parasite ecology has matured as a discipline and begun to integrate with ecology in the broader sense. ===Quantitative ecology=== When considering the distribution of a single parasite species, one finds that parasite individuals exhibit an [[aggregated distribution]] among host individuals. This means that most hosts harbour a few or no parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology: the use of [[parametric statistics]] should be avoided. [[Log-transformation]] of data before the application of parametric test, or the use of [[non-parametric statistics]] is recommended by several authors; however, these give rise to further problems. Therefore, modern day [[quantitative parasitology]] is based on more advanced biostatistical methods. ===Diversity ecology=== Hosts represent discrete habitat patches that can be occupied by parasites. A hierarchical set of terminology has come into use to describe parasite assemblages at different host scales. An '''infrapopulation''' is all the parasites of one species in a single individual host A '''metapopulation''' is all the parasites of one species in a host population An '''infracommunity''' is all the parasites of all species in a single individual host A '''component community''' is all the parasites of all species in a host population A '''compound community''' is all the parasites of all species in all host species in an ecosystem. The diversity ecology of parasites differs markedly from that of free-living organisms. That is, the determinants of species richness and relative abundance animals. For free-living organisms, diversity ecology features many strong conceptual frameworks including Macarthur and Wilson's [[island biogeography|theory of island biogeography]], Diamond's [[assembly rules]] and, more recently, null models such as Hubbell's [[neutral theory]] of biodiversity and biogeography. Frameworks are not so well-developed for parasites and in many ways they do not fit the free-living models. For example, island biogeography is predicated on fixed spatial relationships between habitat patches ("sinks"), usually with reference to a mainland ("source"). Parasites inhabit hosts, which represent mobile habitat patches with dynamic spatial relationships. There is no true "mainland" other than the sum of hosts (host population); in this way, parasite component communities in host populations are [[metacommunities]]. Nonetheless, different types of parasite assemblages have been recognised in host individuals and populations, and many of the patterns observed for free-living organisms are also pervasive among parasite assemblages. The most prominent of these is the interactive-isolationist continuum. This proposes that parasite assemblages occur along a cline from interactive communities, where niches are saturated and interspecific competition is high, to isolationist communities, where there are many vacant niches and interspecific interaction is not as important as stochastic factors in providing structure to the community. Whether this is so, or whether community patterns simply reflect the sum of underlying species distributions (no real "structure" to the community), has not yet been established. ===Transmission=== Parasites inhabit living organisms, and as a result face problems that free-living organisms do not. Hosts, the only habitats in which parasites can survive, actively try to avoid, repel and destroy parasites. Parasites employ numerous strategies for getting from one host to another, a process sometimes referred to as parasite '''transmission''', or the colonization of new hosts. Many endoparasites infect their host by penetrating its external surface, while others must be ingested by the host. Once inside the host, adult endoparasites need to shed offspring into the external environment in order to infect other hosts. Many adult endoparasites reside in the host’s [[gastrointestinal tract]], where offspring can be shed along with host [[excreta]]. Adult stages of [[Cestoda|tapeworms]], [[Acanthocephala|thorny-headed worms]] and most [[Trematoda|flukes]] use this method. Among [[protozoa]]n endoparasites, such as the [[malaria]]l parasites and [[trypanosome]]s, infective stages in the host’s [[blood]] are transported to new hosts by biting-[[insect]]s, or [[Vector (biology)|vectors]]. [[Larva]]l stages of endoparasites often infect sites in the host other than the blood or [[gastrointestinal tract]]. In many such cases, larval endoparasites require their host to be consumed by the next host in the [[parasitic life cycles|parasite’s life cycle]] in order to survive and reproduce. Alternatively, larval endoparasites may shed free-living transmission stages that migrate through the host’s tissue into the external environment, where they actively search for or await ingestion by other hosts. The foregoing strategies are used, variously, by larval stages of [[Cestoda|tapeworms]], [[Acanthocephala|thorny-headed worms]], [[Trematoda|flukes]] and parasitic [[Nematoda|roundworms]]. Many ectoparasites, such as [[Monogenea| monogenean]] worms, rely on direct contact between hosts to colonize new hosts, but other methods are also used. Ectoparasitic [[arthropod]]s may rely on host-host contact (e.g. many [[anoplura|lice]]) shed eggs that survive off the host (e.g. [[Siphonaptera|fleas]]) and/or wait in the external environment for an encounter with a host (e.g. [[tick]]s). Some aquatic [[leech]]es locate hosts by sensing movement and only attach when certain temperature and chemical cues are present. Some parasites modify host behaviour to make transmission to other hosts more likely. For example, in [[California]] salt marshes, the fluke ''[[Euhaplorchis californiensis]]'' reduces the ability of its [[killifish]] host to avoid predators.<ref>Lafferty, K. D. and A. K. Morris. 1996. Altered behavior of parasitized killifish increases susceptibility to predation by bird final hosts. Ecology 77:</ref> This parasite matures in [[egret]]s, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan ''[[Toxoplasma gondii]]'', a parasite that matures in [[Felis silvestris catus|cats]] but can be carried by many other [[Mammalia|mammals]]. Uninfected [[Rattus rattus|rats]] avoid cat odours, but rats infected with ''T. gondii'' are drawn to this scent, a change which may increase transmission to feline hosts.<ref>{{cite journal |author=Berdoy M, Webster JP, Macdonald DW |title=Fatal attraction in rats infected with Toxoplasma gondii |journal=Proc. Biol. Sci. |volume=267 |issue=1452 |pages=1591–4 |year=2000|doi=10.1098/rspb.2000.1182 | url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=}}</ref> ===Roles in ecosystems=== Modifying the behaviour of infected hosts to make transmission to other hosts more likely is one way parasites can affect the structure of [[ecosystem]]s. For example, in the case of ''[[Euhaplorchis californiensis]]'' discussed above, it is plausible that the abundance of local predator and prey species would be different if this parasite were absent from the system. Although parasites are often omitted in depictions of [[food web]]s, they usually occupy the top position. Parasites can function like [[keystone species]], reducing the dominance of superior competitors and allowing competing species to co-exist. 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. In this sense, the parasites in an ecosystem reflect the “health” of that system. ==Disease== Parasitic diseases account for a large proportion of human morbidity and mortality, and doubtlessly contribute significantly to morbidity and mortality among all animal populations as well. In this sense, parasitic disease is an important ecological force shaping the [[biosphere]]. ==Treatment== Some major parasitic diseases of humans include [[malaria]], [[sleeping sickness]], [[schistosoma|schistosomiasis]], [[leishmaniasis]], [[limerence]], [[Ascaris|ascariasis]], [[pinworm|enterobiasis]], [[entamoeba|entamoebiasis]], [[elephantiasis]], [[Onchocerciasis|river blindness]], [[giardia]]sis and [[Cryptosporidium|cryptosporidiosis]], as well as minor afflictions like [[lice]], [[mites]], [[chiggers]], [[botfly|bot flies]], [[bed bugs]], [[ticks]], [[Eye Worm|eye worms]], [[Paragonimus|lung worm]], and [[guinea worm]]. Humans are also subject to a myriad zoonotic diseases including ''[[Diphyllobothrium]]'', [[hydatid]] disease, [[trichinellosis]], ''[[Taenia]]'' infections, and [[anisakis|anisakiasis]]. ==See also== * [[List of parasitic organisms]] * [[Intestinal parasite]] * [[Macroparasite]] * [[Plasmodium]] * [[Myco-heterotrophy]] * [[Parasitic plant]] * [[Parasitic wasp]] * [[Pinworm]] * [[Superparasitism]] * [[Teratology]] * [[Toxoplasmosis]] * [[The Extended Phenotype]] * [[Symbiosis]] ==Further reading== *[[Carl Zimmer|Zimmer, Carl]] 2001. ''Parasite Rex''. Free Press. ISBN -X *[[Claude Combes|Combes, Claude]] 2005. ''The Art of Being a Parasite.'' The University of Chicago Press. ISBN-10: *[[Robert Desowitz|Desowitz, Robert]] 1998. ''Who Gave Pinta to the Santa Maria?'' Harvest Books. ISBN-10: ==External links== *[http://www.cdc.gov/search.do?action=search&queryText=toxoplasmosis&x=0&y=0 Toxoplasmosis] *[http://www.parazytologia.pl Parasitology Parasites Zoonoses] - (Polish/English) over 50 movies (Filmoteka) and over 250 photos (Fotogaleria/Photogallery) with human and animal parasites. *[http://www.aber.ac.uk/~mpgwww/Edu/EduIndex.html Aberystwyth University: Parasitology] – class outline with links to full text articles on parasitism and parasitology. *[http://www.k-state.edu/parasitology KSU: Parasitology Research] - parasitology articles and links. *[http://pathmicro.med.sc.edu/book/parasit-sta.htm Medical Parasitology] – online textbook. *[http://www.cdc.gov/ncidod/dpd/ Division of Parasitic Diseases], [[Centers for Disease Control and Prevention]] *[http://www.vcu.edu/csbc/vpp/ VCU Virtual Parasite Project] - Virtual Parasite Project at Virginia Commonwealth University's Center for the Study of Biological Complexity {{evo ecol}} ==References== <references/> [[Category:Parasitology]] [[Category:Symbiosis]] [[Category:Parasitism|*]] [[ar:تطفل]] [[bg:Паразитизъм]] [[cs:Parazitismus]] [[da:Snylter]] [[de:Parasitismus]] [[el:Παρασιτισμός]] [[es:Parasitismo]] [[eo:Parazitismo]] [[eu:Bizkarroi]] [[fa:انگل]] [[fr:Parasitisme]] [[gd:Dìosganach]] [[ko:기생 (생물학)]] [[id:Parasitisme]] [[it:Parassitismo]] [[he:טפיל]] [[lt:Parazitizmas]] [[hu:Élősködő]] [[ml:പരാദ സസ്യങ്ങള്‍]] [[nl:Parasitair]] [[ja:寄生]] [[no:Parasittisme]] [[om:Maxxantumma]] [[pl:Pasożytnictwo (biologia)]] [[pt:Parasita]] [[ru:Паразитизм]] [[simple:Parasitism]] [[sk:Parazitizmus]] [[sr:Паразитизам]] [[fi:Loinen (biologia)]] [[sv:Parasit]] [[ta:ஒட்டுண்ணி வாழ்வு]] [[uk:Паразитизм]] [[zh:寄生]]