Parasitism
Parasitism is a
There are six major parasitic strategies of exploitation of animal hosts, namely parasitic 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 of
.Parasites reduce host
People have known about parasites such as
Etymology
First used in English in 1539, the word parasite comes from the
Evolutionary strategies
Basic concepts
Parasitism is a kind of
Within that scope are many possible strategies.
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
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] exhaled carbon 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]
Major strategies
There are six major parasitic
A perspective on the evolutionary options can be gained by considering four key questions: the effect on the 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.[23]
Host fitness | Single host, stays alive | Single host, dies | Multiple hosts |
---|---|---|---|
Able to reproduce (fitness > 0) |
Conventional parasite Pathogen |
Trophically-transmitted parasite[a] Trophically-transmitted pathogen |
Micropredator Micropredator |
Unable to reproduce (fitness = 0) |
----- Parasitic castrator |
Trophically-transmitted parasitic castrator Parasitoid |
Social predator[b] Solitary predator |
Parasitic castrators
Directly transmitted
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
Trophically transmitted
Vector-transmitted
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, namely protozoa, bacteria, or viruses, often intracellular pathogens (disease-causers).[22] Their vectors are mostly hematophagic arthropods such as fleas, lice, ticks, and mosquitoes.[22][29] For example, the deer tick Ixodes scapularis acts as a vector for diseases including Lyme disease, babesiosis, and anaplasmosis.[30] Protozoan endoparasites, such as the malarial 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.[31]
Parasitoids
-
Idiobiont parasitoid wasps immediately paralyse their hosts for their larvae (Pimplinae, pictured) to eat.[22]
-
Koinobiont parasitoid wasps like this braconid lay their eggs via an ovipositorinside their hosts, which continue to grow and moult.
-
altering its behaviour.
Micropredators
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.
Transmission strategies
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.[35] Examples to illustrate some of the many possible combinations are given in the table.
Parasite | Host | Transmission method | Ecological context |
---|---|---|---|
monogenean ) |
Poecilia reticulata (guppy) |
physical contact | social behaviour |
Nematodes e.g. Strongyloides |
Macaca fuscata (Japanese macaque) |
fecal–oral |
social behaviour |
Heligmosomoides polygyrus (a nematode) |
Apodemus flavicollis (yellow-necked mouse) |
fecal–oral | sex-biased transmission (mainly to males) |
Amblyomma (a tick) |
Sphenodon punctatus (tuatara) |
free-living infectious stages | social behaviour |
Plasmodium (malaria parasite) |
Birds, mammals (inc. humans) |
Anopheles mosquito vector, attracted by odour of infected human host[36] |
— |
Variations
Among the many variations on parasitic strategies are hyperparasitism,[37] social parasitism,[38] brood parasitism,[39] kleptoparasitism,[40] sexual parasitism,[41] and adelphoparasitism.[42]
Hyperparasitism
Hyperparasites can control their hosts' populations, and are used for this purpose in agriculture and to some extent in medicine. The controlling effects can be seen in the way that the CHV1 virus helps to control the damage that chestnut blight, Cryphonectria parasitica, does to American chestnut trees, and in the way that bacteriophages 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 parasitism
Social parasites take advantage of interspecific interactions between members of
Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation.[51]
Brood parasitism
In
Kleptoparasitism
In
Sexual parasitism
A unique approach is seen in some species of
Adelphoparasitism
Adelphoparasitism, (from Greek ἀδελφός (adelphós), brother[56]), 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 lay haploid eggs in the fully developed larvae of their own species, producing male offspring,[57] while the marine worm Bonellia viridis has a similar reproductive strategy, although the larvae are planktonic.[58]
Illustrations
Examples of the major variant strategies are illustrated.
-
A hyperparasitoidbraconid wasp
-
The large blue butterfly is an ant mimic and social parasite.
-
Inbrood parasitism, the host raises the young of another species, here a cowbird's egg, that has been laid in its nest.
-
The great skua is a powerful kleptoparasite, relentlessly pursuing other seabirds until they disgorge their catches of food.
-
The malesexual parasitepermanently attached below the female's body.
-
citrus blackfly(lower left), is also an adelphoparasite, laying eggs in larvae of its own species
Taxonomic range
Parasitism has an extremely wide taxonomic range, including animals, plants, fungi, protozoans, bacteria, and viruses.[59]
Animals
Phylum | Class/Order | No. of species |
Endo- paras. |
Ecto- paras. |
Invert def. host |
Vert def. host |
No. of hosts |
Marine | Fresh- water |
Terres- trial |
---|---|---|---|---|---|---|---|---|---|---|
Cnidaria | Myxozoa | 1,350 | Yes | Yes | 2 or more | Yes | Yes | |||
Cnidaria | Polypodiozoa |
1 | Yes | Yes | 1 | Yes | ||||
Flatworms |
Trematodes |
15,000 | Yes | Yes | 2 or more | Yes | Yes | Yes | ||
Flatworms |
Monogeneans | 20,000 | Yes | Yes | 1 | Yes | Yes | |||
Flatworms |
Cestodes |
5,000 | Yes | Yes | 2 or more | Yes | Yes | Yes | ||
Horsehair worms | 350 | Yes | Yes | 1 or more | Yes | Yes | ||||
Nematodes |
10,500 | Yes | Yes | Yes | 1 or more | Yes | Yes | Yes | ||
Acanthocephala | 1,200 | Yes | Yes | 2 or more | Yes | Yes | Yes | |||
Annelids |
Leeches |
400 | Yes | Yes | 1 | Yes | Yes | |||
Molluscs |
Bivalves |
600 | Yes | Yes | 1 | Yes | ||||
Molluscs |
Gastropods |
5,000 | Yes | Yes | 1 | Yes | ||||
Arthropods |
Ticks |
800 | Yes | Yes | 1 or more | Yes | ||||
Arthropods |
Mites |
30,000 | Yes | Yes | Yes | Yes | 1 | Yes | Yes | Yes |
Arthropods |
Copepods |
4,000 | Yes | Yes | Yes | 1 | Yes | Yes | ||
Arthropods |
Lice |
4,000 | Yes | Yes | 1 | Yes | ||||
Arthropods |
Fleas |
2,500 | Yes | Yes | 1 | Yes | ||||
Arthropods |
True flies |
2,300 | Yes | Yes | 1 | Yes | ||||
Arthropods |
Twisted-wing insects | 600 | Yes | Yes | 1 | Yes | ||||
Arthropods |
Parasitoid wasps | 130,000[61] - 1,100,000[62] | Yes | Yes | Yes | 1 | Yes |
Parasitism is widespread in the animal kingdom,
Plants
A
Species within the
Many
Fungi
Parasitic
Plant pathogenic fungi are classified into three categories depending on their mode of nutrition: biotrophs,
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.[74] One example of a potent fungal animal pathogen are Microsporidia - obligate intracellular parasitic fungi that largely affect insects, but may also affect vertebrates including humans, causing the intestinal infection microsporidiosis.[75]
Protozoa
Protozoa such as Plasmodium, Trypanosoma, and Entamoeba[76] are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, and amoebic dysentery—and have complex life cycles.[31]
Bacteria
Many bacteria are parasitic, though they are more generally thought of as
Viruses
Evolutionary ecology
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.[83] Humans have 342 species of helminth parasites, and 70 species of protozoan parasites.[84] Some three-quarters of the links in food webs include a parasite, important in regulating host numbers. Perhaps 40 per cent of described species are parasitic.[83]
Fossil record
Parasitism is hard to demonstrate from the
Coevolution
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.[89] But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities in host–parasite coevolution.[90]
Coevolution favouring mutualism
Long-term coevolution sometimes leads to a relatively stable relationship tending to
Lynn Margulis and others have argued, following Peter Kropotkin's 1902 Mutual 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 the symbiogenesis which formed the eukaryotes from an intracellular relationship between archaea and bacteria, though the sequence of events remains largely undefined.[93][94]
Competition favouring virulence
Competition between parasites can be expected to favour faster reproducing and therefore more virulent parasites, by natural selection.[90][95]
Among competing parasitic insect-killing bacteria of the genera
Cospeciation
A parasite sometimes undergoes
An example is between the
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 whether
Parasites infect sympatric hosts (those within their same geographical area) more effectively, as has been shown with digenetic trematodes infecting lake snails.[99] This is in line with the Red 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 to allopatric hosts, those from different geographical regions.[99]
Modifying host behaviour
Some parasites
Trait loss
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 including
Host defences
Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates,[106] the immune system of mammals,[107] insects actively removing parasites,[108] and defensive chemicals in plants.[109]
The evolutionary biologist
Vertebrates
The physical barrier of the tough and often dry and waterproof
Insects
Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp
Plants
Plants respond to parasite attack with a series of chemical defences, such as polyphenol oxidase, under the control of the jasmonic acid-insensitive (JA) and salicylic acid (SA) signalling pathways.[109][113] 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.[114][113] 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.[114] 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.[114] When damaged, such as by lepidopteran caterpillars, leaves of plants including maize and cotton release increased amounts of volatile chemicals such as terpenes that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.[115]
Biology and conservation
Ecology and parasitology
Parasitism and parasite evolution were until the twenty-first century studied by
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 a food 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.[84]
Parasites can play a role in the proliferation of non-native species. For example, invasive green crabs are minimally affected by native trematodes on the Eastern Atlantic coast. This helps them outcompete native crabs such as the Atlantic Rock and Jonah crabs.[117]
Ecological parasitology can be important to attempts at control, like during the campaign 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.[118]
Rationale for conservation
External videos | |
---|---|
"Why you should care about parasites", 12.14.2018, Knowable Magazine |
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.[119] Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change.[120] 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.[121]
An ectoparasite, the California condor louse, Colpocephalum californici, became a well-known conservation issue. A major and very costly captive breeding program was run in the United States to rescue the California 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.[122]
Although parasites are often omitted in depictions of food webs, 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.[84][123][124]
Quantitative ecology
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 renders
History
Ancient
Medieval
In his Canon of Medicine, completed in 1025, the Persian physician Avicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.[126]
In his 1397 book Traité 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 wrote the first description of a trematode endoparasite, the sheep liver fluke Fasciola hepatica.[127][128]
Early modern
In the
In 1681,
Parasitology
Modern parasitology developed in the 19th century with accurate observations and experiments by many researchers and clinicians;[127] the term was first used in 1870.[132] In 1828, James Annersley described amoebiasis, protozoal infections of the intestines and the liver, though the pathogen, Entamoeba histolytica, was not discovered until 1873 by Friedrich Lösch. James Paget discovered the intestinal nematode Trichinella spiralis in humans in 1835. James McConnell described the human liver fluke, Clonorchis sinensis, in 1875.[126] Algernon Thomas and Rudolf Leuckart independently made the first discovery of the life cycle of a trematode, the sheep liver fluke, by experiment in 1881–1883.[127] In 1877 Patrick Manson discovered the life cycle of the filarial worms that cause elephantiasis transmitted by mosquitoes. Manson further predicted that the malaria parasite, Plasmodium, had a mosquito vector, and persuaded Ronald Ross to investigate. Ross confirmed that the prediction was correct in 1897–1898. At the same time, Giovanni Battista Grassi and others described the malaria parasite's life cycle stages in Anopheles mosquitoes. Ross was controversially awarded the 1902 Nobel prize for his work, while Grassi was not.[126] In 1903, David Bruce identified the protozoan parasite and the tsetse fly vector of African trypanosomiasis.[133]
Vaccine
Given the importance of malaria, with some 220 million people infected annually, many attempts have been made to interrupt its transmission. Various methods of
Biological control
Several groups of parasites, including microbial pathogens and parasitoidal wasps have been used as
Resistance
Poulin observes that the widespread
Cultural significance
Classical times
In the
Society
Parasitism has
In everyday speech, the term 'parasite' is loaded with derogatory meaning. A parasite is a sponger, a lazy profiteer, a drain on society.[142]
The
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 proceeds ad 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.[144]
Fiction
In
Disgusting and terrifying
See also
- Antiparasitic
- Carcinogenic parasite
- Effects of parasitic worms on the immune system
- List of parasites of humans
Notes
- ^ 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.
- ^ The wolf is a social predator, hunting in packs; the cougar is a solitary predator, hunting alone. Neither strategy is conventionally considered parasitic.[23]
References
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- ISBN 978-1-78243-857-1.
- ISBN 978-0-472-10760-5.
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.
- ^ ISBN 978-0-19-157934-9. 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.
- PMID 35538778.
- ISBN 978-0-472-11213-5.
- ^ "Parasitism and Symbiosis". The Encyclopedia of Science Fiction. 10 January 2016.
- ^ Dove, Alistair (9 May 2011). "This is clearly an important species we're dealing with". Deep Sea News.
- ^ Pappas, Stephanie (29 May 2012). "5 Alien Parasites and Their Real-World Counterparts". Live Science.
- ^ Sercel, Alex (19 May 2017). "Parasitism in the Alien Movies". Signal to Noise Magazine.
- ^ Nordine, Michael (25 April 2017). "'Alien' Evolution: Explore Every Stage in the Xenomorph's Gruesome Life Cycle. Celebrate Alien Day with a look at the past, present and future of cinema's most terrifying extraterrestrial". IndieWire.
Sources
- ISBN 978-0-691-12085-0.
Further reading
- ISBN 978-0-226-11438-5.
- ISBN 978-0-15-600585-2.
- ISBN 978-0-7432-0011-0.
External links
- Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance at Wikibooks
- Aberystwyth University: Parasitology—class outline with links to full text articles on parasitism and parasitology.
- Division of Parasitic Diseases, Centers for Disease Control and Prevention
- KSU: Parasitology Research—parasitology articles and links
- Parasitology Resources on the World Wide Web: A Powerful Tool for Infectious Disease Practitioners (Oxford University Press)