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Ectoparasites are those that occur on the body surface of the host, like leeches, ticks, lice, and mites. Endoparasites are those that live inside the body of the host, such as hookworms that live in the gut of a host and blood parasites.

Many endoparasites acquire hosts through entrance of the tissue, as well as through consumption of raw foods, such as the nematode Ascaris lumbricoides, an endoparasite of the human intestine. A. lumbricoides produces large numbers of eggs, which are passed from the host's digestive tract into the external environment, relying on other humans to inadvertently ingest them in places without good sanitation. Ectoparasites, on the other hand, often have elaborate mechanisms and strategies for finding hosts. Some aquatic leeches, for example, locate hosts by sensing movement and then confirm their identity through skin temperature and chemical cues before attaching.

An epiparasite is a parasite that feeds on another parasite. The parasite that is being parasitized by another organism is known as hyperpasasite or secondary parasite, and this relationship is sometimes referred to as "hyperparasitoidism," especially in the insect world. For example a wasp or fly larva may be an endoparasite of an Ichneumon wasp larva, which is in turn an endoparasite of a wood-boring beetle larva. Therefore, the ovipositing adult female hyperparasitoid must find the host of her host, namely the beetle larva, and oviposit into that beetle larva, after which her egg hatches within the beetle larva and seeks out the Ichneumon larva, ultimately burrowing into it and becoming an endoparasite. Hyperparasitoidism can be used for biological control of the pest and parasites.

Macroparasites are multicellular parasites that are visible to the naked human eye, such as helminth parasites (parasitic worms, such as flukes, tapeworms, and roundworms, or nematodes). Microparasites are small, generally, unicellular and invisible to the naked eye, such as protozoan parasites.

Mesoparasites are the ones that penetrate external openings, such as the buccal cavity, cloaca, external ear, and so forth.

Necrotrophs are parasites that use another organism's tissue for their own nutritional benefit until the host dies from loss of needed tissues or nutrients. Necrotrophs are also known as parasitoids. Biotrophic parasites cannot survive in a dead host and therefore keep their hosts alive. Many viruses, for example, are biotrophic because they use the host's genetic and cellular processes to multiply.

Temporary parasites (leeches, bed bugs) visit their host only for a short period of time. Permanent parasites spend the duration, or a part, of their life cycle in the host.

Facultative parasites can survive without the parasitic mode of life, but can adapt to it if placed in such a situation; they are opportunistic. For example, the parasitic nematode, Strongyloides stercoralis, can also be free living. The majorities of parasites are obligatory parasites and are totally dependent on the host for food, shelter, and/or protection; they cannot survive without the host. Accidental parasites are those that happen to infect unusual hosts, other than the normal definite host. Wandering or aberrant parasites, instead of arriving at the site of infection in the definitive host, reach an unusual place as a dead end, becoming unable to complete the life cycle. For example, the tapeworm Taenia solium may migrate to the brain and remain there unless removed via an operation.

Monogenic parasites complete the whole life cycle in one host, such as with Entamoeba histolytica. A digenetic parasite needs, in addition to a primary host, also a secondary host to complete the entire life cycle. Plasmodium vivax (malaria parasite) completes its asexual part of life cycle in people and the sexual part in the female Anopheles mosquito.

Some parasites are social parasites, taking advantage of interactions between members of a social host species such as ants or termites, to the hosts' detriment. Kleptoparasitism involves the parasite stealing food that the host has caught or otherwise prepared. A specialized type of kleptoparasitism is brood parasitism, such as that engaged in by many species of cuckoo. Many cuckoos use other birds as lifetime "babysitters"; cuckoo young are raised and fed by adults of the host species, while adult cuckoos fend for themselves.

Cheating or exploitation types of parasitism are often found in situations where there is generalized, non-specific mutualisms between broad classes of organisms, such as mycorrhizal relationships between plants and many types of fungi. Some myco-heterotrophic plants behave as "mycorrhizal cheaters," establishing mycorrhiza-like interactions with a fungal symbiont, but taking carbon from the fungus (which the fungus, in turn, gets from other plants) rather than donating carbon.

Types of Hosts

A definitive host is usually the main host. For digenetic parasites, it is the host for the adult stage and for the completion of sexual part of life cycle. An intermediate or secondary host is a temporary environment, but one that is essential for the completion of a particular parasite's life cycle. Such as host is found only in the case of digenetic parasites for the completion of larval stage, asexual reproduction, and for transmission to the definitive host. An accidental host may be one that can function as the normal host, but is infected only occasionally for some reason, for example due to the lack of exposure or means of transmission.

A vector is usually the intermediate host playing an active role in the transmission of the parasite.

A permissive host is either a definitive, intermediate, or accidental host that allows the parasite to complete its life cycle in part or the whole. A non-permissive host, on the other hand, is a host organism other than true definitive host, which receives the parasite but the parasite finds itself in a dead end.

A paratenic host or transport host refer to a host organism other than true intermediate host that receives the parasite in the position of intermediate host so that the parasite is helped to go to the definitive host. For example Echinococcus granulosus normally passes to a dog through an intermediate host, such as a goat or sheep. But the parasite, instead of passing through the intermediate host, may come to infect a human being and remain, causing hydatiditis, and a dog has no chance to get it from a person. Reservoir hosts are permissive host alternatives to definitive hosts, such that the infective stage can be passed from the host to the population of the definitive host.

Importance

Biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa. Price (1977) maintains that parasitism is the prevalent means of obtaining food among organisms, that over 50 percent of the organisms living today are parasitic, and that there are more species of parasites than all non-parasite species combined.

The hosts of parasites often have defensive mechanisms as well. Plants often produce toxins, for example, which deter both parasitic fungi and bacteria, as well as herbivores. Vertebrate immune systems can target most parasites through contact with bodily fluids. On a behavioral level, the itching sensation, and resulting scratching behavior, is also used to fend off parasites. Many parasites, particularly microorganisms, have adaptations to a particular host species; in such specific interactions, the two species generally have a relatively stable relationship that does not kill the host quickly or at all (since this would be detrimental for the parasite as well).

Sometimes, the study of parasite taxonomy can elucidate how their hosts are similar or related. For instance, there has been a dispute about whether Phoenicopteriformes (flamingos) are more closely related to Ciconiiformes (storks and related groups) or to Anseriformes (waterfowl and related groups). Flamingos share parasites with ducks and geese, so these groups are thought to be more closely related to one another than either is to storks. (Modern DNA methods, however, have suggested that flamingos are not closely related to Anseriformes either.)

It is important to note that "benefit" and "harm" in the definition of parasitism apply to lineages, not individuals. Thus, if an organism becomes physically stronger as a result of infection but loses reproductive capabilities (as results from some flatworm infections of snails), that organism is harmed in a reproductive sense and is thus parasitized. The harm caused to a host by a parasite can take many forms, from direct pathology, including various specialized types of tissue damage, such as castration, to more subtle effects, such as modification of host behavior.

References

  • Margulis, L., and D. Sagan. 1987. Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. HarperCollins. ISBN 004570015X
  • Price, P. W. 1977. General concepts on the evolutionary biology of parasites. Evolution 31(2): 405-420.
  • Towle, A. 1989. Modern Biology. Austin: Holt, Rinehart, and Winston. ISBN 0030139198
  • Zimmer, C. 2001. Parasite Rex. Free Press. ISBN 074320011X
Patterns of evolution: Convergent evolution • Evolutionary relay • Parallel evolutionColour and shape: Aposematism • Mimicry • CrypsisInteractions between species: Mutualism • Cooperation • Predation • Parasitism

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