In nature, hosts are frequently infected by multiple parasite species. Depending on environmental conditions and the order of infection, the different species present in the same individual host, at the same time, can interact or not, depending on whether they have overlapping or non-overlapping niches (the range of environmental conditions experienced by the species). Two kinds of interaction have received much attention: interspecific competition and host-defense-mediated interactions.
Opportunities for competition between two species exist when both species exploit the same resource in the host (food, space) (see Competition and Behavior). The performance (in terms of growth, reproduction, and survival) of one species is reduced when the second species is present, either because the amount of available resources has been depressed (competition through exploitation) or because one species prevents the other from exploiting some resources via direct interaction (competition through interference). Asymmetrical competition has been often reported for parasites, when the species A suffers from the presence of species B whereas species A has no effect on species B. This pattern suggests a one-sided interference rather than exploitative competition, but it could also be due to host-mediated effects involving the immune response.
Competition is thought to be one of the main factors responsible for structure and assemblage of communities. When two parasite species with overlapping realized niche in isolation co-occur in the same host, the realized niche of at least one of them might be changed (compared to when occurring alone) to minimize competition. This process is called interactive site segregation. There are many examples of site segregation in helminths inhabiting the digestive tract of their hosts, sometimes leading to a more or less regular spreading of species along the digestive tract. If there is some genetic basis for the niche preference, and if the cost of competition (in terms of fitness) outweighs the cost of realized niche change (because of less optimal environmental conditions), then this change may be selected for and become genetically fixed. This can lead to a complete segregation of the fundamental niche, where species have nonoverlapping niches whether alone or together in the same host.
Host immunity can also mediate and affect the outcome of the competition between parasite species. As mentioned above, immunosuppression is a common strategy adopted by parasites to persist within the host. Host immunosuppression can, however, have profound effects on the colonization and the population dynamics of other parasites. The spread of opportunistic diseases following infection with HIV virus is one of the most striking examples of this host-immunity-mediated interaction.
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