Parasite adaptations and evolution
Host–parasite co-evolution is a special case of co-evolution, which is defined as the reciprocal adaptive genetic change of two antagonists (e.g. different species or genes) through reciprocal selective pressures. In the particular case of host–parasite co-evolution the antagonists are different species of host and parasite. Hosts and parasites exert reciprocal selective pressures on each other, which may lead to rapid reciprocal adaptation. For organisms with short generation times host–parasite co-evolution can be observed in comparatively small time periods, making it possible to study evolutionary change in real-time under both field and laboratory conditions.
Specificity patterns are shaped during the course of parasite evolution. Phylogenetic specificity is determined by ecological or physiological factors or a combination of both.
The evolutionary basis of host specificity is supported by the fact that primitive hosts habour primitive parasites. Hosts of monophyletic (single) are parasitized by an exclusive group of parasites. Parasites that evolved with their hosts are known as heirlooms while those that are picked up through ecological links or contact are known as souvenirs. E.g adults of family Taenidae parasitize only mammalian hosts. Elasmobranchs are exclusive hosts of the tapeworms of the order Tetraphyllida and tetrarhyncha.
Host resistance and Host protection
Free-living organisms struggle predominantly with abiotic factors within the environment for their survival. Biotic and abiotic factors of parasite habitats affect the establishment, maintenance and survival in the host.
They experience varied destructive factors in their host and their ability to establish, survive and maintain their presence in their host is always under threat. They are affected by host immunity which constantly produce antibody against parasite.
Host resistance refers to the unsuitability of a host. This is however never absolute. It may result from the presence of some physical or chemical barriers that prevents parasite from establishing in a host. A number of host characteristics such as physiology, behavior and structure influence host-parasite interactions, thereby influencing host resistance. Host natural resistance can also arise through geographical distribution, behavioural characteristics of the host or nutritional habits as the host may not come in contact with the infective stages.
Natural resistance encompasses a physiological incompatibility between parasite and host, preventing parasite invasion, establishment and survival, without the intervention of immunological- based protective responses. It is influenced by factors such as host’s genetic composition, age, nutritional status and gender.
Organisms are able to distinguish between self and non-self using immune response. While many parasites have evolved successful means of evading host immune response, the ability of the host to control the parasite is not absolute. Various host exhibit different degrees of susceptibility or resistance for parasitic agents or specific infections.
Hosts are protected from infection by:
Non-specific resistance and
Specific immunity or protection.Non-specific resistance is also known as innate or natural immunity. It includes mechanisms which the host is genetically endowed with at birth and thereby protected from certain infections.