Host switch

In parasitology and epidemiology, a host switch (or host shift) is an evolutionary change of the host specificity of a parasite or pathogen. For example, the human immunodeficiency virus used to infect and circulate in non-human primates in West-central Africa, but switched to humans in the early 20th century. All symbiotic species, such as parasites, pathogens and mutualists, exhibit a certain degree of host specificity. This means that pathogens are highly adapted to infect a specific host - in terms of but not limited to receptor binding, countermeasures for host restriction factors and transmission methods. They occur in the body (or on the body surface) of a single host species or – more often – on a limited set of host species. In the latter case, the suitable host species tend to be taxonomically related, sharing similar morphology and physiology. Speciation is the creation of a new and distinct species through evolution and so unique differences exist between all life on earth. It goes without saying that dogs and birds are very different classes of animals – for one, dogs have fur coats and birds have feathers and wings. We therefore know that their fundamental biological makeup is as different as their physical appearance, this ranges from their internal cellular mechanisms to their response to infection, and so species-specific pathogens must overcome multiple host range barriers in order for their new host to support their infection. Recent studies have proposed to discriminate between two different types of evolutionary change in host specificity. According to this view, host switch can be a sudden and accidental colonization of a new host species by a few parasite individuals capable of establishing a new and viable population there. After a switch of this type, the new population is more-or-less isolated from the population on the donor host species. The new population does not affect the further fate of the conspecific parasites on the donor host, and may finally lead to parasite speciation.