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- 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. (en)
- 在寄生虫学和流行病学中，宿主转换是针对宿主的特异性进化性演化。 所有共生物种，例如寄生虫、病原体和互利生物，都表现出一定程度的宿主特异性。 它们寄生在单一寄主物种的身体中(或身体表面) ，或者更常见的情况是是寄生在一组有限的寄主物种上。 在后一种情况下，合适的寄主物种往往是在分类学上密切相关联的，它们往往具有相似的形态和生理学性状。 例如，人类免疫缺陷病毒（HIV）曾经出现在非洲中西部的非人灵长类动物身上，并在20世纪早期转变为以人类为宿主的病毒。 最近的研究提出，将两种具有宿主特异性的进化性演化类型区分开。 根据这种观点，寄主转换是一些能够在新的寄主上建立一个新的可行的种群的寄生个体对一个新的寄主物种的寄生造成的。而这种新种群的形成往往是意外和突然的。 在完成对新寄主的寄生之后，新的种群或多或少地与原来的种群产生了隔离。 它不影响同种寄生虫在供体宿主上的进一步命运，并可能最终导致新寄生生物的物种形成。 寄生物种的转换更有可能瞄准那些人口日益增长的宿主种群，和寄生者/病原体动物相对贫乏的种群。例如入侵物种的先驱种群。 相反，在多宿主寄生物种情况下，宿主转移是一个特定宿主物种作为初级宿主与次级宿主的相对作用的逐渐变化。 原主寄主逐渐成为次要寄主，甚至完全被遗弃，而原次要寄主则成为新的主要寄主。 这个过程是缓慢和更可预测的，且不会增加寄生虫的多样性。 可以预见的是，它通常发生在寄主种群数量减少，且寄主种群数量相对较少 (考虑到寄主种群规模较小) ，寄生物种 / 病原动物数量较多的情况下。 (zh)
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