Trichinella spiralis: immunity, ecology, and evolution.

Parasites exploit the environment provided by their hosts in the same way that free-living animals exploit their physical and biotic environments. There is, however, 1 important difference: the host environment can respond adaptively to infection in ways that may be disadvantageous to the parasite. Immunity provides the most effective adaptive host response and exerts a major selective pressure in parasite evolution. Among parasites many ways of maintaining the ability to exploit the host in the face of adaptive responses have evolved; such responses additionally may promote survival by enhancing transmission. The nematode Trichinella spiralis provides an excellent model with which to explore immuno-parasite-ecological ideas. Its biology is well known, and host immunity is described in detail. Immunity acts to limit parasite survival, but it also helps to ensure transmission by allowing the host to survive until infective stages mature. Host and parasite engage in a complex interplay, mediated by immunity, in which the genetic variability of each partner plays an important and necessary role.