Genetic Variation in Resistance to Parasites

Evidence for genetic variation to parasites has been found both within, and between, host populations and for a number of important parasite species including nematode worms, coccidian protozoa, flies and ticks. For this genetic variation to be exploited successfully, the requirements of livestock farmers must be identified clearly and an appropriate breeding objective established. The consequences of breeding for parasite resistance need also to be assessed for resistance to diseases that are not the subject of selection, and productivity in terms of cash, investment security, social and religious characteristics, and avoidence of risks such as the development of resistance to drugs used for disease control. The concepts of resistance and resilience can be more clearly defmed in terms of the criteria used for selection, the breeding objective and the environment in which the selection takes place. 'Environment' in this context should include nutritional status, exposure to disease, age and immune status as well as usual features such as temperature, humidity and rainfall. T here is ample evidence from studies on humans, domestic livestock and experimental animals for genetic variation in resistance to internal and external parasites, both within and between populations. We are, however, almost completely ignorant of the nature of the genes that confer resistance to diseases caused by parasites. As we start to exploit genetic variation to improve resistance to parasites and reduce their impact on small ruminant production a major question arises. Can we, knowing so little about the genetic basis of resistance, predict the outcome of breeding programs? The answer to that question is a qualified 'yes'. Genetic variation to parasitism must be viewed in the context of successful breeding programs for other production traits. The criteria for inclusion of a trait in a breeding program-being heritable, variable and able to be measured-are met by resistance to several diseases of small ruminants. The prospects for breeding for resistance are therefore very good. But this interaction of host and parasite should also be seen in an evolutionary context. Host and parasites have evolved together and we should certainly not be complacent that parasites will not adapt to genetic changes in the host imposed by breeding programs. There is no evidence (Woolaston and Eady, this volume) that suggests this can happen but it is worthwhile to be reminded that genetic adaptation by the parasite, genetic resistance to anthelmintics, has stimulated much of the research on breeding approaches to parasite control. Breeding for Resistance to Infectious Diseases in Small Ruminants There have been a number of comprehensive reviews of genetic variation in host response to parasites (Chandler 1932; Wakelin 1988, 1989; Oargie 1982; Mitchell 1979; Albright and Albright 1984; Grencis 1990). In this paper more practical aspects of breeding for resistance to parasites will be addressed. Genetic Differences Between Hosts