Genetics of disease resistance in sheep and goats

This review summarises the current state of knowledge of the genetic control of resistance to infectious and metabolic diseases in small ruminants. Diseases covered include gastrointestinal nematode infections, diseases due to mycotoxins, bacterial diseases including footrot and mastitis, ectoparasites such as flies and lice, and scrapie, the small ruminant transmissible spongiform encephalopathy. In all cases there is well-documented evidence for between-animal genetic variation in resistance to the disease and, in the case of some of the infectious diseases, resistance to infection. These heritable differences between animals lead to opportunities to breed animals for enhanced resistance to the disease. For some diseases, including nematode parasite infections, resistance to various forms of mycotoxin poisoning and fly-strike, the feasibility of breeding for resistance has been demonstrated in experimental flocks. Importantly, in other cases, including (again) nematode parasite infections and some forms of mycotoxin poisoning, but also mastitis, footrot and scrapie, there are now breeding programmes selecting commercial animals for enhanced resistance. In almost all cases, the evidence for genetic variation and the documented success of breeding for resistance occurs in sheep rather than goats. For nearly all the diseases discussed, there are now concerted efforts to find genetic markers associated with resistance to infection, potentially allowing selection for increased resistance in the absence of infection. Such selection is now implemented for scrapie, as a part of national scrapie eradication programmes, and selection using genetic markers is currently being explored for nematode parasite resistance, footrot resistance and resistance to facial eczema, a mycotoxin poisoning example. Functional genomics, combining gene mapping and gene expression studies, is now being applied to a number of diseases, an example being the identification of genes whose expression underlies genetic differences in resistance to nematode parasites. It is likely that integrated studies combining quantitative and functional genomics, large-scale data collection (both within and between breeds) and epidemiological prediction will lead to new opportunities for breeders to select sheep and goats for enhanced resistance to a variety of diseases.