Genetic resistance to haemonchosis in sheep

Parasitic nematodes cause huge economic losses to the domestic livestock industry. Control requires skilled farm management and heavy use of anthelmintics, but breeding for resistant animals has long been held a possibility. For about 20 years after World War ll, the genetics of association between sheep breeds and Haemonchus contortus - the main parasite of importance in sheep - was studied in the USA and Europe. Since the early 1970s, however, such studies have received greater emphasis in Australia, where anthelmintic resistance has become widespread. In this article, Douglas Gray discusses the background to breeding programmes for resistance to hoemonchosis - paying homage to the 'Golden Ram', a Merino believed to have carried a single major gene for immunological resistance to Haemonchus and Trichostrongylus.