The Influence of Genetic Factors on the Resistance of Ruminants to Gastrointestinal Nematode and Trypanosome Infections

During the past 80 years there has been a steady flow of reports comparing various breeds of sheep and cattle, as well as individuals within breeds, in terms of their resistance to gastrointestinal nematodes and trypanosomes. The vast majority indicate that genetic factors play a part in influencing resistance to the parasites concerned. In view of the economic impact of the diseases caused by these parasites, the possibility of improving livestock production by developing genetically resistant animals has aroused considerable interest. This chapter reviews genetic resistance to parasitic infections in cattle and sheep, and includes a discussion of underlying mechanisms, from the standpoint of resistance to the parasites themselves and of resistance to the diseases they produce. An attempt is also made to assess the potential value of resistant livestock against the background of existing methods of parasite control.