Selective breeding for the control of nematodiasis in sheep.

Genetic manipulation of sheep by selective breeding offers a means to reduce the current reliance on chemotherapy for the control of gastro-intestinal nematodes. Simulated epidemiological studies support this view as, compared to lambs of 'normal' susceptibility, those 'selected' for resistance to Trichostrongylus colubriformis have lower worm burdens and reduced production losses. Considerable genetic variation exists both between and within breeds of sheep, and a number of breeding programmes have demonstrated that selection for animals with heightened levels of resistance to nematodes is feasible. Animals from these selection experiments are currently being used to investigate the nature of this genetic regulation and the economic benefits that can be achieved. An understanding of the mechanisms of resistance, facilitated by having animals with defined extremes of responsiveness, is crucial for studies into the specificity of selection, identification of predictive markers with resistance, and determination of suitable vaccines and vaccination strategies in unselected populations. Immunity plays a major role in host resistance to parasites, and from studies with selected animals, it appears that a broad range of immune responses are under genetic control. Genetic diversity within the parasite population may manifest itself in adaptation to withstand host resistance mechanisms. Such an occurrence could limit the effectiveness of the genetic approach.