PROSPECTS OF BREEDING SMALL RUMINANTS FOR RESISTANCE TO INTERNAL PARASITES

Resistance to nematode parasites can be improved by selection, but eff orts to include appropriate traits in commercial Livestock breeding pr ograms are only a recent development. Procedures for including resista nce in breeding programs are similar to those involving other traits. The steps are described with special reference to sheep, and areas are highlighted where particular considerations exist. Three approaches a re described and contrasted: breeding for resistance (reduced parasite numbers, as determined by faecal worm egg count); resilience (product ion during parasitism); or number of treatments required during parasi tism. It is necessary, but difficult, to assess the economic benefits of improving resistance relative to other traits. Disease costs vary w idely depending on the prevalence of the disease and on the availabili ty, effectiveness and sustainability of alternative control measures. Costs of treatment and control are relatively simple to estimate for a given situation, but production losses are more difficult. Methods of dealing with this problem are discussed. Breeding for disease resista nce usually requires that either selection candidates, or their relati ves, are exposed to the pathogen so that resistance levels can be comp ared. Parasitic diseases generally create no special ethical problems in a breeding program unless natural challenge levels are insufficient to enable discrimination between hosts in their susceptibility. In th e longer term, it is desirable that selection criteria for all major d iseases be developed that will be informative in healthy animals. Mole cular genetic markers offer promise, but simple genetic markers have s o far been as elusive as physiological traits to predict resistance in undiseased animals. In the longer term, useful genetic markers will b e found and techniques for combining these with phenotypic information need to be developed. Commercial breeding programs for sheep which in clude resistance to gastrointestinal roundworms are now operating in A ustralia and New Zealand, and issues related to breeding in the tropic s are discussed. Copyright (C) 1996 Australian Society for Parasitolog y. Published by Elsevier Science Ltd.