THE GENETIC-BASIS OF RESISTANCE TO OSTERTAGIA-CIRCUMCINCTA IN LAMBS

The relationship between Ostertagia (Teladorsagia) circumcincta and sh eep is one of the best understood host-parasite relationships in any s pecies. The key components of resistance have been quantified, the ext ent of genetic control has been established for lambs, and methods now exist to breed lambs which will be both more resistant to worms and m ore productive than unselected lambs. A major gene for resistance has been identified within or around the major histocompatibility complex, and this gene appears to be the strongest yet identified for resistan ce to any parasite species. The most important mechanisms of resistanc e are local Igh responses which regulate worm fecundity and immediate hypersensitivity responses which regulate worm burdens. IgA responses develop before effective immediate hypersensitivity responses. Good si mulation models now exist to predict, for example, outbreaks of diseas e and the response of sheep to selection. The challenge now is to use our improved understanding of the population biology to develop even b etter simulation models and to produce expert systems based on these m odels which can be used by veterinarians and others to determine optim al procedures for individual farms to control disease and reduce sub-c linical economic losses.