THE IMMUNOGENETICS OF RESISTANCE TO TRICHOSTRONGYLUS-COLUBRIFORMIS AND HAEMONCHUS-CONTORTUS PARASITES IN SHEEP

Three possible immunogenetic markers for resistance to intestinal para sites in sheep have been studied. Allotypes of the major histocompatib ility complex (MHC) of the sheep have been investigated as markers, us ing serological typing or gene probes, for associations between alloty pes and resistance to parasites in selected high responder and low res ponder lines of sheep. Only the serologically-determined class I ovine leucocyte antigen (OLA) types SY 1a and SY 1b have been found to be c onsistently associated with increased resistance to Trichostrongylus c olubriformis, but this association has not extended to the immunologic ally distinct Haemonchus contortus parasite. Gene probes of the sheep DRB, DQB and DQA MHC class II loci have detected animals with increase d susceptibility to T. colubriformis. Eosinophilia was investigated as a marker and found to be associated with increased resistance to para sites in lines of Australian Merinos and New Zealand Romneys selected for resistance on the basis of low faecal egg count. Blood eosinophili a was distinct from eosinophil infiltration of the gut which was poorl y associated with resistance. The mechanism of parasite resistance app eared to involve the release of vasoactive amines and leukotrienes int o intestinal mucus, since the selected high responder sheep to T. colu briformis and H. contortus had significantly increased amounts of thes e agents in their gut mucus, compared with selected low responder or r andom-bred sheep. Antibodies to T. colubriformis and H. contortus have also been used as markers to select high responder sire groups of lam bs in contact with the parasites, for the first time, at weaning. This assay had the advantage of detecting distinct antigens for the two pa rasites, which would allow resistance to the species of parasite to be selected in the lambs. Vaccines have been developed against H. contor tus using 'novel' gut antigens from the parasite, but variable respons iveness of the host sheep seemed to result in varying degrees of prote ction which were stimulated by these vaccines.