The relationship between ovine lymphocyte antigens and parasitological and production parameters in Romney sheep

The occurrence of ovine lymphocyte antigen (OLA) types in two flocks of New Zealand Romney sheep was examined in relation to resistance to nematode parasites [as judged by faecal egg counts (FEC)], plasma pepsinogen levels, liveweight and weight gains. A panel of OLA-typing antisera (SY 1-16) which determine class 1 MHC antigens of the sheep was used. The OLA combination SY 1a + 1b was found exclusively in Romney sheep of both flocks having below average FEC, but was present in low frequency (5%). In one flock, sheep possessing this antigen combination had consistently lower FEC from weaning to 1 year of age than sheep without this combination. SY 6 occurred significantly more frequently in above average FEC sheep and was associated with significantly higher FEC during secondary challenge infection. Plasma pepsinogen levels were significantly lower in those sheep possessing SY 2 or SY 3 but these OLA types were not associated with lower than average FEC. No OLA type was associated with above average weight gain but in one flock sheep with SY 1b and SY 1a + 1b had significantly lower weight gains between weaning and 6 months of age than sheep without these OLA types. A similar association was not found in the other flock. Sheep in one flock with SY 16 were significantly heavier than those without this antigen. No other associations between OLA types and liveweight were found.