The effects of maedi-visna virus infection on productivity in ewes

Five sets of data from four separate sheep flocks (1445 ewes) were used to assess the effects of subclinical maedi-visna virus infection (as determined by serological procedures) on productivity in ewes. In general, infection with maedi-visna virus lowered productivity. Infection reduced conception rates, with the chances of an infected ewe conceiving being only approximately 0.67 times those of a non-infected ewe. For ewes which did conceive, there was no effect of infection on the number of lambs born. The birth weights of lambs born to middle-aged (3-4 years) infected ewes were 3-6% lower than the weights of lambs born to non-infected ewes of the same age. There was very little effect of infection in younger (1-2 years) and older (5+ years) ewes: some factors which may have biased these results are discussed. Lamb weights at 25-50 days of age were reduced by maedi-visna infection in the ewe but the results were not statistically significant..