The effect of a natural maedi-visna virus infection on the productivity ofSouth African sheep

A cohort study was conducted in order to measure the effect of the chronic indurative lymphocytic mastitis caused by the South African strain of maedi visna virus (MVV) on the pre-weaning growth of lambs born either of natura lly infected or uninfected ewes kept under similar conditions. Fifty natura lly infected ewes as well as another 40 from a maedi-visna-free source to b e used as control animals, were purchased and kept in separate flocks which were managed in a similar way. All the ewes were of the same breed and 3-4 years old. During the adaptation period, and through the mating, pregnancy and lactation periods they were periodically monitored for the presence of MVV serum antibodies. The lambs were weighed at birth and thereafter every 2 weeks until the age of 90 days, when they were weaned. The ewes were the n slaughtered, and their udders examined histologically and the number of l ymphocytic follicles were counted and assessed. Although the calculated val ues indicated a correlation between the number of follicles in the udder an d the reduction in the growth rate of the lambs, this was not statistically significant. Similarly, despite higher counts of lymphoid follicles in the udders of sere-positive ewes as compared to those that were sere-negative and the lower ewe productivity indexes in infected ewes, no statistically s ignificant differences were found in the indexes of ewes in different folli cle categories.