Importance of ewe/lamb relationship and breed in the epidemiology of maedi-visna virus infections

Sheep from a closed experimental breeding flock containing Finnish Landrace, Ile de France, their F1 crossbreeds, and a new breed were tested for antibodies to maedi-visna virus in 1975 and 1985-86. Over the years, the percentage of seropositive sheep increased: the Ile de France, however, remained virtually negative. Since exposure to the virus was similar, the results indicated a breed-associated difference in susceptibility to maedi-visna virus infection. Analysis of the serological results of 1985-86 yielded a total of 173 dam/progeny pairs. A significant (P less than 0.05) association between the test results of the dams and their progeny was found: seropositive (infected) dams produced 36.6 per cent positive progeny, whereas the seronegative dams had 20.0 per cent positive progeny (odds ratio = 2.3). Further analysis showed that this association was most apparent between dams and their youngest progeny, which is explained by the fact that the chance of infection by horizontal transmission increases with age. These results provide evidence that the ewe/lamb relationship plays a role in the epidemiology of maedi-visna virus infections.