Control of bovine leukaemia virus transmission by selective culling of infected cattle on the basis of viral antigen expression in lymphocyte cultures

Abstract

The use of viral antigen expression in lymphocyte cultures to prioritize the culling of bovine leukaemia virus (BLV) infected cattle was evaluated as a means of controlling the spread of infection in heavily infected herds. Selective culling was implemented in five commercial dairy herds containing between 126 and 304 cattle with infection prevalences, based on serological testing using the agar gel immunodiffusion test, of 19.4%, 20.3%, 20.6%, 20.6% and 39%. All seropositive cattle were tested for BLV antigen expression in lymphocyte cultures, and 51% found to express detectable quantities of viral antigens. In the four herds with 19% to 21% infection prevalence, all antigen-positive animals were culled immediately. Antigen-negative animals were retained in the herds for at least 16 months. Only two new infections were recorded in these four herds after antigen-positive animals had been culled, despite the continued presence of the antigen-negative animals. In the herd with 39% infection prevalence, a rapid reduction in the incidence of infection was achieved, even though only those animals with the highest levels of antigen expression were culled initially. Experimental transmission from seropositive cattle indicated that sheep could be infected from an antigen-positive cow with fewer than 10 3 lymphocytes, whereas more than 10 6 lymphocytes were required to transmit infection from an antigen-negative caw. Estimation of the amount of integrated BLV DNA in serial dilutions of blood from antigen-positive and antigen-negative cattle provided an explanation for the higher infectivity of antigen-positive cattle. By using the polymerase chain reaction to amplify proviral DNA, it was shown that blood from antigen-positive cattle could contain up to 10 6-fold more integrated provirus than blood from antigen-negative cattle. This work demonstrates that selective culling of BLV-seropositive cattle, on the basis of viral antigen expression in lymphocyte cultures, provides a useful management option in herds where economic considerations preclude the immediate culling of all infected cattle.