CONTROL OF BOVINE LEUKEMIA-VIRUS TRANSMISSION BY SELECTIVE CULLING OFINFECTED CATTLE ON THE BASIS OF VIRAL-ANTIGEN EXPRESSION IN LYMPHOCYTE-CULTURES

The use of viral antigen expression in lymphocyte cultures to prioriti ze the culling of bovine leukaemia virus (BLV) infected cattle was eva luated as a means of controlling the spread of infection in heavily in fected herds. Selective culling was implemented in five commercial dai ry herds containing between 126 and 304 cattle with infection prevalen ces, based on serological testing using the agar gel immunodiffusion t est, of 19.4%, 20.3%, 20.1%, 20.6% and 39%. All seropositive cattle we re tested for BLV antigen expression in lymphocyte cultures, and 51% f ound to express detectable quantities of viral antigens. In the four h erds with 19% to 21% infection prevalence, all antigen-positive animal s were culled immediately. Antigen-negative animals were retained in t he herds for at least 16 months. Only two new infections were recorded in these four herds after antigen-positive animals had been culled, d espite 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 hi ghest levels of antigen expression were culled initially. Experimental transmissions from seropositive cattle indicated that sheep could be infected from an antigen-positive cow with fewer than 10(3) lymphocyte s, whereas more than 10(6) lymphocytes were required to transmit infec tion from an antigen-negative cow. Estimation of the amount of integra ted BLV DNA in serial dilutions of blood from antigen-positive and ant igen-negative cattle provided an explanation for the higher infectivit y of antigen-positive cattle. By using the polymerase chain reaction t o amplify proviral DNA, it was shown that blood from antigen-positive cattle could contain up to 10(6)-fold more integrated provirus than bl ood from antigen-negative cattle. This work demonstrates that selectiv e culling of BLV-seropositive cattle, on the basis of viral antigen ex pression in lymphocyte cultures, provides a useful management option i n herds where economic considerations preclude the immediate culling o f all infected cattle.