Bovine leukemia virus: A herd-based control strategy

A study was conducted in order to develop a herd-based control strategy for bovine leukemia virus (BLV). Michigan State University's closed lactating dairy herd of 114 cows was utilized for the study. Ninety-five percent (95%) of the cows were positive for BLV antibodies, as determined by the agar gel immunodiffusion (AGID) test in November 1979. To develop the control strategy program, the following management practices were instituted: there was a complete physical separation of BLV positive from BLV negative animals for 3 years after which the two groups were physically mixed. Different and sterile supplies and equipment were utilized for any veterinary-medicine-related activity. Personnel working on the farm were constantly made aware of the transmission of the disease and how to minimize it. A constant vector control program and other miscellaneous management practices were implemented. The BLV seronegative animals were examined monthly for BLV antibodies starting at 6–7 months of age. The reactor animals were separated from the seronegative animals when the reactors had 2 positive AGID tests consecutively. A positive BLV serotest was not a criterion for culling. Following a 3-year complete separation of positive animals from negative animals, the overall point prevalence decreased from 95 to 34%. The results were further categorized according to 4 age groups of 6–15, 16–23, 24–47 and 48 months and older. The percentage of BLV-positive animals decreased from 19 to 17, 58 to 14, 90 to 33 and 100 to 90% for the respective age groups. Following a 10-month discontinuation of physical separation of positive animals from negative animals, a decrease in the overall point prevalence rate was still observed. The limitations and practical applications of the program are discussed.