Reducing bovine leukemia virus prevalence on a large midwestern dairy farm by using lymphocyte counts, ELISA antibody testing, and proviral load

Cattle infected with BLV have disrupted immune systems, associated with reduced milk production, shortened lifespan, and predisposition to lymphoma. The objective of this exploratory case study was to develop a disease control testing and management protocol to reduce the prevalence of bovine leukemia virus (BLV) within a large commercial dairy herd. Three diagnostic tests were available: lymphocyte count (LC), ELISA for BLV antibodies, and proviral load (PVL) as determined by quantitative polymerase chain reaction (qPCR). Both testing and management protocols evolved over time as the BLV prevalence decreased and we learned how to reduce labor and unnecessary or redundant diagnostic testing. Test results were used to inform culling and pen assignments for cows most likely to transmit BLV or develop disease. Significant decreases in the percentage of cows with LC≥10.0x103/μL (4.22% to 1.04%) and PVL\u003e0.5 were observed for all lactations during the 4 quarters of intervention. By October 30, 2020, 6 of ~3,000 cows remained with a detectable PVL. Additionally, it was found that LC and PVL were associated with clinical lameness, but not with clinical mastitis.