Evaluation of bovine leukaemia virus infection control measures in Japanese Black beef cattle breeding farms: A 7-year longitudinal study based on proviral load.

Background: Bovine leukaemia virus (BLV) infection remains a major problem for Japanese Black (Wagyu) cattle, and effective and sustainable control measures are essential to prevent its spread in breeding herds. This study aimed to monitor the BLV infection status of all cattle at a Japanese Black breeding farm in Kagoshima Prefecture, Japan, over 7 years. Methods: BLV serological testing was conducted at baseline in 2018 (Y0, study year 0), followed by real-time polymerase chain reaction (PCR) testing in six survey years between 2019 and 2024 (Y1‒Y6; no testing in 2023). The infection status and proviral load (PVL) were assessed. Cattle were classified as calves (<10 months) or breeding cattle (≥10 months), reflecting standard Japanese production practices. After high-PVL cattle were identified in Y1, new barns were constructed stepwise from Y2 onwards to physically separate BLV-positive and BLV-negative animals. In addition, strategic culling of BLV-positive cattle, BLV screening of introduced cattle, and early separation of calves from their dams were implemented. Results: At Y0, BLV seroprevalence among breeding cattle was 78.6%. At Y1, PCR detected BLV in 28.0% of calves and 74.1% of breeding cattle. By Y6, BLV prevalence had significantly decreased to 39.2% in breeding cattle compared with Y1, while it was 21.6% in calves, with no significant reduction observed. Conclusions: Early identification of high-PVL cattle, physical separation of BLV-positive and BLV-negative cattle, and culling of BLV-positive cattle was effective in reducing BLV prevalence in a Wagyu breeding herd. These findings propose a feasible, farm-based approach for managing endemic BLV under commercial conditions.