Importance of continuous monitoring of bovine leukaemia virus infection on farms in bovine leukaemia virus-endemic areas of Japan.

BACKGROUND: Enzootic bovine leukosis (EBL), caused by the bovine leukaemia virus (BLV), has exhibited increasing infection rates in Japan. While many farms implement various infection prevention measures, practical challenges-such as barn structure and availability of replacement cattle-can hinder farm purification efforts. This study aimed to determine whether continuous monitoring of BLV proviral load (PVL) in cattle on a farm could be effective in tracking BLV infection dynamics, identification of BLV infection sources and the dynamics of BLV PVL enrichment on-farm. METHODS: Bovine leukaemia virus infection was monitored over a 3-year period in approximately 10% of cattle at a test farm located in Miyazaki Prefecture, an endemic area for BLV in southern Kyushu, Japan. Eight calves and five adult cows were included to assess vertical and horizontal transmission of BLV. RESULTS: All calves developed new BLV infections, which were traced to the nursing/growing barn, the dry/new milking barn at the Miyazaki farm, and the point of deposit in Hokkaido, Japan, for rearing and artificial insemination. In BLV-positive adult cows, an increasing trend in PVL was observed. These findings indicate that continuous monitoring of a subset of cattle enables effective tracking of infection dynamics and identification of infection sources; thus, supporting the implementation of targeted prevention measures. CONCLUSIONS AND CLINICAL IMPORTANCE: Despite heightened awareness among farmers regarding livestock quarantine, consistent and long-term control of BLV infections remains challenging. The results underscore the necessity of expert-guided management strategies tailored to BLV control, informed by ongoing infection monitoring. This study highlights husbandry management factors, including barn structure and cattle boarding practices, and provides new insights into the transmission routes of BLV in Japan.