Ten-year absence of new bovine leukemia virus infections under integrated bovine leukemia virus control measures, including 4.8‑m spatial separation, in a Japanese Black cattle breeding herd

Abstract Bovine leukemia virus (BLV) is primarily transmitted through infected lymphocytes. When vertical and iatrogenic routes are effectively controlled, horizontal transmission is the major driver of within-farm spread. Although physical separation has been proposed to reduce insect-mediated horizontal transmission, long-term field evidence of this approach in actual farming conditions remains limited. In this 10-year observational study, we evaluated whether maintaining a fixed 4.8-m physical separation between BLV-positive and BLV-negative cattle could prevent horizontal BLV transmission in a commercial Japanese Black cattle breeding herd. On the study farm, BLV-positive and BLV-negative cattle were housed in separate sections of the same barn. A 4.8-m corridor and a 4.8-m vacant pen were consistently maintained as buffers between the cattle groups. Vertical and iatrogenic transmission was minimized through colostrum management, calf segregation, and strict hygiene protocols. BLV testing was performed 21 times over the 10‑year observation period. Cows in the test barn were tested twice annually using real-time polymerase chain reaction (PCR) and proviral load (PVL) was quantified. Across the 21 rounds of PCR testing, no BLV-negative cows became PCR-positive, even though high-PVL cattle were present throughout the study, blood-sucking insects were abundant, and no insect control measures were implemented in the test barn. These findings suggest that a 4.8-m physical separation, combined with strict compartmental segregation and accurate diagnostic testing, may help to prevent horizontal BLV transmission under specific farm conditions.