Epidemiological survey of bovine leukemia virus in beef cattle in the Tohoku region of Japan.

The incidence of enzootic bovine leukosis, which is due to bovine leukemia virus (BLV), has been increasing in Japan. However, large-scale surveys have not been conducted in recent years, and the impact of cattle harboring high BLV proviral loads on within-farm BLV prevalence remains unclear. To clarify the situation, questionnaires and PCR and serological testing for BLV were conducted between 2022 and 2024 on 95 beef breeding farms (3,082 cattle) and 9 Japanese Black fattening farms (140 cattle) in the Tohoku region. We also examined the effects of various factors on within-farm proportions of BLV-positive cattle. The proportions of PCR-positive and seropositive cattle were 34.3% and 35.9% in beef breeding cattle and 15.7% and 15.0% in Japanese Black fattening cattle, respectively. Among the 74 farms with at least one PCR-positive animal, the proportion of PCR-positive cattle was significantly higher on farms with at least one animal with a high proviral load (≥400 copies/10 ng DNA) than on farms without any such cattle. Similarly, farms where high-proviral-load cattle accounted for≥10% of the herd had significantly higher proportions of PCR-positive cattle. In addition, farms that used public pastures, practiced on-farm grazing, or had experienced enzootic bovine leukosis within the past 5 years had significantly higher proportions of PCR-positive cattle. These findings suggest that BLV prevalence in beef cattle in the Tohoku region remains comparable to that reported approximately 10 years ago, indicating that BLV transmission in this region may be under control. Furthermore, cattle with high proviral loads may adversely affect within-farm BLV prevalence, indicating that targeted control measures focusing on such animals are essential for effective BLV control.