No evidence of bovine leukemia virus proviral DNA in a widely used commercially frozen semen in Japan

Bovine leukemia virus (BLV) is a global concern that significantly affects the productivity of dairy and beef cattle, leading to considerable economic losses. BLV is primarily transmitted through infected lymphocytes via both horizontal and vertical routes. Vertical transmission occurs through the colostrum, uterus, and placenta. Although the possibility of BLV transmission through infected semen during artificial insemination is a recognized concern. No such cases have been reported in Japan, while studies from other countries reported a low risk. In this study, we aimed to evaluate the presence of BLV provirus in commercially frozen semen to assess the potential risk of infection in Japan. A sufficient quantity of genomic DNA with adequate purity was extracted from 191 widely used commercially frozen semen samples obtained from top-ranking bulls in the annual market survey in Japan between 2000 and 2022 using the phenol-chloroform extraction method. No BLV provirus was detected in any of the 191 frozen semen samples using either BLV-CoCoMo-qPCR-2 targeting the BLV long-terminal repeat region or nested polymerase chain reaction targeting BLV gp51 env gene. Similarly, the results are consistent with reports from other countries. Therefore, the findings of this study suggest a low risk of BLV transmission through frozen semen in Japan. Our study further supports that artificial insemination combined with the selection of non-infected cows for breeding are effective measures to prevent the spread of infection.