Impact of amino acid 233 in Tax on bovine leukemia virus infection in Japanese Black cattle

Bovine leukemia virus (BLV) is an economically important pathogen that both causes fatal enzootic bovine leukosis (EBL) and reduces lifetime milk production, reproductive efficiency, carcass weight, and longevity in dairy cows. The virus can be divided into two categories based on the amino acid at position 233 in Tax protein, which activates viral transcription and probably plays crucial roles in leukemogenesis. We recently reported that early-onset EBL in Japanese Black (JB) cattle was frequently caused by L233-Tax–carrying virus. This study examined the impact of BLV infection, the proviral load (PVL), and amino acid 233 in Tax on the outcomes of JB cattle. We measured PVL in cattle enrolled between February 2016 and December 2018, determined the Tax type of the isolates, and performed follow-up until March 2022. The results demonstrated that BLV infection increased the risk of involuntary culling and mortality in JB cattle in a PVL-dependent manner. Infection with L233-Tax–carrying virus increased the likelihood of mortality by 1.6-fold compared with the effects of P233-Tax–carrying virus infection. Intrauterine and perinatal infections were frequently caused by L233-Tax–carrying virus, and these infections were likely to influence the early onset of EBL in JB cattle. Conversely, breeding cows infected with P233-Tax–carrying virus were often eliminated by involuntary culling. These findings indicate that amino acid 233 in Tax has importance in terms of preventing economic loss attributable to EBL in JB cattle.