Increased T-cell responses that control bovine leukemia virus proviral load in beef cattle under dietary vitamin A restriction for marbling.

Bovine leukemia virus (BLV) proviral load is controlled by T-cell responses, which require vitamin A (VA) derived from food. However, whether dietary VA restriction for marbling impairs the T-cell responses that control BLV proviral load in beef cattle is unknown. We assessed T-cell subsets, interferon (IFN)-γ gene expression, and BLV proviral load in naturally BLV-infected Japanese Black cattle that were fed a diet with decreased VA levels. We found that the percentage of CD4+ T cells increased over time during dietary VA restriction. In addition, BLV proviral load was negatively correlated with the percentage of CD4+ T cells and with the level of IFN-γ gene expression. These observations suggest that dietary VA restriction for marbling enhances T-cell responses that control BLV proviral load and thus does not promote leukemogenesis in fattening beef cattle.

Effectiveness of on-farm continuous flow high-temperature short-time pasteurization for inactivation of bovine leukemia virus in milk.

The effectiveness of on-farm continuous flow high-temperature short-time (HTST) pasteurization (i.e., 72°C for 15 s) for the inactivation of bovine leukemia virus (BLV) in milk was investigated with a sheep bioassay. Four sheep that had been inoculated with completely pasteurized milk containing approximately 3.4 × 107 BLV-infected peripheral blood mononuclear cells (PBMC) and treated by either HTST pasteurization or laboratory-scale low-temperature long-time (LTLT) pasteurization (i.e., 60°C for 30 min), remained negative for BLV for at least 17 weeks after inoculation. In contrast, all sheep inoculated with unpasteurized or inadequately pasteurized milk containing the same number of BLV-infected PBMC were tested positive for BLV and anti-BLV antibodies within 3 weeks after inoculation. These results suggest that on-farm continuous flow HTST pasteurization was equivalent value with inactivated BLV on the LTLT procedure and can effectively inactivate BLV in the milk. Therefore, on-farm HTST pasteurization of the pooled colostrum or milk used in automated feeding systems is likely to protect group-housed preweaned calves from BLV infection, thereby improving animal health on dairy farms.

Development of a microchip electrophoresis-based, high-throughput PCR-RFLP method to type Tax 233 variants of bovine leukemia virus in Japan.

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL). We used microchip electrophoresis in combination with automatic image analysis to develop a novel high-throughput PCR-RFLP to type the gene sequences that encode BLV Tax 233. This method revealed that 233L-Tax is more prevalent than 233P-Tax in cattle in Japan. The proportion infected with BLV carrying the gene encoding 233L-Tax was significantly higher in Holstein cattle than in Japanese Black cattle. Holsteins infected with BLV encoding 233L-Tax had higher proviral loads than did Holsteins infected with BLV encoding 233P-Tax and Japanese Blacks infected with BLV encoding 233L-Tax or 233P-Tax. The novel method developed in this study will be a useful tool for identifying cattle harboring BLV with a higher risk of EBL and viral transmission.