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Abstract Background Subclinical infection with bovine leukemia virus (BLV) in cows can cause economic losses in milk and meat production in many countries, as BLV-related negative effects. The volatile fatty acids (VFAs) and microbiota present in the digestive tracts of cows can contribute to cow health. Here, we exploratorily investigated the VFAs and microbiota in the rumen and gut with respect to subclinical BLV infection using cows housed at a single farm. Results We analyzed a herd of 38 cows kept at one farm, which included 15 uninfected and 23 BLV-infected cows. First, the analysis of the VFAs in the rumen, gut, and blood revealed an absence of statistically significant differences between the uninfected and BLV-infected groups. Thus, BLV infection did not cause major changes in VFA levels in all tested specimens. Next, we analyzed the rumen and gut microbiota. The analysis of the microbial diversity revealed a modest difference between the uninfected and BLV-infected groups in the gut; by contrast, no differences were observed in the rumen. In addition, the investigation of the bacteria that were predominant in the uninfected and BLV-infected groups via a differential abundance analysis showed that no significant bacteria were present in either of the microbiota. Thus, BLV infection possibly affected the gut microbiota to a small extent. Moreover, bacterial associations were compared between the uninfected and BLV-infected groups. The results of this analysis suggested that BLV infection affected the equilibrium of the bacterial associations in both microbiota, which might be related to the BLV-related negative effects. Thus, BLV infection may negatively affect the equilibrium of bacterial associations in both microbiota. Conclusions Subclinical BLV infection is likely to affect the rumen and gut microbiota, which may partly explain the BLV-related negative effects.

Abstract Bovine leukemia virus (BLV) infects cattle and causes serious problems for the cattle industry, worldwide. Vertical transmission of BLV occurs via in utero infection and ingestion of infected milk and colostrum. The aim of this study was to clarify whether milk is a risk factor in BLV transmission by quantifying proviral loads in milk and visualizing the infectivity of milk. We collected blood and milk from 48 dams (46 BLV seropositive dams and 2 seronegative dams) from seven farms in Japan and detected the BLV provirus in 43 blood samples (89.6%) but only 22 milk samples (45.8%) using BLV-CoCoMo-qPCR-2. Although the proviral loads in the milk tended to be lower, a positive correlation was firstly found between the proviral loads with blood and milk. Furthermore, the infectivity of milk cells with BLV was visualized ex vivo using a luminescence syncytium induction assay (LuSIA) based on CC81-GREMG cells, which form syncytia expressing enhanced green fluorescent protein (EGFP) in response to BLV Tax and Env expressions when co-cultured with BLV-infected cells. Interestingly, in addition to one BLV-infected dam with lymphoma, syncytia with EGFP fluorescence were observed in milk cells from six BLV-infected, but healthy, dams by an improved LuSIA, which was optimized for milk cells. This is the first report demonstrating the infectious capacity of cells in milk from BLV-infected dams by visualization of BLV infection ex vivo. Thus, our results suggest that milk is a potential risk factor for BLV vertical spread through cell to cell transmission.