The transmission of bovine leukemia virus to calves occurs mostly through colostrum and milk.

Abstract

Background and aim: Enzootic bovine leukemia is highly prevalent in most dairy farms, and strategies to reduce both vertical and horizontal transmission are being investigated. In this study, we aimed to investigate the rate of in utero infection, transmission of bovine leukemia virus (BLV) to calves through colostrum and milk, and the effectiveness of colostrum and milk pasteurization in reducing BLV transmission to calves.

Materials and methods: This study included four groups of calves from seropositive and seronegative cows. Group 1: Calves from BLV-positive cows (n = 11) were fed pasteurized colostrum and milk; Group 2: Calves from BLV-negative cows (n = 9) were fed pasteurized colostrum and milk; Group 3: calves from BLV-positive cows (n = 16) were fed unpasteurized colostrum and milk; and Group 4: calves from BLV-negative cows (n = 9) were fed unpasteurized colostrum and milk. In utero infection was evaluated using blood samples collected from calves before colostrum ingestion (day 0), and BLV transmission through colostrum and/or milk was evaluated by collecting blood samples after colostrum ingestion (days 1, 7, and 30). Samples seropositive on days 0 and/or 30 were also analyzed for the presence of viral DNA by nested polymerase chain reaction (nPCR).

Results: All calves born to BLV seronegative cows (Groups 2 and 4) tested negative on days 0 and 30, indicating a lack of virus transmission via tank milk. Among the calves from Group 1, we found one in utero infection, and among the nine calves serologically positive on day 30, we found four positives by nPCR. Within Group 3, we found one in utero infection, and among the 10 calves serologically positive on day 30, we found 7 also positive by nPCR.

Conclusion: The transmission of BLV through colostrum is central to the persistence of this virus in dairy cattle. Molecular detection of BLV in seropositive calves during the 1^st month of life, followed by culling, may be a valuable eradication strategy.