Avoidance of Natural Suckling from Dams with Bovine Leukemia Virus Is a Low Priority Countermeasure against Postnatal Transmission.

Although natural suckling from dams with bovine leukemia virus (BLV) has not been recommended in Japan, the frequency of BLV transmission through natural suckling under natural conditions is still unclear. The purpose of this study was to elucidate the risk of BLV transmission through natural suckling. Dams with BLV were classified into three groups (high, middle, low) based on the proviral loads (PVLs). PCR positivity of their colostrum and the correlations between the ratios of calves with BLV and types of feeding milk were analyzed. In dams with low PVLs, no colostrum or calves were confirmed to have BLV. In dams with middle and high PVLs, 17 out of 25 (68.0%) colostrum were PCR positive, and 10 out of 23 (43.4%) and 13 out of 29 (44.8%) calves with natural suckling and artificial rearing were infected with BLV, respectively. No difference was confirmed between the infection rates of natural-suckled and artificially reared calves. Thus, we concluded that the avoidance of natural suckling from dams with BLV and the introduction of artificial rearing were low priority countermeasures against BLV transmission.