Role of colostrum and milk in the natural transmission of the bovine leukemia virus.
The role of colostrum and milk in the transmission of the bovine leukemia virus (BLV) was examined by monitoring the development of BLV infection in calves that were fed since birth on colostrum and milk from their BLV-positive dams and then reared in complete or partial isolation from infected cattle. Twenty-one of the 25 calves raised in complete isolation remained negative for BLV until the last evaluation. At this time, 14 calves were older than 28 months of age, and seven were 16 to 20 months old. Three calves in this group became BLV positive before the age of five months, and one became so at the age of 12 months. Of the 16 calves raised in partial isolation, two were positive at the ages of 11 and 18 months, respectively. The other 14 calves remained negative during the 26 to 29 months of observation. It is not known if the six animals that became BLV positive in these two groups were infected by milk, or prenatally, or during contact with their infected dams. While only six of the 41 calves raised in isolation became infected, all 18 calves raised in contact with BLV-positive cows became infected before the age of 26 to 29 months of age, and 12 were positive at 16 to 20 months of age. Thus, it is apparent that, under natural conditions, milk-borne transmission of BLV, if it occurs at all, is much less frequent than contact transmission, despite the fact that, as shown in previous studies, infectious BLV is present in the colostrum and milk of most BLV-positive cows. Passively acquired maternal antibodies to BLV were probably responsible for the resistance of the calves to milk-borne infection.