Postnatal and Prenatal Transmission of the Bovine Leukemia Virus Under Natural Conditions2

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Abstract
Cattle in a well-characterized multiple-case leukemia herd (herd BF) were examined for bovine leukemia virus (BLY) infection and BLY antibodies from birth to adulthood. According to standard husbandry practices for dairy herds, the BF calves were separated from the adult herd after they were weaned up to the ages of 14–16 months. The results showed that after the age of 30 months all cattle in the herd were infected with BLY and had antiviral antibodies. However, only 18% of the BF calves were positive at birth and before their first colostral meal. Thus prenatal transmission of BLY was relatively infrequent. Several observations indicated that prenatal transmission occurred only through the placenta. After ingestion of colostrum and for the first 3–5 months of life, all the BF calves had passively acquired maternal antibodies to BLY. During this time and until the ages of 7–10 months, the incidence of BLY infection among the BF calves did not change significantly. Subsequently, the incidence increased rapidly, and when the calves reached 2 years of age, it was 93%. By contrast, BLY infection was detected in only 3 of 17 BF calves of similar ages that were virus-free at birth and kept in isolation after weaning from their BLY-infected dams. These data indicate that the large majority of the BF cattle were infected with BLY by contact following exposure to the adult herd and that, during the first weeks of life, they were protected against milk-borne or contact infection by passively acquired maternal antibodies. The results also indicate that BLY infection in cattle persisted despite the presence of actively produced or passively acquired BLY-neutralizing antibodies.