Eradication of bovine leukosis from a heavily infected herd by the use of own offspring.

In a Holstein-Friesian dairy herd (n = 1,248) kept in loose housing system and showing 62% prevalence of infection with bovine leukosis virus (BLV), the newborn calves were separated from their dams and placed into individual pens. They were given freeze-stored colostrum derived from a BLV-free farm and then a calf starter diet. Strict hygienic measures of leukosis control were observed. Serum samples were taken from the calves and tested by agar gel immunodiffusion (AGID) before and after colostrum uptake, at 2.5 and 6 months of age, and then at the heifer-rearing farm at 2-month intervals. Animals giving a positive reaction were culled. No AGID positivity occurred before the uptake of colostrum. Subsequently, six times as many calves became positive among the offspring of infected cows than among calves born to negative animals. As no separate barn was available, the infected farm was divided into a "black" and a "white" part with a foil to prevent aerogenic infection. The negative heifers being in the 7th month of pregnancy were placed into the "white" part. A separate calving house was established for them, as the joint calving house proved to be a source of infection. By this method a leukosis-free progeny herd was obtained from 90% of the heifer calves of the cow population showing a high prevalence of infection. In this way the original infected herd was gradually replaced.