Bovine leukemia's hidden toll.

Bovine leukemia's hidden toll

Bovine leukemia virus kills cattle, saddling the U.S. dairy industry with annual losses of about $44 million from the fullblown disease. However, this toatl may greatly understate the damage, according to the first study of the infection in its advanced inapparent stages, says immunogeneticist Harris A. Lewin of the University of Illinois in Urbana-Champaign.

The new findings indicate the virus is most prevalent among cows bred to produce large amounts of milk, and that it thins the milk of infected cows that have developed abnormally proliferating white blood cells. Thus, Lewin says, it squeezes profits on two fronts in addition to its lethality. Dairy farmers, he notes, are paid on the basis of milk fat and volume.

Lewin and co-workers Ming-Che Wu and Roger D. Shanks sampled blood from 219 outwardly healthy cows over a two-year period, looking for antibodies to the virus as well as for an abnormally high level of B lymphocytes, which indicates a stage of the ailment known as persistent lymphocytosis. Lymphocytosis occurs in about 30 percent of virus-infected animals and increases a cow's chances of developing terminal disease, but it does not make them visibly sick, Lewin says.

The scientists matched the blood data with estimates of each cow's genetic potential for milk and fat production, actual milk and fat production and milk-fat percentage. They found that estimates of genetic potential for milk production were significantly greater for both groups of infected cows than for noninfected cows and that the genetic potential for fat production was greater in coes with lymphocytosis.

"These results show that cows with superior genetic potential for milk and fat yield were more susceptible to subclinical progression of [bovine leukemia virus] infection," the researchers report in the February PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCE (Vol. 86, No. 3).

Lewin believes stress accounts for the greater infection rate among high milk producers, some of which churn out as much as 65 pounds of milk in a day. "Producing lot of milk is stressful for them, and stress influences the progression of the disease," he suggests. Cows with persistent lymphocytosis produced milk with about 9.2 percent less fat than that of uninfected cows, probably because they "are using fat to make more lymphocytes," Lewin says.

To confirm and refine his results, Lewin is conducting a five-year study of how milk production and fat content change in individual cattle as the disease progress.