USDA confirms that cow-to-cow transmission is a factor in the spread of bird flu. By Reuters

By Tom Polansek

CHICAGO (Reuters) – The U.S. Department of Agriculture said this week that cow-to-cow transmission is a factor in the spread of bird flu in dairy herds, but it still does not know exactly how the virus is spread.

Farmers and veterinarians have been waiting for confirmation on how the virus is transmitted to better control its spread. Dairy cattle herds in eight states tested positive in the past month, along with one dairy worker in Texas.

“Those of us who have worked with influenza for a long time were pretty quick to say, ‘Yes, it’s moving from cow to cow,’” Jim Lowe, an associate professor at the University of Illinois College of Veterinary Medicine, said Friday. “You can’t explain the epidemiology any other way.”

Wild migratory birds are believed to be the original source of the virus. But the USDA said its investigation into cow infections “includes some cases in which the spread of the virus has been linked to livestock movements between herds.” There is also evidence that the virus has spread from dairy farms “via an unknown route back to nearby poultry farms,” the department said.

The USDA said cows shed the virus in high concentrations in milk, so anything that comes into contact with unpasteurized milk can spread the disease. Respiratory transmission is not considered the main way the virus spreads among livestock, the department added.

Despite the uncertainty surrounding transmission, USDA has not imposed quarantines to restrict the movement of livestock around infected dairy farms, as it has with chickens and turkeys around infected poultry farms. Infected livestock appear to recover, while bird flu is usually fatal to poultry.

The USDA said it expects that minimizing the movement of livestock and testing the animals that need to be transported, along with safety and cleaning practices on farms, should avoid the need for regulatory restrictions.

Officials reported last month that bird flu mainly affected older cows, although additional data now indicates that younger cattle are affected, the USDA said.