USDA: Bird flu transmission path remains unclear

Federal investigators still haven't found a specific pathway to explain the spread of bird flu on poultry farms.

The U.S. Department of Agriculture says a survey of 81 Midwest turkey farms affected by avian flu shows that farm equipment and people likely played a role in spreading the deadly virus on commercial farms. Starlings, sparrows and other wild birds could have contributed, as well.

In another study of egg-laying chicken flocks in Iowa and Nebraska, the government said farms located within 10-km control zones appeared to have a higher risk of being infected with avian flu. Farms that shared company trucks, trailers and other equipment were also associated with an increased risk of infection.

"We're not surprised that there's not a silver bullet that says, 'Here's how it is, and if you do this, it's going to keep the virus out,'" said Steve Olson, executive director of the Minnesota Turkey Growers Association. "There's several avenues where a disease can get into a building and we're looking at how do we shut down all of those."

The USDA's study shows that farms located more than 100 yards from a public gravel or dirt road were associated with a lower infection risk.

Having wash stations for vehicles on the farm also appeared to reduce the risk of flu transmission.