Minnesota has become the epicenter of a national bird flu outbreak, with the highly lethal virus hitting two more of the state's commercial turkey farms and perplexing animal health regulators with its rapid spread.

The bird flu has struck seven Minnesota farms, with regulators Monday announcing tainted flocks in the heart of turkey country, one each in Kandiyohi and Stearns counties. Three farms in Stearns County have been hit in the past 10 days. So far, 340,000 birds statewide have died or been killed out of precaution.

"This is getting to be disturbing," said Carol Cardona, a veterinary biosciences professor at the University of Minnesota.

The bird flu has been reported recently in eastern South Dakota, Arkansas and Missouri, and earlier this year in several western states. Though chicken flocks are susceptible to the H5N2 bird flu strain, it's particularly hard on turkeys, and Minnesota is the nation's largest turkey producer.

"Turkeys are exquisitely sensitive to it," Cardona said.

With the recent outbreaks, the state has called in an "incident management team" from the U.S. Department of Agriculture. About 30 USDA researchers started working in Minnesota this week on the turkey bug, joining about 20 state animal health workers, said Bill Hartmann, state veterinarian for the Minnesota Board of Animal Health.

The bird flu thrives in colder, damper weather, Hartmann said, so spring should help as temperatures rise.

While the H5N2 bird flu poses a low health risk to humans and isn't a food safety issue, it's a major threat to Minnesota's turkey industry, which churns out 46 million birds a year. The industry is boosting "biosecurity" measures: limiting farm access for people and machinery, and intensely cleaning both if they go into a barn.

Waterfowl transmit disease

The bird flu is believed to be spread by wild birds that carry the virus but don't get sick from it. "We know that wild waterfowl are a reservoir of the disease," Hartmann said.

Minnesota, of course, has lots of lakes and waterways and therefore lots of waterfowl. They transmit the virus through feces, which are tracked into enclosed turkey barns. Researchers aren't exactly sure how.

"It's kind of unpredictable, but I do know if we get bio­security in these turkey barns, that reduces the problem," Hartmann said.

Hartmann and other researchers believe the virus is not being spread from farm to farm, and cite as evidence the flu's erratic movements. It has decimated one barn and passed over another on the same property, and surfaced at farms that are miles away from each other.

The first outbreak came at a turkey breeding farm in Pope County in early March. Since March 26, the flu has touched down six more times, including in Lac qui Parle and Nobles counties. The state's animal health board said state law doesn't allow the release of farm names or exact locations.

100,000 more affected

On Monday, the state said the H5N2 flu struck a commercial flock of 76,000 turkeys in Stearns County and hit another 26,000 turkeys at a breeding farm in Kandiyohi County. Kandiyohi and ­Stearns are, respectively by rank, the state's No. 1 and 2 turkey-producing counties.

The Minnesota Department of Natural Resources is testing wild birds in 6-mile radii around the infected farms. Its review of the Pope County site — which included 148 feces samples from ducks and swans — found no trace of highly pathogenic H5N2. Nor did the DNR find any dead wild turkeys or raptors that might have eaten infected birds.

"The question we are really trying to ask is, can we identify H5N2 in these areas?" said Michelle Carstensen, the department's wildlife health supervisor. With all the recent bird flu strikes, the natural environment "should be pretty saturated with the virus," she said. So it should be more apt to show up in fresh feces samples, she said.

Curiously, back-yard turkey flocks in Minnesota haven't been hit hard by the disease so far. "They are at greater risk," a puzzled Carstensen said. Unlike commercial birds, they don't spend their whole lives in barns and are more exposed to wild bird droppings.