The commercial turkey industry in Minnesota, the nation's largest, has been struck again by bird flu, and animal health experts are stumped as to exactly how the virus invades a farm.

The U.S. Department of Agriculture confirmed the detection of the disease in a second Stearns County flock of 71,000 turkeys, the fifth in a commercial operation. A Nobles County turkey farm became the fourth such case in the state in recent weeks.

The bird flu has led dozens of countries to stop buying turkey meat from Minnesota and other states. About 12 percent of Minnesota's turkey production — mostly dark meat — is exported.

The Nobles County facility had 21,000 birds — 7,000 in each of three barns — at the time of the infection, the Minnesota Board of Animal Health said. The farmer noted an increase in deaths in one of three barns and reported it to the board. Birds in the other two barns were expected to be killed Thursday as a precaution.

The H5N2 strain of highly pathogenic bird flu first surfaced a month ago in Minnesota at a turkey breeding farm in Pope County. Since then, the same virus appeared in commercial turkey farms in Lac qui Parle and Stearns counties and now at a farm near the Iowa border.

The bird flu also has been reported recently in eastern South Dakota, Arkansas and Missouri, and earlier this year in several western states. Turkey farms have been noticeably hit by the virus, though chicken flocks are also susceptible.

The lethal flu is believed to be spread by migratory wild birds that carry the virus but don't appear to get sick themselves. The question vexing animal scientists: How is the flu virus making its way into enclosed turkey barns?

"So far our investigations haven't found an answer to that," said Bill Hartmann, state veterinarian for the Minnesota Board of Animal Health. "But what we do know is there is no connection between these [four] farms."

State officials are prevented by law from disclosing the exact locations of the farms affected, board spokeswoman Bethany Hahn said. The number of birds destroyed now totals 170,000. However, that's still a small number considering Minnesota churns out about 46 million turkeys annually.

The risk of bird flu to the public is "very low" and there is no food safety concern connected to the virus, the board said. "Only people with close contact with infected birds would be at risk," said Joni Scheftel, state public health veterinarian at the Minnesota Department of Health.

Between the four infected turkey farms, there have been 19 workers who worked closely with birds. None of them has shown any sign of the virus, Scheftel said.

The Minnesota Department of Natural Resources has been canvassing the areas around the affected farms, looking for any sign of highly pathogenic H5N2 in wild birds.

None have tested positive, and the DNR hasn't found any dead wild turkeys or dead raptors that could also indicate the presence of the flu, said Michelle Carstensen, the department's wildlife health supervisor. Raptors that eat birds with the lethal virus could themselves die.

Minnesota's turkey industry is on high alert. "We are concerned about this continuing to happen and getting into more farms," said Steve Olson, executive director of the Minnesota Turkey Growers Association.

The state's several hundred turkey farmers have upped biosecurity measures. Special attention is being paid to vehicle tires and workers' shoes — possible conveyances for the virus, which would originate through wild bird feces. Vehicles are often hit with disinfectant cleaners soon after entering a turkey farm.

Minnesota's turkey industry exports to about 50 countries, Olson said. About 40 have slapped turkey import bans on the nation as a whole or on Minnesota and other states. However, some of those bans are limited, Olson said. Some countries will ban turkey products only from counties with confirmed cases, he said. Mexico has banned imports of whole birds, but not of turkey parts.

Farmers face a big blow if their flocks are hit by the flu. The federal government will pay the cost of euthanizing birds that are not yet sick, but on the same farm as diseased birds. But turkey growers absorb the financial loss of birds felled by the virus.

"For those farms affected," Olson said, "it has been pretty huge."