Tyson Foods Inc. has begun killing and burying the carcasses of 15,000 hens in northwest Arkansas that tested positive for exposure to a strain of the avian flu that is not harmful to humans, state officials said Tuesday.

Jon Fitch, director of the state's Livestock and Poultry Commission, said routine blood tests conducted Friday found the possible exposure. Further tests done by the state and the U.S. Department of Agriculture found the birds did not have active infections, but rather were exposed to a subtype of the disease.

Fitch said the company immediately began disposing of the birds.

"There is absolutely no human health threat," Fitch said. "But we take this very seriously."

Fitch said state officials decided against announcing the infection to the general public because the birds tested positive for exposure to the H7N3 strain of the virus. The strain that ravaged Asian poultry stocks in late 2003 was H5N1 bird flu virus. That version of the virus has killed 240 people worldwide and scientists worry it could mutate into a form that spreads easily among people.

Gary Mickelson, a spokesman for Springdale-based Tyson, said the hens showed no signs of sickness before their pre-slaughter blood tests. He said the exposed birds all came from a contractor.

"As a preventive measure, Tyson is also stepping up its surveillance of avian influenza in the area," Mickelson said in a statement. "The company plans to test all breeder farms that serve the local Tyson poultry complex, as well as any farms within a 10-mile radius of the affected farm."

Matt DeCample, a spokesman for Gov. Mike Beebe, said the governor was alerted about the tests Monday.

Stock in Tyson, the world's largest meat producer, fell by 8 percent in trading Tuesday, down $1.47 to $16.98 per share.