LITTLE ROCK (AP) - Tyson Foods Inc. has begun killing and burying the carcasses of 15,000 hens from a flock that tested positive for exposure to a strain of the bird flu in northwest Arkansas, state officials said Tuesday. Tyson said preliminary tests on the flock indicated the presence of antibodies for H7N3, a less virulent strain of the virus.

Routine blood tests conducted Friday found the possible exposure, said Jon Fitch, director of the state's Livestock and Poultry Commission. Further tests done by the state and the U.S. Department of Agriculture found the birds did not have active infections.

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.

However, the Centers for Disease Control and Prevention in Atlanta said a 2004 outbreak of H7N3 at a poultry plant in British Columbia, Canada, did sicken two workers there. The CDC said the two workers recovered after treatment with the antiviral medication.

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.

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