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. Matt DeCample, a spokesman for Gov. Mike Beebe, said the governor was alerted about the tests Monday. Farms within a 6.2-mile radius of the contractor in West Fork will have their poultry checked for the bird flu strain, Fitch said. Only one farm falls within that range. "That's one of the fortunate things, that there are no farms really close," Fitch said. The 15,000 hens will be killed by carbon-dioxide gas and buried at the farm to avoid spreading the disease, Fitch said. So far, he said officials have a working theory about how the virus spread to the hens. "The speculation at this point in time was that a large group of Canadian geese made home on a pond very near this facility," Fitch said. "Our speculation is someone stepped into some of those droppings and carried it into the poultry house." Fitch said it was the first outbreak of a bird-flu strain in Arkansas, which mandates bird-flu testing of all flocks bound for slaughter. In this case, Fitch said the birds were tested Friday before a planned killing and processing Sunday night. He acknowledged there may have been a need to make a public announcement about the cases because many have fears about bird flu. "In retrospect, maybe we should have," Fitch said. Stock in Tyson, the world's largest meat producer, fell by 8% in trading Tuesday, down $1.47 to $16.98 per share. Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more