LITTLE ROCK, Ark.—Tyson Foods Inc. has begun killing and burying the carcasses of 15,000 hens that tested positive for exposure to a strain of the avian flu in northwest Arkansas, 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.

Fitch said state officials decided against announcing the infection to the general public as 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 an 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 avian flu strain, Fitch said. Only one farm falls within that range.

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.

Fitch said it was the first outbreak of a bird-flu strain in Arkansas, which mandates avian-flu testing of all birds bound for slaughter. In this case, Fitch said the birds were tested Friday before a planned killing and processing Sunday night.

However, he acknowledged there may have needed to be a public announcement about the cases, as many have fears about the avian flu.

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