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CHICAGO - Tyson Foods Inc, the second largest U.S. chicken producer, said on Tuesday it will eradicate about 15,000 chickens in Arkansas exposed to a mild strain of bird flu, and while there was no risk to human health the news sent its shares lower.

The affected chickens, which will not enter the food supply, had antibodies of a mild or low pathogenic strain of bird flu called H7N3.

It is the deadly high pathogenic H5N1 strain, which has never been found in the United States, that worries scientists because it can spread to humans and has killed 241 people around the world.

The Arkansas chickens were not sick and the antibodies were discovered as part of pre-slaughter surveillance by the company, Tyson said in a statement.

Shares of Tyson Foods fell more than 9 percent on New York Stock Exchange trading on Tuesday, but by early afternoon were down about 7 percent, or $1.38, at $17.07.

While tests showed the birds had antibodies for the mild bird flu strain, "there was no indication the birds currently have the virus," the company said.

The deadly H5N1 strain has spread to humans overseas who have been in close contact with infected chickens. So far there have been 376 human cases worldwide including 238 deaths.

A major worry among health experts is the H5N1 strain will mutate into a form that can be transmitted from human to human, raising the threat of a global pandemic that could kill millions.