A Texas chicken flock was diagnosed with an "extremely infectious and fatal" form of bird flu Monday, and federal health officials began monitoring area farmworkers as a precaution against the first U.S. outbreak of a severe form of the disease in 20 years.
Although the strain in Texas is considered a low health threat to humans and is different from the one blamed for the recent deaths of at least 22 people in Asia, officials could not rule out a risk.
A team of federal human and animal health experts went into action after weekend tests showed the Texas flock had a more virulent flu virus, known as H5N2, than a mild strain found this month in Pennsylvania, Delaware and New Jersey.
"Past experience with H5N2 viruses has indicated there is a low threat to public health," Dr. Nancy Cox of the Centers for Disease Control and Prevention said.
She said there were no known cases of the strain infecting humans, but added, "Nevertheless, as we move forward with this situation, we must keep an open mind and really monitor the situation as we go."
U.S. animal health experts said consumers should not be concerned as bird flu cannot be spread by eating poultry. Mild heating will kill the virus.
The Asian outbreak has alarmed scientists, who say it shows that a deadly strain of bird flu can jump species. Thai officials also have confirmed the deaths of two house cats from bird flu, the first domesticated mammals known to have contracted the disease in that outbreak.
The last time a highly pathogenic strain of bird flu was found in the United States was in 1984. In that outbreak, more than 17 million birds were killed at a cost of nearly $65 million.
Shares of poultry companies, including the nation's largest poultry producer, Tyson Foods Inc., fell in response to the Texas case. Tyson shares slipped 46 cents to $15.40 on the New York Stock Exchange.
At the Chicago Mercantile Exchange, some cattle and hog futures prices fell amid concerns that bans on U.S. poultry exports could cause a meat glut.
Farmers fear bird flu because sick birds produce fewer eggs, which are often misshapen or soft-shelled. A mild form of the virus, commonly found in migratory birds, is spread through the birds' feces or mouth secretions.
The infected Texas flock was discovered Friday and initially was thought to have a mild form of the virus. Workers involved in destroying the flock of 6,600 birds were urged to monitor their own health for the next 10 days. Symptoms of bird flu in humans include fever, sore throat, muscle aches and pneumonia.
More than two dozen countries -- including Russia, the top buyer of U.S. poultry -- have banned imports of at least some American poultry since bird flu was found in U.S. flocks this month.
Industry officials fear that the Texas case may prompt Russia to expand its ban to cover all U.S. poultry. Canada, also a major buyer of U.S. poultry, halted imports from Texas on Monday.
Texas ranked No. 6 in U.S. chicken production in 2002, with 2.88 billion pounds, according to industry figures.
The bans on poultry shipments came on the heels of mad cow disease discovered in December in a Washington state dairy cow, which halted virtually all U.S. beef exports.