A bird flu virus has been found in a pair of wild swans in Michigan, but U.S. officials said on Monday the birds have not contracted the deadly H5N1 strain that has spread through birds in Asia, Europe and Africa, killing more than 135 people.

Routine tests conducted in a Michigan gaming area found two of 20 swans have what government officials believe is likely a low-pathogenic strain of H5N1. Confirmatory test results are underway and will be available in about two weeks, but officials stressed there is no threat to human health.

"We can definitely say this is not the H5N1 highly pathogenic virus that´s been found in Asia and other parts of the world," said the U.S. Agriculture Department´s Ron DeHaven, head of the Animal and Plant Health Inspection Service. "It´s not the introduction of that virus into North America."

The swans had shown no sign of sickness and test results indicate this is low pathogenicity avian influenza.

Pathogenicity refers to the ability of the virus to produce a disease. A low-pathogenic strain produces less disease and mortality in birds than does a high-pathogenic version.

Health officials and industry analysts said they do not believe the infected swans came in contact with U.S. commercial poultry.

Shares of Tyson Foods Inc. and Pilgrim´s Pride Corp. shrugged off the news as the top two U.S. chicken companies´ shares rose 3 cents to $13.48 and 4 cents to $23.20, respectively, to close at the New York Stock Exchange.

"This is a nonissue because it is not the bad flu and because it is not in chickens," said Paul Aho, an economist with Poultry Perspective, a consulting firm for the poultry industry.

The National Chicken Council said this showed the monitoring and surveillance program is working and could detect the Asian bird flu strain if it reached the United States.

Rep. John Dingell, a Michigan Democrat who represents Monroe County where the swans were found, said the Bush administration has not done enough to develop a comprehensive U.S. public health preparedness plan for bird flu.

"This discovery does reinforce the urgent need for the Bush administration to devote sufficient federal resources to help every state develop a comprehensive plan for public health emergencies, including pandemic influenza," he said.

The low-pathogenic strain of H5N1 has been found six times in the United States since 1975, most recently in 2002. A similar low-pathogenic strain was found in Canada last year. It is common for mild and low pathogenic strains of bird flu to appear in the United States and other countries.

The infected swans were found as part of an increased surveillance program. In late December, USDA received $91 million in supplemental funding from Congress for bird flu with much of it going toward the surveillance of wild birds. So far, more than 10,000 wild birds have been tested.

Just last week, the program was expanded beyond Alaska -- targeted because of its close proximity to the Pacific Flyway with Asia -- to the rest of the country.

"We see no cause for public health and medical authorities to take any special actions as a result of this information," said Bill Raub, science advisor to the Department of Health and Human Services.

The latest H5N1 bird flu strain in Asia, Europe and Africa is known to have killed 138 people and forced hundreds of millions of birds worldwide to be destroyed. It has yet to be found in the United States.