Tests results released on Monday found H5 and N1 subtypes in the birds, said Ron DeHaven, administrator of the U.S. Department of Agriculture's Animal and Plant Health Inspection Service, but "this is not the highly pathogenic avian influenza that has spread through much of other parts of the world."

Pathogenicity, high or low, indicates the ability of the virus to cause illness. The swans showed no signs of sickness, nor did any birds around them, DeHaven said. Further testing is being done at a national veterinary lab in Ames, Iowa.

TRACKING THE FLU:Bird-flu testing migrates to Calif.

A highly pathogenic H5N1 bird flu virus that emerged in Hong Kong in 1997 and spread through Asia and into Africa and Europe has not been found in the USA. It has raised alarm bells because the virus, although still mainly a disease of birds, has managed to infect 238 people since 2003, killing 139. There has been a constant fear that the virus could mutate to the point at which it spreads easily among people and ignites a flu pandemic.

Low-pathogenic versions of the H5N1 virus were found in wild ducks in the USA in 1975 and 1986, and again in ducks in Manitoba, Canada, last year.

Flu viruses mutate all the time, but among the many forms of low-pathogenic bird flu, only H5 flu and H7 strains have been known to mutate into highly pathogenic forms, DeHaven said. "Any H5 or H7 virus is of concern from a commercial standpoint in that it has the potential to mutate" and become more dangerous, he said. But, he said, the affected swans had had no known contact with commercial poultry.

The National Chicken Council said in a statement that the discovery "creates no problems for commercial poultry operations or consumers" and that commercial growers already protect their poultry from contact with wild birds.

The two mute swans, which don't migrate, lived in the Pointe Mouillee State Game Area on the coast of Lake Erie and were among 20 swans killed in a population-reduction program. Samples were taken Aug. 8 as part of an expanded testing program for wild birds.

Sue Haseltine, associate director for biology at the Department of the Interior's U.S. Geological Survey, said surveillance for all forms of avian flu has increased this year, and more than 8,000 wild birds have been tested since June. Less than 2% had any form of bird flu, she said, which is "about standard."

She said much of the testing this summer has been done in Alaska, where migrating birds congregate for breeding, and testing will continue as birds return to the Pacific Islands.

At the same time, DeHaven said, "we'll be doing a lot of testing in the lower 48 (states) beginning late August and through the fall migration season."

Enlarge By Sven Nackstrand, AFP/Getty Images The two infected birds in Michigan were among 20 mute swans killed in a population-reduction program.