Two Michigan swans were carrying an apparently harmless version of the H5N1 bird flu virus. It is not the same H5N1 bird flu that has killed 139 people in Asia.

"We do not believe this virus is a threat to human health," Ron DeHaven, an administrator at the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, said in a news conference.

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William Raub, PhD, a science advisor at the U.S. Department of Health and Human Services, appearing at the same news conference, agreed, saying the evidence suggests the virus found here is not dangerous for humans.

Both swans carrying the virus were healthy. They were among 20 tested as part of the expanded U.S. bird flu surveillance program.

Genetic tests show the virus infecting the birds is what biologists call "low pathogenicity" bird flu, meaning it is not particularly dangerous to birds and is no health threat to humans. In contrast, the H5N1 bird flu circulating in Asia is a "high pathogenicity" H5N1 strain.

It's not the first time a low-pathogenicity H5N1 bird flu has been seen in the U.S. It popped up in wild ducks here in 1975 and in 1986. Last year it was detected in wild ducks in Manitoba, Canada.

Flu viruses are characterized by their H and N surface proteins. But viruses with the same H and N surface proteins can be very different.

Flu viruses mutate easily, and it is possible the Michigan virus could mutate into a high-pathogenicity one.

But there's no evidence that is happening. And even if it did, the virus would still not be the same as the Asian H5N1 virus.

The two swans were captured Aug. 8 at the Mouillee state game area south of Detroit, on the coast of Lake Erie in Monroe County, Mich. They live in that area, rather than migrate across the country.

The U.S. Department of Agriculture is continuing tests on the swans’ virus at its central facility in Ames, Iowa.

The tests will include growing the virus and injecting it into chickens. If it kills six or more of eight baby chicks, the virus will be reclassified as highly pathogenic bird flu.

Those tests will be completed in two weeks.

"We found this virus in several other wetland situations around the U.S. and Canada over the last 20 years," DeHaven said. "This is not a precedent, just a normal avian influenza event. Given what we know, this is no surprise."

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By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: News conference, U.S. Department of Agriculture and U.S. Department of the Interior. News release, USDA.