Wild birds can carry bird flu into new regions, threatening new bird populations and increasing the risk to humans. The great concern is that the virus will mutate into a form that will allow it to spread quickly and easily from human to human.

The bird flu virus H5N1 has devastated flocks of domestic poultry in 10 Asian countries. More than 25 million chickens have been killed in a futile effort to stop the virus, which has infected more than 100 people.

Researchers reported Wednesday in the journals Nature and Science that an outbreak in April and May at a nature reserve called Qinghai Lake in western China had killed 1,500 birds, most of them bar-headed geese, a species known to migrate in the fall south into Myanmar and then into India.

"This outbreak may help to spread the virus over and beyond the Himalayas" and along migration routes to parts of southern Asia not yet infected, as well as into Europe, say the authors of the Nature report, who are members of a Joint Influenza Research Center at Hong Kong University and Shantou University Medical College in Shantou, China.

Global Threat

The virus has been found before in wild birds close to poultry farms, the authors say, suggesting "it was possible these birds were dead-end hosts of virus acquired from poultry."

The UN Food and Agriculture Organization, which says more than 6,000 wild birds at the Qinghai Lake reserve have died since early May, issued a statement Monday, attributed to its Chief Veterinary Officer Joseph Domenech, that advises against killing wild birds to contain the outbreak.

Instead, it recommends surveillance and prevention efforts focused on vaccination of poultry in areas at risk and keeping domestic animals separate from wild birds.

The lake where the outbreak occurred is a "major breeding center for migrant birds" from Southeast Asia, Siberia, Australia and New Zealand, Beijing researchers say in Science. Its emergence in migrating waterfowl "indicates that this virus has the potential to be a global threat."

'Very Concerning Development'

Infectious-disease specialist Gregory Poland of the Mayo Clinic in Rochester, Minn., says wild birds can carry the virus into new regions, threatening new bird populations and increasing the risk to humans. "The real concern," he says, is that the virus will mutate into a form that allows its "rapid and easy transmission from human to human."

The H5N1 virus has undergone some changes already, says virologist Frederick Hayden of the University of Virginia. "Unlike most instances, this virus is showing lethality for migratory birds," which usually are not killed by avian flu viruses, he says. "It's a very concerning development."

Hayden says he can't predict what will happen next. "We don't have good precedents to follow here. We're seeing nature play out."