WASHINGTON—One day in the next few weeks, flocks of wild birds from Asia will wing northeast across the Bering Strait to Alaska, where they'll join other birds heading north from their winter homes in the United States and points south.
As they embark on their annual spring migration, Asian ducks and geese may be carrying some unwelcome baggage—the highly virulent H5N1 avian-flu virus—that they could pass on to their American neighbors.
The deadly bug has killed millions of birds and almost 100 humans since it appeared in China about 10 years ago. It's been detected in 40 countries in Europe, Africa and the Middle East, as well as in its Asian homeland. A new outbreak is reported almost every week.
According to the University of Alaska's Institute of Arctic Biology in Fairbanks, "Alaska could provide an unusual mixing ground for the evolution of new strains of bird flu—strains that could spread to lower latitudes and possibly jump to other species, including humans."
So far, the virus hasn't reached North or South America, but experts say its arrival is only a matter of time.
"It is certainly within the next six to 12 months; it could be earlier," David Nabarro, the coordinator of the United Nations bird-flu program, told a news conference March 8, according to Reuters.
"Spread to new geographical areas can be anticipated when migratory birds begin returning to their breeding areas," the World Health Organization warned last month.
Although H5N1 still mainly affects birds, scientists fear that the fast-spreading virus might mutate to a form that passes easily among people, threatening a worldwide pandemic.
The flu danger is multiplied when the virus jumps from wild birds to domestic chickens and ducks living near people. The transfer happens when visiting and local birds share a water hole or feeding ground. A bit of mucus, blood or feces can carry the virus from one bird to another.
"The virus is able to spread from wild birds to poultry and vice versa. The infection can go in either direction," said Hon Ip, the director of the Diagnostic Virology Laboratory at the U.S. Geological Survey's National Wildlife Center in Madison, Wis. "If H5N1 does arrive in Alaska, wild birds are the likely suspect."
Ip added, however, that the virus also can be carried by birds that are shipped from Asia to the United States, or by humans who've come in contact with infected poultry there.
"We should check the large amount of air traffic and air cargo between the United States and Asia through Alaska," Ip said. "We should investigate each outbreak like a crime scene."
The transmission of H5N1 from wild to domestic birds is happening throughout the Old World. Migrating birds have passed a lethal strain of the virus that originated in the Qinghai Lake in central China in 2005 to poultry in Europe, the Middle East and Africa.
"Viruses from the most recent outbreaks in Nigeria, Iraq and Turkey, as well as from earlier outbreaks in Russia, Kazakhstan and Mongolia, are virtually identical to the Qinghai Lake viruses," the WHO reported.
Scientists say bird flu's most likely point of entry into the Western Hemisphere is Alaska, which lies at the intersection of the Asian and North American "flyways" for migratory birds.
A flyway is a route that wild fowl repeatedly travel between their winter and summer feeding and breeding grounds. There are dozens of flyways around the world, including four major ones between the continental United States and Arctic regions in Canada and Alaska.
The flyways provide an "extensive overlap between the Old World and New World migration systems as a disease pathway," said Kevin Winker, an ornithologist at the University of Alaska, in Fairbanks.
Last summer, researchers collected samples from about 6,000 birds that travel between Asia and Alaska to test for H5N1 infection. About 10 percent of the birds had a low-level strain of the virus that usually doesn't kill, but could mutate into the lethal version at any time.
Government and university scientists plan to check at least 75,000 more migrant birds this year. They also will do what they call "sentinel sampling" of Alaskan chickens, ducks and geese to see if they've been infected.
All the recent avian flu cases occurred along wild bird migratory routes.
"I can't think of any outbreak that was not on a migratory flyway," Ip said. "Birds have most of the globe covered. They don't leave any niches unexplored."
In some cases, domestic birds catch the virus directly from wild fowl. Once infected, they easily pass it to others.
"Direct transmission from poultry to poultry is considered to be the most common method of transmission for avian influenza," Maria Zampaglione, an official at the World Organization for Animal Health in Paris, said in an e-mail message.
So far, all 175 known cases of human H5N1 infection have been traced to domestic chickens or ducks.
"To date, no human cases have been linked to exposure to wild birds," the World Health Organization reported last month. "Close contact with infected poultry and other domestic birds remains the most important source of human infections."
For more on avian flu:
(c) 2006, Knight Ridder/Tribune Information Services.
PHOTOS (from KRT Photo Service, 202-383-6099): AVIANFLU
GRAPHIC (from KRT Graphics, 202-383-6064): 20060314 AVIANFLU US
Need to map