As spring approaches in the Northern Hemisphere and millions of birds begin their ancient long-distance migrations, scientific evidence is mounting that the deadly Asian strain of H5N1 "bird flu" virus is flying with them.
If so, the virus may soon wing its way into Alaska - where biologists are establishing an unprecedented surveillance network as part of an aggressive, $29 million early warning campaign with a new focus on birds in the wild. Until now, scientists' greatest focus has been on domestic flocks.
From Alaska, scientists fear, the virus will spread into all the Americas and ultimately become a global presence - raising the odds it will mutate and touch off a new human flu pandemic.
"I think it is more likely than not that we are going to see [H5N1] bird flu in the Western Hemisphere," said Dr. William Schaffner, chairman of the department of preventative medicine at Vanderbilt University in Nashville, Tenn.
"Whether it takes place during this migratory season or the next is uncertain," he said. But "I wouldn't be at all surprised if we get some introduction of the virus during this ... season."
Scientists already suspect wild swans of carrying the H5N1 virus last month onto an island in northern Germany, where more than 100 of the graceful birds were found dead.
The virus later killed a house cat on the same island, and Dutch scientists have evidence that cats can spread the virus to one another in the laboratory. Meanwhile, Thai scientists have found that dogs and cats there could also be carrying the bug.
All of these findings raise new questions about whether a virus hitherto spread by wild birds can now infect and spread among the mammals people live with.
"Probably not," said Vanderbilt's Schaffner. "In the real world, unlike the research lab, we see no mammalian die-offs, and believe me, they would have been noticed.
"But this is something we have to keep watching, both in animal populations and in people," he said.
Confined for years to poultry flocks in Southeast Asia, the highly pathogenic strain of the H5N1 avian flu virus has been moving west since May.
Just since Feb. 1, according to the World Health Organization, it has turned up for the first time in wild birds and poultry in India, Iran, Egypt, Niger, Nigeria, Azerbaijan, Bulgaria, Slovakia, Hungary, Greece, Slovenia, Bosnia and Herzegovina, Serbia and Montenegro, Italy, Austria, Germany, France and Switzerland.
It's now present in at least 35 countries, and exports of birds or poultry products from those nations have been banned.
Although human commerce and travel can explain some of the spread of the virus, its velocity in recent months has scientists increasingly convinced that wild birds, and perhaps bird migration, are also playing a significant role.
"I think the evidence is now quite strong indicating that migratory birds are involved in serving as at least one carrier of the H5N1 subtype," said Dirk V. Derksen, supervisory wildlife biologist at the U.S. Geological Survey's Alaska Science Center, in Anchorage.
Avian influenza is not uncommon in waterfowl. There are many strains, and they are commonly passed around by the birds through their respiratory secretions and feces.
"What is uncommon is for migratory birds, particularly waterfowl, to be affected by it," said Paul G. Slota of the National Wildlife Health Center, in Madison, Wis. "In this case, there are instances where wild birds are dying from the H5N1 Asian strain."
It's a strain that veterinary health officials call highly pathogenic to poultry, or "high-path." More common "low-path" strains produce little or no illness in poultry flocks, and low mortality rates.
The big worry among global health authorities is that this "high-path" strain of H5N1 will infect so many poultry farms that it will eventually mutate into a form that can pass from person to person.
Humans generally have had no previous exposure to this bird virus, and health officials fear that its spread would trigger a global pandemic, potentially killing tens of millions of people.
So far, the virus doesn't have that capacity, and its human toll remains low.
Since 2003, at least 174 people - in Southeast Asia, Turkey and Iraq - have been reported with H5N1 infections. Nearly all were ascribed to direct contact with infected poultry or contaminated surfaces, according to the WHO. Ninety-four of those have died - a fatality rate of 54 percent.
No one has reported a human H5N1 infection from contact with wild or migratory birds. But wild birds do appear to be spreading it, and scientists think the most likely route to the Americas will be through Alaska.
The 49th state is an avian mixing bowl. Migrants winging along flyways from Asia, the central Pacific and western North America converge there to forage and breed in the northern summers.
And that's where the federal government is focusing its new $29 million Interagency Strategic Plan for early detection of high-path Asian H5N1.