As spring approaches in the Northern Hemisphere and millions of birds begin their 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 soon might wing its way into Alaska - where biologists are establishing an unprecedented surveillance network as part of an aggressive, US$29 million (HK$226 million) early warning campaign with a new focus on birds in the wild. Until now, scientists' greatest focus has been on domestic flocks.

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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's more likely than not that we're going to see [H5N1] bird flu in the Western Hemisphere," said Dr William Schaffner, chairman of the department of preventive medicine at Vanderbilt University in Tennessee. 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 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 also could be carrying the bug.

All of these findings raise new questions about whether a virus hitherto spread by wild birds can infect and spread among the mammals people live with. "Probably not," said 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 February 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 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 virus' spread, its velocity in recent months has scientists increasingly convinced that wild birds, and perhaps bird migration, also play 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 Derksen, supervisory wildlife biologist at the US Geological Survey's Alaska Science Center in Anchorage.

Avian influenza is not uncommon in waterfowl. There are many strains, and they commonly are passed around by the birds.

"What's uncommon is for migratory birds, particularly waterfowl, to be affected by it," said Paul Slota of the National Wildlife Health Center, in Madison, Wisconsin. "In this case, there're instances where wild birds are dying from the H5N1 Asian strain."

It's a strain 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 eventually it will mutate into a form that can pass from person to person.

Humans generally have had no 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 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.

But not everyone is ready to lay blame for the virus' spread solely on migrating birds. One of them is Hon Ip, director of the Diagnostic Virology Lab at the National Wildlife Health Center, one of several federal labs that will test the samples from Alaska and the Pacific. Wild birds might be a factor in the virus' spread across Europe this winter, he said, but maybe not via migrations. Instead, he suggests that severe cold simply might have moved non-migratory species to new locations. Will their presence now ignite new outbreaks among local birds and poultry flocks? Or have they just flown a short distance and died, with no further consequences?

"It could go either way," Ip said. Ip also questions whether an Asian strain of bird flu is capable of becoming a long-term problem in the Americas. "Historically, there's been a clear [genetic] separation between North American avian influenza viruses and those in Europe and Asia," he said. To a geneticist, "it's almost like a fingerprint. You can tell them apart."

If they mixed regularly, they would be indistinguishable, Ip argues.

The government's surveillance work in Alaska might resolve the issue. Nearly 30 species have been targeted - mostly waterfowl and shore birds, from the Aleutian cackling goose to Steller's eider and the sharp-tailed sandpiper.

What public health officials fear most is that wild birds eventually will spread the virus to huge poultry flocks and rural backyard coops. If that happens, poultry deaths from infection, or from culling to stop the spread of the disease, will exact a heavy toll on the food industry. A spreading H5N1 epidemic among poultry would bring the virus into contact with more people, increasing the risk that it would swap genes with an ordinary human flu virus and launch a deadly pandemic.

The discovery that a domestic cat in Germany was killed by the H5N1 virus has renewed worry that the avian virus already might be acquiring the ability to infect and spread among the mammals people live with.

But Schaffner isn't worried about a threat to humans from their pets. "We can take it as a general theme that this bird flu virus can get into mammals. But it isn't really transmitted," he said.

The cat-to-cat transmission in the Dutch experiments is "instructive," he said. But virologists know that lab findings don't always translate to the real world. "There's one sort of animal we ought to keep our eye on," he said. It's pigs, whose cells can harbor both avian and human flu viruses, and where the feared viral recombination might well occur.

Even with all the H5N1 outbreaks in Asia, where poultry and pigs frequently live in close proximity, Schaffner said: "We've seen no pig die- off."

THE BALTIMORE SUN