Scientists have been unable to link the spread of the virus to migratory patterns, suggesting that the thousands of wild birds that have died, primarily waterfowl and shore birds, are not primary transmitters of bird flu.
If that holds true, it would suggest that shipments of domestic chickens, ducks and other poultry represent a far greater threat than does the movement of wild birds on the wing.
It also would underscore the need to pursue the virus in poultry farms and markets rather than in wild populations of birds if a possible pandemic is to be checked, U.S. and European experts said.
The H5N1 strain has infected millions of poultry throughout Asia and parts of Europe since 2003. Theat least 71 people, many of whom had close contact with poultry.
To date, the virus hasn't been shown to spread from person to person, but many fear that it could mutate into a strain that could, potentially killing millions in a global pandemic.
While the prospect that migrating birds could carry the virus worldwide still worries health authorities, that sort of scenario doesn't appear to be playing out.
"There is more and more evidence building up that wild migratory birds do play some role in spreading the virus, but personally I believe, and others agree, that it's not a major role," said Ward Hagemeijer, a wild bird ecologist with Wetlands International, a conservation group in Wageningen, Netherlands. "If we would assume based on this evidence that wild birds would be a major carrier of the disease we would expect a more dramatic outbreak of the disease all over the world."
Reports this summer and fall of the spread of the H5N1 strain strongly suggested wild birds were carrying the disease outward from Asia as they followed migration patterns that crisscross the Earth. The timing and location of outbreaks in western China, Russia, Romania, Turkey and Croatia seemed to point to wild birds en route to winter grounds.
That put places like Alaska, where birds from the Old and New Worlds gather each summer to create what some call an "international viral transfer center," on alert that the virus could arrive this coming spring. And from there, species like the buff-breasted sandpiper and others that split their time between North and South America could in theory transport the virus farther afield.
Since the early fall, however, there have been only scattered reports of more outbreaks. The disease has been glaringly absent, for example, from western Europe and the Nile delta, where many presumed it would crop up as migrating birds returned to winter roosts.
That suggests the strain has evolved to specifically exploit, whose short lives spent in tight flocks mean a virus has to skip quickly from bird to bird if it is to survive, said Hon Ip, a virologist with the U.S. Geological Survey's National Wildlife Health Center in Madison, Wisconsin.
That also means that while the virus can pass from domestic to wild birds, the latter may not be suited as transmitters of the strain — at least so far.
"By the timing of the spread and the pattern of outbreaks within a country and between countries, it does not make sense relative to a role for migratory birds as a means of spreading the virus," Ip said.
For example, the virus killed thousands of bar-headed geese in May and June at Lake Qinghai in western China. The deaths raised immediate fears that the virus was on the move, jumping among hosts in the wild. In the Aug. 19 issue of the journal Science, scientists wrote that the virus "has the potential to be a global threat."
But Ip and others suggest the lake is not as remote and pristine as initially portrayed, and that poultry raised in the area could have been the source of the flu strain that killed the geese.