International health officials continue to report new outbreaks of the dangerous H5N1 strain of avian flu among domestic and wild birds, and among a small but growing list of human victims. There are concerns that migratory bird populations could be hastening the spread of the disease. Those concerns are growing as tens of millions of wild birds begin their spring migrations to the United States and other regions all across the northern hemisphere.

Nearly 100 people have died since the H5N1 outbreak in southeast Asia in 2003, mostly due to close contact with infected birds. Health officials worry the virus could one day mutate and be passed easily between humans -- which is why they're trying to stop its spread now among birds.

Many experts suspect that migratory birds are the main culprits in the steady advance of the disease to the west from Asia into Europe and Africa.

Murray Trostle, a top official with the avian flu response unit at the U.S. Agency for International Development, or USAID, says the agency -- and other international health organizations -- are monitoring spring migration routes.

"The evidence is pretty good at this point that the wild birds are carrying the virus and playing a role in spreading it," he says. "To that end, we have entered into an agreement [with international health and other government organizations] to start tracking bird migrations, looking at the landing zones, where they stop on the migration paths to rest and feed and they eventually stop for breeding purposes -- and conduct sampling of birds in those areas to see if they are carrying the virus and identify the areas that may be at high risk for spreading the virus."

But Trostle says very little is known about the precise role migratory birds might be playing in the spread of avian flu. "It's a detective story and it's played out on the world stage in areas where we have little information and knowledge about what's going on," he says.

Hon Ip, an expert on bird diseases, puts it this way: "It's not a simple thing that's cut and dried that makes one banner headline: 'SCIENTISTS FIND SO AND SO?' It's the nature of an early investigation that you have."

Hon Ip is the director of the diagnostic virology laboratory at the National Wildlife Health Center in Madison, Wisconsin. He and dozens of U.S. scientists will be tracking migratory birds to see if there is a direct link between where the birds finally land and breed --and where the virus crops up.

But he believes the spread of the H5N1 virus since last October from Asia to Europe and Africa, was probably not mainly caused by migratory birds. "When the initial outbreaks occurred in Europe last year, we had a spate of outbreaks in Croatia, Romania, Turkey -- all of that occurred around October", Ip explains. "Everybody said, 'Ah-hah! They [those countries] are right in the location for the Black Sea-Mediterranean flyway. It is the fall. Birds are migrating through that area on their way to the wintering grounds. That's why there are outbreaks there.' But if birds were bringing it to that area, they should continue bringing it to the rest of the migratory route. Yet during that period of time, we don't have any outbreaks in the Mediterranean basin and even in Africa until February -- January and February."

So, if not migratory birds, then where does the blame for the spread of avian flu lie? There are several possible sources of contagion, according to US Agriculture Department official Ron DeHaven. "We're looking at all pathways as potential," he says. "We need to be addressing all of them -- whether it be the movement of migratory birds, whether it be the accidental movement of international passengers bringing poultry or poultry products, or those who might for business purposes or otherwise, be smuggling poultry or poultry products."

But the USDA official says scientists are still keeping an eye on the migratory flyways. "You can only use migratory birds as an early warning system. You look for healthy bird die-offs. And you test them to see if the virus is present," DeHaven points out. "If it is, it gives you an early warning signal. If you find migratory birds carrying the virus, then that is reason for you to beef up your biosecurity [health precautions taken around domestic poultry], increase your surveillance around domestic populations of birds, and otherwise be prepared for the possibility that the virus would find its way into your country and as the case may be, onto your poultry farm."

Avian flu experts note that countries like South Korea, Taiwan, and Hong Kong -- and more recently countries in Europe -- have imposed strict measures to quarantine their poultry flocks when infected wild birds died near -- or on --their soil. The result was these countries successfully stopped the spread of the virus within their borders. It's unclear whether those measures will be adequate once millions of birds move over the northern hemisphere during spring migration.