What's Behind Bird Flu: The Chicken or the Duck? Health officials say migratory waterfowl like ducks and geese are spreading the H5N1 bird flu virus from Asia to Europe and Africa. Bird experts aren't so sure; they point to an illegal trade in infected poultry.

What's Behind Bird Flu: The Chicken or the Duck?

What's Behind Bird Flu: The Chicken or the Duck?

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Ornithologists say it's only a theory that migratory birds are spreading avian flu; so far no outbreaks have been traced to waterfowl. Daniel Mihailescu/AFP/Getty Images hide caption

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Daniel Mihailescu/AFP/Getty Images

The Asian bird flu is turning up in Europe. Mute swans killed by the H5N1 virus have been found in Greece, Italy, Austria, Slovenia, Bulgaria and Germany. Dead swans found in Denmark are still being tested. Health officials say these discoveries show that waterfowl are moving the virus around. Ornithologists say this case is far from proven.

It's long been known that ducks, swans and geese can carry bird flu viruses, says Peter Marra, a migration expert at the National Zoo in Washington, D.C.

Melody Kokoszka, NPR
A map of the spread of bird flu.
Melody Kokoszka, NPR

Flyways or Highways: See how the path of bird flu matches up with Asian trucking routes and migratory routes.

"About 30 percent of ducks might be carrying some form of low pathogenic avian influenza, which doesn't hurt them or other ducks or animals that they might interact with," says Marra.

It used to be assumed that the virulent H5N1 virus killed migrating waterfowl before they traveled anywhere, says Marra. But that assumption has taken some hits in recent months.

Last fall, the virus started spreading along migratory pathways that took it west to the Black Sea and then south into Africa. Just last week, a study published in the Proceedings of the National Academy of Sciences showed that some migratory waterfowl can live with the virus inside them.

"We know that about .15 percent of waterfowl sampled will actually have live virus and be symptomatic," says Marra. "That's really important because we know they can actually carry the virus."

Other studies have also shown that birds given the virus in captivity are infectious for up to seven days -- and are asymptomatic at the same time, says Marra.

These developments strengthened a theory that Marra would have waved off a year ago: Migrating waterfowl are delivering the dreaded Asian bird flu to poultry farms, by flying overhead and defecating on flocks or in nearby waters.

Marra says the public health community now treats this theory as a fact, to the point where uninfected poultry found miles from dead swans are being slaughtered in parts of Europe. Unconfirmed reports that waterfowl are being shot are now in circulation, too.

These reports have put ornithologists on the defensive, to say the least.

"I think that wild birds are victims of the bird flu, but I don't think these birds are really spreading the disease," says Michael Fry, a migration expert with the American Bird Conservancy.

Fry says there are several problems with the waterfowl-as-carriers theory. First, while it's clear that migrating birds pick this virus up at poultry farms, there's no evidence linking their droppings to subsequent die-offs.

Also, if H5N1 really is being spread around by waterfowl, why did the virus follow only one of the many migratory routes leading out of Asia?

"There are many places that waterfowl go that if they were with any frequency carrying the disease, this disease would be everywhere," says Fry.

Fry says health officials need to focus less on ducks, swans and geese, and more on other ways the virus may be spreading. He says they might start by making sure poor poultry farmers are compensated when they kill infected flocks, lest they sell off birds that don't appear sick.

"If you don't compensate them, you are going to have a whole underground movement of birds away from quarantined areas because these farmers will try to save their birds in the hope that they are not infected," says Fry.

Fry suspects that sales like these have been driving the spread of H5N1 virus to a much greater extent than migrating waterfowl.