Flu Spread Is No Reason to Kill Wild Birds, Study Says

James Owen in London
for National Geographic News

April 20, 2006

Wild ducks and other migratory birds could be important carriers of deadly bird flu, researchers say.

Even so, the infectious-disease experts say there is no solid basis for killing wild birds to protect poultry and minimize the risk of human infection.

The European team investigating the global spread of the H5N1 strain of avian influenza says certain duck species may be infecting wild bird populations.

Geese and wading birds are also possible vectors of the virus, the team says.

The team's study, to be published tomorrow in the journal Science, was led by Björn Olsen of Umeå University in Sweden. Olsen runs Europe's largest wild-bird flu-monitoring program.

Studies have shown that influenza viruses in lake water, generally passed via bird feces, can stay infectious for up to 30 days.

The migration or feeding behavior of dabbling ducks could at least partially explain the spread of the H5N1 strain of bird flu, the researchers add.

This group of duck species includes mallards, teal, pintails, and others that feed at or near the surface, where viruses in water are most likely to be picked up.

Perhaps as a result, dabblers have the highest known rates of avian influenza infection, the study says. For instance, nearly 13 percent of mallards tested positive for bird flu. Other species tested include the American black duck (18.1 percent), blue-winged teal (11.5 percent), and northern pintail (11.2 percent).

However, bird flu viruses appear to exist in ducks in a low-pathogenic form, meaning infection doesn't usually lead to severe illness and death.

Prime Hosts

"Dabbling ducks are for sure the prime hosts for low pathogenic viruses," said study co-author Ron Fouchier, a virologist at the Erasmus Medical Center in Rotterdam, Netherlands.

"But the big question is, How much of our knowledge about these viruses can we translate to high-pathogenic viruses" such as the H5N1 strain of bird flu?

In poultry avian viruses can mutate into more virulent influenza strains, including H5N1.

If this mutated virus then finds its way back into wild populations, the birds could then spread the disease through migration.

Some scientists have argued that wild birds infected with HN51 would be too ill to migrate. Swans, for instance, appear to be particularly vulnerable to the strain.

"Swans apparently drop dead quite easily, but they are unlikely to be the vector because they are not going to fly very far if they are dead," Fouchier said.

But the study team says that some birds that have been purposely infected for the sake of research show that wild birds can survive H5N1.

"For some reason H5N1 has adapted so it no longer kills dabbling ducks," Fouchier said.

This means the ducks may be able to spread the virus over a wide area.

The study team says migratory geese may also be vectors, because they often graze in huge flocks, a practice that could encourage transmission.

Migrating ducks, the researchers add, "could provide an intercontinental bridge" for bird flu to North America, which has not yet had any known cases of H5N1. (See "Bird Flu Will Reach U.S. and Canada This Fall, Experts Predict.")

No Wild Culls Needed?

However, there is currently no reason to cull wild birds to control the spread of H5N1, the study says.

Wild birds, though, should be kept away from poultry, the researchers say.

"For all of the outbreaks that have ever been recorded for bird flu, it's clear that the poultry-production industry itself is responsible for most of the spread through poultry trade [and the] movement of people and equipment between farms," Fouchier said.

"You can prevent your chickens and turkeys from getting into contact with wild birds by simple biosecurity measures" such as keeping farm birds enclosed, he added (photo: "Ducks in Detention").

BirdLife International, a global bird-conservation group based in Cambridge, England, says culling operations may in fact spread the virus to noninfected areas by forcing diseased birds to disperse.

Last week Shafqat Kakakhel of the United Nations Environment Programme spoke at a conference on bird flu and migratory birds in Nairobi, Kenya.

"Blaming avian flu on bird migrations is misleading. And a 'quick fix' of culling migratory birds is certainly not the solution," Kakakhel said.

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