The H5N1 influenza strain circulating the globe now may never seriously threaten humans, but for another subset of the Earth's living creatures, it is already a disaster.
"If you're a chicken," Dr. Julie Gerberding, director of the US Centers for Disease Control and Prevention, said at a recent conference on avian flu, "this is a pandemic. We have to be aware that other species are thinking about this differently."
By some estimates, more than 200 million domestic chickens, ducks and geese have already either died of the disease or been killed on the order of public health authorities to prevent its spread.
But scientists are scratching their heads over how much of a threat the virus presents to the world's birds.
All birds are thought to be susceptible, said Patrick Leahy, acting director of the US Geological Survey, which tracks wild bird movements in the country. The survey's National Wildlife Health Center lists all the 87 species from which infected birds have been found in Asia, Africa and Europe. The species include sparrows, eagles and flamingoes. But in most cases, it has been a dead bird here and a dead bird there.
While the virus can race through a chicken farm, killing tens of thousands of birds in a few days, there have been very few die-offs of large numbers of wild birds in any one spot. Nor have ornithologists who capture live birds along international flyways and take samples from birds shot by hunters found many infections.
And bird experts cannot yet point to any species they think is likely to become extinct.
"As a conservationist, I'm not concerned about it wiping out whole populations," said Colin Poole, director of the Asia program for the Wildlife Conservation Society, which runs the Bronx Zoo and others. "I'd say the biggest threat is things like Russian politicians saying people should go to the borders and shoot migrating birds. There's plenty of that kind of nonsense going around."
In May last year, Chinese researchers reported that 6,000 dead birds had been found in Qinghai Lake nature reserve, among them many bar-headed geese (Anser indicus). In theory, that could mean that five percent to 10 percent of the world's population of bar-headed geese was wiped out, but foreign teams were not allowed near the lake, and several bird experts said that the sampling data China released was sketchy about which species were affected.
Shortly afterward, reports of a large die-off at Erkhel Lake in Mongolia were investigated by Wildlife Conservation Society veterinarians. They found only 100 dead birds, many of them swans, and counted 6,500 live ones of 55 species. Only one sample from about 800 they gathered turned out positive for the virus.
"It had a very low impact," Dr. William Karesh, one of the veterinarians, said at the time. "The disease is self-limiting in wild birds."
According to Birdlife International, a bird-protection group based in England, members of two other threatened species, the red-breasted goose (Branta ruficollis) and the Dalmatian pelican (Pelecanus crispus), have been found dead of the disease. But only a handful were found, and there are an estimated 88,000 of the geese and 15,000 of the pelicans remaining.
There are at least 15 types of avian flu, and wild ducks are a natural host. The viruses, which are shed in feces and linger in water, turn Arctic breeding ponds into a duck soup of influenza, and migratory birds carry antibodies to many strains.
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