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.
PHOTOS: AGENCIES
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.
But when a new virus gets into a barn packed with thousands of young chickens that have been genetically selected for their plump breasts rather than their ability to survive in the wild, it leaps from bird to bird, mutating slightly each time, and sometimes morphs into a lethal strain -- just as the 1918 Spanish flu was believed to become more deadly as it passed through crowded American military camps during the cold winter of 1917.
The new, lethal virus may then be passed back to migratory birds when they visit ponds or rice paddies used by domestic flocks -- or perhaps through intermediaries like crows or mynahs that steal food from poultry operations. But then, that wild infection may snuff itself out again because dying birds cannot fly far.
This may help account for the unpredictable hopscotch pattern the virus is following, experts said.
"It's hard to know what's going on," said Rob Fergus, science coordinator for the National Audubon Society. "But we're not seeing many wild birds dying nearby every time we find outbreaks in poultry."
Another possible route is the international trade in day-old chicks. After the outbreak in Nigeria, the Nigerian agriculture minister, Adamu Bello, said poultry producers there import chicks from China and Turkey, where the disease is widespread. Although the river deltas of southern Nigeria shelter many birds that migrate from Europe, ducks there are not dying, and the first infected poultry operations were in the arid north.
The chick trade "has made the chicken the most migratory bird in the world," said Adrian Long, a spokesman for Birdlife International. "When the United Nations puts every outbreak on wild birds, they're not exactly being Sherlock Holmes in their inquiries."
Two localized die-offs of wild birds, one in Azerbaijan and one in Germany, have been reported. And from Italy to Sweden, small numbers of dead, infected swans have been found, while many other species sharing waters with them appear healthy.
Many of the dead were mute swans (Cygnus olor), Poole said, which often live close to humans in park lakes. Unlike whistling swans, which migrate thousands of miles to the Arctic, mutes tend to circle Europe following food crops, the way Canada geese circle North America, and might have caught their infections from poultry in Turkey or Romania.
Human overreactions have proved more of a threat. In January 2004, the Thai government briefly laid plans to kill 70,000 open-billed storks in two nature reserves, fearing they were spreading the disease after 200 of them died. It canceled the plan a month later after only two of the dead birds turned out to have had the flu.
In Romania, villagers were reported to have attacked exhausted wild birds on an icy lake. In Maine, Kathleen Gensheimer, the state epidemi-ologist, said her department had fielded calls from citizens asking if they should shoot all the Canada geese landing in their backyards.
No bird with the deadly H5N1 strain has yet been found in North or South America.
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