Sabin Russell, (SF) Chronicle Medical Writer, reports:

An outbreak of deadly bird flu among wild geese at a remote mountain lake in China is adding to the international concern about a rogue strain of influenza that could evolve into one capable of killing millions of humans.

Teams of American and Chinese researchers published two separate reports Wednesday on the incident at Qinghai Lake, in central China, where 1,500 birds perished in May from a strain the so-called H5N1 flu that has killed millions of domestic ducks and chickens in Vietnam and Thailand, a thousand miles to the south.

Although H5N1 is a bird disease, it has stricken 108 people in Southeast Asia since December 2003, killing half of them. There is no evidence to date that the bird flu virus can be transmitted readily among humans, but epidemiologists fear that it could easily mutate into one that does.

To scientists concerned about the spread of bird flu, the outbreak at the lake is alarming on many levels. It demonstrated that the epidemic can move from domestic ducks and chickens to several species of wildfowl; it raises questions about how the disease turned up so far from the epicenter in Southeast Asia; and its presence at this particular site raises the prospect that wild birds could carry the virus far beyond the lake to other parts of the world.

Qinghai (pronounced ching-high) Lake is the largest saltwater lake in China. It is located some 9,000 feet above sea level and serves in particular as a summer haven for wild birds that eventually head south, over the Himalayas, to India and Burma for the winter.

“There is a danger that (H5N1) might be carried along the birds’ winter migration routes to densely populated areas in the south Asia subcontinent, a region that seems free of this virus, and spread along migratory flyways linked to Europe,'’ wrote University of Hong Kong researcher Yi Guan and colleagues from China and the United States in the British science journal Nature.

A second paper on the outbreak by another group of Chinese scientists was published simultaneously in an online edition of the journal Science. A team led by George Gao of the Institute of Microbiology in Beijing concluded that the virus found at the lake appears to be a hybrid, blending genes from older and more recent strains of H5N1. The group speculates that the virus emerged from a mixing of strains in birds “overwintering” in Southeast Asia.

This virus, they concluded, “has the potential to be a global threat: Lake Qinghai is a breeding center for migrant birds that congregate from Southeast Asia, Siberia, Australia and New Zealand.'’

Both papers noted that the strain of influenza that struck bar-headed geese in Qinghai contained “virulence genes” that lab tests have shown make this virus particularly deadly to both chickens and mice.

Professor Yang Xiaojun, who studies bird ecology at the Kunming Institute of Zoology in China’s Yunnan province, said that birds stricken at Qinghai were all very common at the lake. The lake is a well-known habitat for bar- headed geese, which bore the brunt of the outbreak, as well as for the great black-headed gull and the brown-headed gull, which were also stricken but in far fewer numbers.

“The lake is very far away from the nearest city,'’ said Yang, who was visiting the California Academy of Sciences in San Francisco. “There are very few people living there.”

The bar-headed geese fly to an island on Qinghai to breed. The site tends to be the northernmost point of their annual migration. Most of the birds head for India in the winter.

“The government is aware of the outbreak, and is trying to prevent people from going to this area,'’ Yang said. He said he knew of no reports of people coming down with bird flu in Qinghai. However, he said Chinese papers had reported that local farmers had had to kill some of their birds on orders from the government.

UC Davis veterinarian Dr. Carol Cardona, an expert on avian influenza, said that reports about the outbreak at the lake in China were worrisome. A key question, she said, is whether any of the birds that contracted the flu at the lake are still living with it, becoming potential carriers. “The situation could be dire if the virus figures out how to go from migratory bird to migratory bird, and those birds survive,” she said. “Someone needs to test those birds, and I’m not sure that’s actually happening.'’

The evidence that the virus found at the lake is a hybrid made of genes from different H5N1 strains is also disturbing news but not surprising, she said. “What it means is that there are multiple viruses circulating,” she said. “It is showing the plasticity of the virus — its ability to change.'’

But Cardona said she remained skeptical that the flu virus had been carried to the lake by a migrating bird. “This virus has been around for nearly a decade, and human movement of domesticated birds usually explains it, ‘’ she said. “If birds get sick, they don’t walk far, but people walk far with them.'’

Cardona said that scientists were nevertheless watching for signs that migratory birds from Asia might be carrying the virus to the United States. “We look at wild birds in Alaska, where birds from America and birds from Asia commingle,'’ she said.

But as the bird flu continues to spread throughout Southeast Asia, chances also increase that smugglers might bring the virus in with an exotic bird. Last year, customs officials in Belgium seized a pair of eagles that tested positive for H5N1. They were stowed in carry-on luggage on a flight from Bangkok.

“Birds are not like cows. Some are very small, easily drugged and can fit in small containers,'’ said Cardona, who also acts as spokeswoman for the National Center for Foreign Animal and Zoonotic Disease Defense, a consortium of university researchers who work with federal and state agencies to monitor threats posed by overseas animal diseases. “There is a number of ways the virus could come into the United States, and we need to begin to understand how to control it.'’

E-mail Sabin Russell at .

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