A live chicken to be slaughtered is pulled out from its cages at a farm northeast of Tokyo where bird flu has been detected

Scientists are warning that a recent outbreak of deadly bird flu among migrating birds in western China could spread the virus rapidly beyond its current Southeast Asian stronghold.

The sick and dying waterfowl can be easily spotted at the Lake Qinghai nature reserve. If not paralyzed, they stagger and have tilted necks, classic symptoms of avian flu in such birds.

The outbreak was first detected on April 30 and within three weeks, it killed about 1,500 birds, mostly geese, but also two gull varieties. Now, the World Health Organization estimates that about 6,000 have died.

In a new paper in the journal Nature, Chinese scientists say this is the first evidence that avian flu transmission has spread beyond domesticated poultry into wild bird populations far away from farms. Hong Kong University researcher Yi Guan says the expansion could signal an even further spread of the disease once the flocks begin migrating from the Chinese lake. "You already know we have a big problem in Southeast Asia. Now this has become a new challenge for us," he said.

Mr. Yi and colleagues say the infected bar-headed geese at the lake are capable of flying over the Himalayas at a range of 16 hundred kilometers a day.

That migration will not be long in coming. Microbiologist George Gao of the Chinese Academy of Sciences in Beijing says the birds breed at the lake from the end of April only until the end of July.

"So they are flying out, soon. The problem is if there is any carrier, they might carry this to Tibet, India, and Southeast Asia. So that is something we're worried [about]. But whether or not some survivors will carry the virus, we don't know yet," he said.

Mr. Gao's team reports in the journal "Science" that they isolated several viruses from the birds and found that each shows the genetic hallmarks of a highly virulent strain. According to the "Nature" article by Yi Guan's group in Hong Kong, it is closely related to the strain that has infected poultry and people in Thailand and Vietnam.

Public health experts fear that it will merge with human influenza in a patient and create a strain that more easily circulates among people and causes a global pandemic that kills millions.

World Health Organization spokesman Dick Thompson says the birds at the Chinese nature reserve should be watched closely. "There's an urgent need to sample and tag and track as many of these species as feasible, especially considering the narrow time frame that we've got available to do it. We need more information on the migratory routes regarding these birds," he said.

The World Health Organization has urged China to increase its testing of the wild geese and gulls.