An outbreak of avian flu in wild geese in western China has raised fears that the virus responsible could soon spread beyond its Asian stronghold.
A dying bar-headed goose displays classic symptoms
Researchers say evidence of the H5N1 pathogen in the geese is a big concern because of the migratory animals' ability to fly huge distances.
Their reports, in the Science and Nature journals, are the first to show viral transmission between wild birds.
Previously, the flu was only seen to move to wild birds from domestic fowl.
World health officials are worried avian influenza virus (AVI) could cause a pandemic of human disease if it ever acquires the ability to pass easily from human to human.
So far, the impact on people has been limited to 54 deaths out of 154 infections in Vietnam, Thailand and Cambodia - again, after contact with domesticated chickens and other infected food birds.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam, but none confirmed
News that the H5N1 viral strain is now being passed around wild geese makes avian flu even more of a global threat than it already is, the scientists say.
"These birds can fly one thousand miles a day at maximum," explained Yi Guan, of the University of Hong Kong, China.
"This means the virus has the opportunity to expand its distribution to currently virus-free areas," he told BBC News.
The reported outbreak was first detected on 30 April in bar-headed geese (Anser indicus) at Lake Qinghai, a protected nature reserve in Qinghai Province.
The animals displayed classic symptoms, such as tremors, diarrhoea, head tilt and paralysis.
By 20 May, the outbreak had claimed some 1,500 birds, report Yi Guan and colleagues in Nature; not just bar-headed geese, but great black-headed gulls (Larus ichthyaetus) and brown-headed gulls (Larus brunnicephalus).
Genetic analysis of the virus extracted from dead birds shows that it is closely related to the strain that has caused human illness in Thailand and Vietnam.
Lake Qinghai is a major breeding centre for migrant birds, say to Jinhua Liu and colleagues in Science magazine. Bar-headed geese are known to move south to Burma and north over the Himalayas to India.
In their report, Liu's team analysed a variety of birds collected from the lake. The scientists managed to isolate four H5N1 virus sub-strains, and tests on mice and chickens showed them to be highly virulent.
Fifteen of the 16 test animals were dead within three days of exposure.
"The occurrence of highly pathogenic H5N1 AIV infection in migrant waterfowl indicates that this virus has the potential to be a global threat," Liu's team write in Science.
Evidence of spread in wild geese means farmers outside of southeast Asia should now be more vigilant for signs of the disease, the research teams say. Avian flu is almost impossible to stamp out once it becomes established in farm poultry populations, they warn.