Scientists believe domestic ducks may have played a key role in the creation of a deadly strain of bird flu that killed 24 people in south-east Asia.
Ducks were slaughtered to curb bird flu spread
Their research suggests the ducks, from southern China, supplied genes that turned the virus into a killer.
The experts, from Hong Kong University, say their work shows how easily the virus can mutate, and warn it has the potential to trigger a human pandemic.
The study is published in the journal Nature.
The virus responsible for the deaths in south-east Asia earlier this year is a particular version of a strain known as H5N1. It is referred to as the Z genotype.
The researchers traced it back to an ancestor which is believed to have been responsible for a previous outbreak of human disease in Hong Kong in 1997.
The same ancestral virus was thought to have struck birds in the region in 2001 and 2002.
By studying the genetics of the differing forms of the virus, the researchers found that the deadly new version emerged after the virus swapped genes with viruses found in the ducks.
At present, the virus cannot pass from human to human, but experts fear that it could easily acquire this ability if it swaps genes with a human flu virus in a similar fashion to the way it was able to trade genes with duck viruses.
If this happens they fear a major worldwide pandemic claiming millions of lives might result.
Spread by wild birds
The researchers wrote: "Our findings indicate that domestic ducks in southern China had a central role in the generation and maintenance of this virus, and that wild birds may have contributed to the increasingly wide spread of the virus in Asia.
"These developments pose a threat to public and veterinary health in the region and potentially the world, and suggest that long-term control measures are required.
"H5N1 virus is now endemic in poultry in Asia and has gained an entrenched ecological niche from which to present a long-term pandemic threat to humans.
"Continued, extensive exposure of the human population to H5N1 viruses increases the likelihood that the viruses will acquire the necessary characteristics for efficient human-to-human transmission through genetic mutation or reassortment with a prevailing human influenza."
The same deadly strain is reported to have re-emerged this week in Thailand and China, although authorities in both countries insist the outbreaks are under control.
The strain has so far spread to at least eight different Asian countries, including Vietnam, Cambodia, Indonesia and Japan.