A deadly new bird flu virus in China evolved from migratory birds via waterfowl to poultry and into people, and there are other bird flu viruses circulating that could follow the same path, scientists have found.
The study - an analysis of the evolutionary history of the H7N9 bird flu that has so far killed 44 people - identified several other H7 flu viruses circulating in birds that the researchers said "may pose threats beyond the current outbreak".
While none of the additional H7 strains they detected has yet been found in humans, some are able to infect other mammals such as ferrets, the researchers said, suggesting a jump to humans may be possible.
"The discovery ... reminds us that even if H7N9 does not return, there are risks lurking amongst the great diversity of avian influenza viruses," said Peter Horby, a bird flu expert at the Oxford University Clinical Research Unit in Hanoi, Vietnam, who was not involved in the research.
H7N9 bird flu, which was unknown in humans until February, has so far infected at least 135 people in China and Taiwan, killing 44 of them, according to the latest World Health Organization (WHO) data.
Most cases have been in people who had visited live poultry markets or had close contact with live poultry.
In recent weeks the number of new infections in people has dropped dramatically, thanks largely, experts say, to the closure by Chinese authorities of many live poultry markets and the summer season. But many virologists fear the human case rate may pick up again with the return of autumn and winter in China.
To trace the evolution of H7N9 and its path into humans, researchers led by Maria Huachen Zhu and Yi Guan of the University of Hong Kong conducted field surveillance around the main H7N9 outbreak region and mapped out, or sequenced, genetic codes of a large number of bird flu viruses they found.
Reporting their findings in the journal Nature on Wednesday, they concluded that H7 viruses probably transferred from ducks to chickens on at least two independent occasions and that a so-called "reassortment" of these viruses with others called H9N2 eventually generated the H7N9 outbreak virus.
They also found another previously unrecognized H7N7 virus strain had emerged and is circulating in poultry in China.
In experiments testing this strain, they discovered it has the ability to infect ferrets - an animal model often used by scientists to find out more about what flu might do in humans - suggesting it could jump into people in future.
Flu experts praised the research, with Horby describing it as using the kind of "microbial forensics" essential for helping scientists piece together where new bird flu viruses come from and what other threats might be out there.
Ian Jones, a virologist at Britain's University of Reading, said the discovery of other H7 bird flu strains was interesting but stressed they were "not of immediate public concern".
"The study identifies the route of adaptation (for H7N9) from migratory birds to local waterfowl to poultry in live markets and then to people," he said.
"Surveillance programs can now focus on key strains in the adaption process and eradicate them before they complete the jump to people."