Hong Kong scientists studying the H7N9 bird flu virus said they had discovered another H7-type virus lurking in chickens in China.
Dubbed H7N7, the virus was able to infect mammals in a lab experiment, said the team, warning H7 viruses "may pose threats beyond the current outbreak".
"The continuing prevalence of H7 viruses in poultry could lead to the generation of highly pathogenic variants and further sporadic human infections," they wrote in the journal Nature .
"If (we) let this H7N7 continue circulating in chickens, I am sure that human infection cases will occur," study co-author Guan Yi from the University of Hong Kong said.
"This virus could cause more severe infection than ... H7N9, based on our animal experiment," he said.
H7N7 spreads easily in birds. It caused one human death and more than 80 cases of mild disease in the Netherlands in 2003.
For the new study, researchers led by Maria Huachen Zhu and Guan took the H7N7 virus they had found in poultry and tested it on ferrets in the lab. The animals, considered a good human model, developed severe pneumonia, suggesting the virus is potentially also infectious for us.
"We think it is scary for humans," said Guan. "Our entire human population almost has no antibodies against the H7 subtype of influenza virus. Thus, if it causes pandemic outbreak, it will kill many people."
Among a sample of 150 chickens tested, 36 carried the H7N7 virus, said Guan. Many birds had both H7N7 and H7N9.
Strains of the H5, H7 and H9 avian influenza subtypes have caused human infections, mainly following direct contact with infected poultry. None of the strains have mutated to become easily transmissible from person to person - the epidemiologist's nightmare.
The best-known strain is the H5N1 virus that has caused 633 confirmed flu cases in humans in 15 countries from 2003 to July this year, of whom 377 died - a death rate of about 60 per cent.
Guan and a team used genetics to determine that H7 precursor viruses were first introduced to southeast China by migratory birds to domestic ducks, where they circulated from 2010 and were then transferred to chickens.
The viruses then mixed with endemic chicken viruses to create the H7N9 and H7N7 variants, which could spread to humans at live poultry markets.
"We need to take samples from different types of poultry regularly to see what kinds of viruses are circulating in these birds," Guan said.