The H7N9 bird flu virus may combine with the swine flu and mutate into a new virus strain, a University of Hong Kong research team warns.
The team's study, reported by the South China Morning Post yesterday, confirmed that the bird flu could be transmitted through the air and suggested it might already be spreading among humans.
"It is possible that this virus can evolve further to form the basis of a future pandemic threat," said Maria Zhu Huachen, an assistant professor of research at HKU's school of public health.
The team, which officially announced their findings yesterday, discovered that both ferrets and pigs could contract H7N9.
Zhu said the more time sick chickens and pigs spent in close contact, the greater the likelihood of the two viruses - the bird flu and swine flu - combining and mutating into a new virus strain.
"A major intervention is to separate pigs and chickens in the market to minimise the risk of the virus spreading to the pig," said HKU virologist Guan Yi.
The team found that, compared with the swine flu, H7N9 spread less easily between species and was also transmitted less efficiently.
The virus can spread from ferret to ferret or from pig to pig, but is inefficient in spreading from pigs to other mammals.
But the study also suggested that it was possible for a larger human population to have already contracted the disease - although the symptoms they suffered might be mild.
"As the virus can be spread among ferrets, it's possible it can be transmitted among humans [too]. Ferrets are the best model for the study in human influenza as [they are] very similar to humans," Zhu said.
"It's possible there may already be some hidden cases spreading among humans. Some animals infected by H7N9 do not develop fever and other clinical signs, indicating that asymptomatic infections among humans are [also] possible."
The team's tests on ferrets found that the virus could be spread among the animals through both direct contact and airborne exposure.
In one test, all three ferrets kept together in a cage with an infected ferret contracted the virus within two days.
The animals developed symptoms such as fever, sneezing, coughing and reduced activity for about five to seven days before their antibodies kicked in, the study showed.
In another test with ferrets kept in different cages, one of three ferrets was infected through airborne exposure within two days.
But that ferret developed milder symptoms of the flu. Zhu said the milder symptoms were a result of a lower toxic level of the virus.
In contrast, swine flu virus attacked all ferrets through close contact and airborne exposure within a few days.
As well as maintaining monitoring of birds, Guan said surveillance of pigs was also necessary to prevent further spreading of the epidemic.
The bird flu has affected 131 people and killed 36, according to the World Health Organisation.
View H7N9 map  in a larger map
Click on each balloon for more information on individual patients infected: blue, patients infected with the H7N9 virus under treatment; red, those infected with H7N9 who have died; yellow, those who have fully recovered; and pink, those infected other types of the Influenza A virus, including H1N1.