HONG KONG (Reuters) - The H7N9 bird flu virus may be capable of spreading from human to human and can be transmitted not only through direct contact but also through airborne droplets, researchers at the University of Hong Kong have found.
The researchers report in the journal Science, and at a news briefing on Friday, that three ferrets - the main animal used for research into human influenza - contracted the virus after having been placed in close contact with ferrets that had been injected with H7N9.
One out of three that were kept in different cages became infected through airborne exposure.
The World Health Organization (WHO) has previously said it has no evidence of "sustained human to human transmission" of the virus, which has killed 36 people in China.
"The findings suggest that the possibility of this virus evolving further to form the basis of a future pandemic threat cannot be excluded," said the research team, led by bird flu expert and microbiologist Yi Guan.
The team also found that some infected animals did not develop fever and other clinical signs, indicating that asymptomatic infections among humans are possible. That would make the virus harder to detect and control.
The virus can also infect pigs, the study showed, but could not be transmitted from pig to pig or from pigs to other animals, although the team urged authorities to maintain surveillance to ensure the virus did not mutate into a more serious one.
The WHO called the study a good one but cautioned that people "have to be very careful about what's going on the ground."
"Studies like that are really helpful for increasing general knowledge and it's really helpful to know that under lab conditions this thing could transfer from person to person," WHO chief spokesman Gregory Hartl told Reuters.
"We've already seen maybe a few limited instances of human to human transmission within close family range, within close contacts, so this is another piece of the puzzle," he said.
The findings come just days after the WHO said the H7N9 virus appeared to have been brought under control in China thanks to restrictions at bird markets.
H7N9 has relatively mild clinical signs in ferrets, according to the study. All the animals infected with the virus in the experiments presented symptoms for no more than seven days and all recovered from the disease.
The researchers said that cases in which humans died or became extremely ill were triggered by additional causes.
"All the deceased or seriously ill patients, (their illnesses) are due to other causes," said Dr Maria Zhu Huachen from the research team.
United Nations experts said this week the bird flu outbreak in China had caused some $6.5 billion in losses to the economy.
The H7N9 virus is known to have infected 130 people in mainland China since it emerged in March, but no cases have been detected since early May.