Testing for the H7N9 virus at the Centre for Diseases Control and Prevention in Beijing. Photograph: Reuters
A third person has died from H7N9 bird flu in China as the total number of confirmed cases rose to nine and concerns were raised that the virus might have mutated to infect other animals and humans more readily.
The latest victim was a 38-year-old cook who fell ill early in March while working in the province of Jiangsu, where five of the other cases were found, authorities said. He died in hospital in on 27 March and a positive test for H7N9 flu came back on Wednesday.
Chinese authorities have said there is no evidence the strain spreads easily between humans but experts say it seems to move through poultry without making them sick, making it hard to track, and possesses genetic markers that indicate it is infectious to people.
In the wake of the illnesses, the Chinese Centre for Disease Control and Prevention shared the genetic sequence of H7N9 with other scientists to help study how it might behave in different animals and situations.
"The tentative assessment of this virus is that it may cause human infection or epidemic," said Dr Masato Tashiro, director of the World Health Organisation's influenza research centre in Tokyo and one of the specialists who studied the genetic data. "It is still not yet adapted to humans completely but important factors have already changed."
Flu viruses evolve constantly and scientists say such changes have made H7N9 more capable of infecting pigs.
Pigs are a particular concern because bird and human flu viruses can mingle there, potentially producing a bird virus with heightened ability to spread between humans, said Dr William Schaffner, a flu expert at the Vanderbilt University School of Medicine. This happened in 2009 with swine flu.
One scientist said the sequence raised concern about a potential global epidemic but it was impossible to give a precise estimate of the likelihood. "At this stage it's still unlikely to become a pandemic," said Richard Webby, director of a WHO flu centre at St Jude Children's Research hospital in Memphis, Tennessee.
"We should be concerned [but] there's no alarm bells ringing yet."
The scientists who inspected the genetic data said that based on information from the genes and Chinese lab testing the H7N9 virus appeared capable of infecting some birds without causing any noticeable symptoms. Without obvious outbreaks of dying chickens or birds authorities could face a challenge in trying to trace the source of the infection and stop the spread.
If there were no obvious symptoms in birds or pigs "nobody recognises the infection in animals around them. Then the transmission from animal to human may occur," Tashiro said. "In terms of this phenomenon it's more problematic."
This behaviour is unlike the H5N1 strain, which set off warnings when it began ravaging poultry across Asia in 2003. H5N1 has since killed 360 people worldwide, mostly after close contact with infected birds.
If the latest virus continues to spread in China and beyond "it would be an even bigger problem than with H5N1, in some sense, because with H5N1 you can see evidence of poultry dying" said University of Hong Kong microbiologist Malik Peiris, who also examined the genetic information.
He urged China to widely test healthy birds for the virus in live animal markets in the parts of the country where the human infections have been reported.