H7N9 avian flu
The influenza A (H7N9) virus is one subgroup among the larger group of H7 viruses that normally circulate among birds. A number of human infections of the H7N9 virus have been reported in eastern China, mostly in the Yangtze River Delta region since late March 2013. Some of the patients have died of severe pneumonia brought on by the virus.
DNA points to poultry markets as H7N9 transmission route
HKU flu virologist who helped crack genetic code of H7N9 says evidence suggests wild birds do not play a role in transmission of the virus
A Hong Kong professor who was among the first to crack the DNA code of the H7N9 virus suspects poultry farms are the direct transmission route of the current outbreak and it has little to do with wild birds.
In order to trace the route of infection and control the virus, the next step would be to investigate poultry markets and then farms - a difficult task that would require extensive sampling, said Malik Peiris, a flu virologist at the University of Hong Kong.
It is unlikely wild birds are involved in the transmission route, as this particular H7N9 gene pattern has not been found in them so far.
However, wild birds may have been part of the gene reassortment process that occurred earlier. The virus most likely originated from wild birds in Europe and Asia.
There is no evidence of human to human transmission at the moment, but that risk would rise if people are infected.
More cases would make mutations more likely, increasing the chance it will become transmittable between humans.
Finding the virus in other mammals such as pigs or dogs would also elevate that risk as the virus might have adapted to mammals, Peiris said.
Poultry markets are most likely to be the direct source of infection in these recent cases in eastern China, given previous studies he had done on bird flu.
"The virus can enter one bird and stay in the market for a long time, and humans have a lot of contact with these poultry," said Peiris, a specialist in animal and human influenza viruses.
"The next question would be how the virus got into the poultry markets in the first place."
Sampling work will need to be done at farms supplying the markets to find which species have been infected.
He suspects mainland authorities are already working on that. H7N9 has already been found in some poultry markets in Shanghai.
"We are trying to understand what allowed the virus to jump to humans and what made it so pathogenic," Peiris said.
The World Health Organisation's Chinese National Influenza Centre in Beijing sequenced the viral DNA from each of the first three human cases - two in Shanghai and the third in Anhui province - and published them in an online flu sequence database on March 31.
Researchers around the world have since been analysing them.
What is known is the virus' eight genes came from three sources - two groups of wild bird viruses and one group of H9N2 poultry virus.
There are genetic characteristics which enabled the virus to bind to humans. H9N2 has been widely spread in poultry over the past 15 years in many parts of Asia and the Middle East.
Gene reassortments are "chance events and unpredictable". It requires two viruses to infect one animal at the same time, which is not very common, Peiris said. "Most of the time they do not cause any problems to birds and humans."
But sometimes the genetic combinations were unsuitable for the virus and it dies, but in other circumstances the new arrangement is advantageous and the virus thrives.
The H7N9 virus has so far claimed seven lives and infected at least 24 people in China. The latest victim being a 64-year-old man in Shanghai who died on Sunday.
In 1997, the H5N1 virus killed six of 18 people who contracted it in Hong Kong and led to an unprecedented mass culling of 1.4 million birds and the closure of the trade for two months.
Scientists said that prevented a pandemic.
Click on each balloon for more information on individual patients infected with the avian flu virus: blue, patients infected with the H7N9 virus under treatment; red, those infected with the H7N9 who have died; and pink, those with H1N1 avian flu virus.