* Case studies trace infection back to chickens from market
* New H7N9 flu strain has infected 109 in China, killed 23
* Controlling virus in poultry should be priority -experts
By Kate Kelland
LONDON, April 25 (Reuters) - Chinese scientists have confirmed for the first time that a new strain of bird flu that has killed 23 people in China has been transmitted to humans from chickens.
In a study published online in the Lancet medical journal, the scientists echoed previous statements from the World Health Organization (WHO) and Chinese officials that there is as yet no evidence of human-to-human transmission of this virus.
The H7N9 strain has infected 109 people in China since it was first detected in March. The WHO warned on Wednesday that this strain is "one of the most lethal" flu viruses and is transmitted more easily than the H5N1 strain of bird flu, which has killed hundreds around the world since 2003.
Kwok-Yung Yuen of the University of Hong Kong, who led the study, said its findings that chickens in poultry markets were a source of human infections meant that controlling the disease in these places and in these birds should be a priority.
"Aggressive intervention to block further animal-to-person transmission in live poultry markets, as has previously been done in Hong Kong, should be considered," he told the Lancet.
He added that temporary closure of live bird markets and comprehensive programmes of surveillance, culling, biosecurity and segregation of different poultry species may also be needed "to halt evolution of the virus into a pandemic agent".
"The evidence ... suggests it is a pure poultry-to-human transmission and that controlling (infections in people) will therefore depend on controlling the epidemic in poultry," he said.
Yuen's findings do not mean all cases of human H7N9 infection come from chickens, or from poultry, but they do confirm chickens as one source.
The WHO has said 40 percent of people infected with H7N9 appear to have had no contact with poultry.
Other so called "reservoirs" of the flu virus may be circulating in other types of birds or mammals, and investigators in China are working hard to try find out.
Yuen's team conducted detailed cases studies on four H7N9 flu patients from Zhejiang, an eastern coastal province south of the commercial hub Shanghai.
All four patients had been exposed to poultry, either through their work or through visiting poultry markets.
To find out whether there was transmission of the virus from poultry to humans, the researchers took swabs from 20 chickens, four quails, five pigeons and 57 ducks, all from six markets likely to have been visited by the patients.
Two of the five pigeons and four of the 20 chickens tested positive for H7N9, but none of the ducks or quails.
After analysing the genetic makeup of H7N9 virus in a sample isolated from one patient and comparing it to a sample from one of the chickens, the researchers said similarities suggest the virus is being transmitted directly to humans from poultry.
The team also checked more than 300 people who had had close contact with the four patients and found that none showed any symptoms of H7N9 infection within 14 days from the beginning of surveillance. This suggests the virus is not currently able to transmit between people, they said.
But they noted that previous genetic analysis shows H7N9 has already acquired some gene mutations that adapt it specifically to being more able to infect mammals - raising the risk that it could one day cause a human pandemic.
"Further adaptation of the virus could lead to infections with less severe symptoms and more efficient person-to-person transmission," the scientists wrote. (Editing by Kevin Liffey)