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Chinese researchers reporting in The Lancet on Thursday confirmed poultry as a source of H7N9 flu among humans but said they found no evidence of person-to-person transmission.
A probe into four cases of human H7N9 influenza in eastern Zhejiang province determined that all the patients had been exposed to poultry, either through their occupation or through visiting so-called wet poultry markets.
The World Health Organisation (WHO) had previously said poultry were the likely source of the virus, which has been linked to at least 22 deaths out of 108 identified cases since February.
A team led by Lanjuan Li of Zhejiang University in Hangzhou and Kwok-Yung Yuen of the University of Hong Kong took rectal swaps from 20 chickens, four quails, five pigeons and 57 ducks all from six live poultry markets likely to have been visited by the patients.
Two of the pigeons and four of the chickens tested positive for H7N9 but the virus was not found in any of the ducks or quails.
The researchers analysed the genetic makeup of H7N9 found in one of the patients and compared it to a sample found in one of the infected chickens.
The similarities "suggest that it is being transmitted sporadically from poultry to humans," The Lancet said.
"This is the first time that definite bird-to-human transmission has been shown for the H7N9 virus."
Doctors also monitored 303 other people who were relatives or co-workers of the patients, as well as 82 healthcare workers.
"Nobody else who came into contact with the H7N9-infected patients began to show any symptoms within 14 days from the beginning of surveillance, suggesting that the virus is not currently able to transmit between human beings," the journal said.
However, further adaptation of the virus could lead to infections with less severe symptoms and "more efficient person-to-person transmission," the researchers cautioned.
The probe was launched after a 39-year-old patient infected with H7N9 was hospitalised.
Li and Yuen carried out tests on 486 other patients who had been admitted to three hospitals with respiratory problems.
This identified three other patients with the virus. Out of the four, two died, the study said.
On Wednesday, Taiwan confirmed its first infection, in a man who had recently returned from working in eastern China where most cases have been reported.
The study added that the virus was presumed to have incubated between three and eight days, and caused fever, difficulty in breathing, coughing and sputum.
The patients were given daily doses of Tamiflu -- the leading pharmaceutical weapon against flu -- and all required breathing support.
The paper recommended "aggressive intervention" to prevent the virus unleashing a pandemic.
"Temporary closure of live bird markets and comprehensive programmes of surveillance, culling, improved biosecurity, segregation of different poultry species, and possibly vaccination programmes to control H7N9 virus infection in poultry seem necessary to halt evolution of the virus into a pandemic agent," it said.