* 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 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
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
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)