By Stephanie Nebehay and Sui-Lee Wee
GENEVA/BEIJING (Reuters) - The World Health Organization said on Wednesday that a number of people who have tested positive for a new strain of bird flu in China appear to have had no contact with poultry, adding to the mystery about a virus that has killed 17 people to date.
Chinese authorities have slaughtered thousands of birds and closed some live poultry markets to try to slow the rate of human infection, but many questions remain unsolved, including whether the H7N9 strain is being transmitted between people.
WHO spokesman Gregory Hartl confirmed that "there are people who have no history of contact with poultry", after a top Chinese scientist was quoted as saying this applied to about 40 percent of those infected.
"This is one of the puzzles still (to) be solved and therefore argues for a wide investigation net," Hartl said in emailed comments.
Hartl an international team of experts going to China soon would include in their investigation the possibility that the virus can be spread between people, although there was "no evidence of sustained human-to-human transmission".
"It might be because of dust at the wet markets, it could be another animal source beside poultry, it could also be human-to-human transmission," he said by telephone.
Wendy Barclay, an influenza expert at Imperial College London, said it could be hard to reveal or rule out exposure to poultry - or to wild birds, which could also be a possible source of infection:
"The incubation time might be quite long, so visiting a market even 14 days before might have resulted in infection."
Hartl said two new suspected cases of possible human-to-human transmission were being investigated.
The first is a couple in Shanghai who tested positive, Hartl said, adding that the wife had died and husband was still sick. A seven-year-old girl in Beijing was the first case in the capital at the weekend and the boy next door has also tested positive, but is not showing symptoms, he said.
NUMBERS TO RISE
The WHO had previously reported two suspected family "clusters", but the first turned out to be a false alarm and the second was inconclusive.
China has warned that the number of infections, 82 so far, could rise. Most of the cases and 11 of the deaths have been in the commercial capital Shanghai.
China reported three new outbreaks to the World Animal Health Organisation (OIE) this week, bringing the total number of locations to 11, the OIE said.
Poultry markets remain the focus of investigation by China and the U.N.'s Food and Agriculture Organization.
But Zeng Guang, chief scientist in charge of epidemiology at the China Disease Prevention and Control Centre (CDPCC), said about 40 percent of human victims had no clear history of poultry exposure, the Beijing News reported.
The centre declined to comment on state media reports saying only 10 of the 77 cases known by Tuesday had had contact with poultry.
A study published last week showed the H7N9 strain was a so-called "triple reassortant" virus with a mixture of genes from three other flu strains found in birds in Asia. One of those three strains is thought to have come from a brambling, a type of small wild bird.
"We can't rule out that this ... has passed through poultry but then been reintroduced to a wild bird population from which some spread to humans might be occurring," Barclay said.
China's poultry sector has recorded losses of more than 10 billion yuan (1 billion pounds) since reports of the strain emerged two weeks ago.
(Reporting by Stephanie Nebehay in Geneva, Sui-Lee Wee, Michael Martina and Huang Yan in Beijing, Kate Kelland in London and Sybille de La Hamaide in Paris; Editing by Jon Hemming)