The World Health Organization said Wednesday that a number of people who have tested positive for a new strain of bird flu in China have had no history of contact with poultry, adding to the mystery about the virus that has killed 16 people to date.
Chinese authorities have slaughtered thousands of birds and closed some live poultry markets to try and stem the rate of human infection, but many questions remain unsolved including whether the H7N9 strain is being transmitted between people.
World Health 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 about 40 percent of those with the H7N9 flu had had no poultry contact.
“This is one of the puzzles still be solved and therefore argues for a wide investigation net,” Hartl said in emailed comments, though he did not know the exact percentage.
Several avenues should be explored by an international team of experts going to China soon, including the possibility that the virus can be spread between people, although there is no evidence of sustained human-to-human transmission, Hartl added. “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.”
Wendy Barclay, a flu expert at Imperial College London, said it was likely to be very difficult to determine and rule out people’s exact exposure to poultry and 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,” she said.
Previously World Health reported two suspected family clusters, but later said the virus was found not to have infected anyone in the first. Tests in the second were inconclusive and experts say the poor quality samples may make it impossible to know.
China has warned that the number of infections could rise from the current 77. The latest victims are from the commercial capital of Shanghai, where the majority of the cases have been found, the official Xinhua news agency said on Tuesday.
China reported three new bird flu outbreaks to the World Animal Health Organization this week, bringing the total number of places to 11, the agency said on its website.
Samples have tested positive in some poultry markets that remain the focus of investigation by China and the U.N.’s Food and Agriculture Organization.
Zeng Guang, the chief scientist in charge of epidemiology at the China disease prevention and control center, said about 40 percent of human victims had no clear history of poultry exposure, the Beijing News reported Wednesday.
China said on Sunday the virus had spread outside the Yangtze River delta region in eastern China, with cases reported in Beijing and the central province of Henan.
World Health said no H7N9 vaccine was currently being produced.
“This is being followed up, but we are not yet there in terms of thinking about producing a vaccine…We need a decision based on the epidemiology,” Jean-Marie Okwo-Bele, the agency’s director of immunization, vaccination and biologicals, told a news briefing in Geneva Wednesday.