More than 50 percent of patients infected with a new type of bird flu in China had no contact with poultry, the World Health Organization said on Friday, further raising questions about whether the virus was transmitted between humans.
The H7N9 virus has so far infected 87 people in China and killed 17, but it remains unclear how they contracted the disease. A Chinese official earlier this week said about 40 percent of patients had been in no contact with poultry.
The WHO's China representative, Michael O'Leary, issued the new data, but said human-to-human transmission was rare.
"As the investigation gets deeper we're finding that more than half have had no contact with poultry", O'Leary told a briefing.
"It's hard to definitively rule in or rule out chickens", he said. "This is still an animal virus that occasionally infects humans. With rare exceptions, we know that people are not getting sick from other people".
O'Leary said investigators had tested 80,000 birds for the flu, but only 40 tested positive and none of those was sick.
"This is unusual in that we haven't been finding sick birds", he said.
An international team of epidemiologists and other experts led by the WHO and Chinese government officials will visit live chicken markets and hospitals over the next several days in Beijing and Shanghai.
Some bird samples in markets have tested positive.
China has culled thousands of birds and shut down some markets selling live poultry to try to stave off the spread of infection.
Widespread culls of poultry would be premature, O'Leary said, though he added that he was not criticizing those that had already taken place.
China has warned that there could be more infections to come. Many of the 87 cases, and 11 of the deaths, have occurred in the commercial center of Shanghai.
The flu is linked to the migration of wild birds, the official China Daily newspaper on Friday quoted an investigator at the Chinese Academy of Sciences as saying.
"The infection time and route coincided with the migration of water birds," He Hongxuan told the newspaper.
China's poultry sector has recorded losses of more than 10 billion yuan ($1.6 billion) since reports of the flu surfaced two weeks ago, an industry official told Reuters this week.