Big questions remain unanswered in the hunt for the source of the spreading bird flu outbreak in China, experts say, as many of the dozens of confirmed cases have no traceable history of contact with poultry.
Around 40 percent of the H7N9 influenza virus cases have no clear history of contact with chickens or pigeons, where the virus is thought to have developed to a form capable of infecting humans, the Beijing News quoted health officials as saying this week.
State media reports suggest the proportion of known cases linked to poultry contact may be as low as one in seven, Reuters reported.
The continuing mystery over the manner of transmission of H7N9, which has killed 17 people in China and sickened 87, has led to fears that this latest strain of avian flu may be capable of sparking a deadly pandemic that is easily passed from person to person.
Zeng Guang, chief epidemiologist at the China Center for Disease Control and Prevention (CDC), said widespread testing would be needed to narrow down the source of the virus, which has now been found in at least 11 apparently separate locations.
"It's important for local authorities to begin screening, to see whether carrying H7N9 is quite widespread in healthy people, or whether it's just a few isolated cases," Zeng told reporters on Tuesday.
"If healthy people can carry the H7N9 avian influenza virus, then this could be a target for prevention efforts," Zeng said.
Poultry markets closed
Chinese authorities have slaughtered thousands of birds and closed live poultry markets in Shanghai and Beijing, sparking estimated losses of more than U.S.$1.6 billion since the outbreak was first reported two weeks ago.
On Thursday, the State Forestry Administration ordered the suspension of wild bird sales to prevent the spread of the virus.
Beijing residents reported via the Twitter-like service Sina Weibo that they had seen an unusual number of dead sparrows in the city in recent days, leading to speculation that the virus is now widespread in wild birds.
A Beijing resident surnamed Zhang, who reported two dead sparrows near his home to the authorities, said many people are increasingly doubtful about expert assurances that the virus isn't being transmitted from person to person.
"There have been a lot of natural disasters lately, including earthquakes, and after natural disasters you often have epidemics," Zhang said. "It's like SARS back when it was just spreading from person to person."
"Now, we are seeing birds dying everywhere for no reason, so of course people are going to be frightened."
An international team of flu experts is traveling to China this week to help with investigations into the H7N9 virus.
"We're still trying to find out more information about the reservoir [of the virus]," Glenn Thomas, a spokesman for the World Health Organization (WHO) told a regular U.N. news briefing on Tuesday.
"From what we know at the moment, the poultry markets have been a focus of attention, but the fact-finding mission will be looking into this as a key target of its research," Thomas said.
According to an update posted on the WHO website on Tuesday, there is still "no evidence" of ongoing human-to-human transmission.
Zeng Jun, director of the Guangzhou No. 1 People's Hospital, said that the transmission route could still emerge, however.
"The H7N9 avian influenza virus is fairly easily transmitted from birds and animals to people," Zeng said. "If this virus exchanged genetic material with a seasonal influenza virus [in the same person], we couldn't rule out the possibility of transmission between people."
"That's why we have to implement strict prevention measures."
He said people should stay away from known sources of the virus, like poultry, especially the carcases of those that had died of influenza.
He also recommended avoiding crowds, as some human carriers haven't yet developed symptoms, and urged frequent hand-washing to combat infection.
Overall, he said that Chinese authorities seem to be doing a better job of informing the public about the virus, compared with the SARS outbreak of 2003.
Reported by Feng Riyao for RFA's Cantonese Service and by Yang Fan for the Mandarin Service. Translated and written in English by Luisetta Mudie.