Indonesian scientists and officials said they were baffled by the "mysterious" behaviour of the bird flu virus here, which has already claimed nine lives this year in the world's worst-hit nation.
Indonesia has reported 126 cases of H5N1 bird flu, 103 of them fatal, since 2005. This year's victims have all come from the capital Jakarta and its satellite cities.
Officials from the ministry of agriculture's bird flu control unit told a media briefing that the risk factors for human infection remained unclear after studies were conducted around victims' homes.
"In some of the cases we found the virus in the water and chickens, but in many other cases the studies showed no signs of the virus in the surroundings," said the unit's Tjahjani Widjastuti at the briefing late Tuesday.
The usual mode of transmission of the deadly H5N1 strain of bird flu is directly from an infected bird -- typically poultry -- to humans.
"The behaviour (of the virus in Indonesia) is mysterious and we are competing with the dynamics of the virus. There needs to be deeper study on why there are more cases in humans, what are the risk factors... so we can cut the chain of infection to humans," Widjastuti said.
Globally, scientists fear that the virus will eventually mutate into a form easily spread between people.
Indonesia has been sharply criticised for being slow to act in its fight to control bird flu, which has spread easily in a nation where many people keep chickens and other birds in their gardens and homes.
Bird flu is endemic in all of Indonesia's 33 provinces except for Gorontalo on Sulawesi island and in North Maluku, said Widjastuti.
Meanwhile, early partial results of a study conducted by the UN's Food and Agriculture Organisation bird flu programme in Indonesia in traditional markets in western Java -- which covers Jakarta and its surrounds -- showed a high level of penetration by the virus.
About half of all markets tested were positive for the virus, said James McGrane, leader of the FAO programme.
"It's a very difficult virus to control," he said, adding that the team could not say why the number of cases had spiked here.