Hundreds of thousands of chickens have been killed to stop the spread. Credit: AP / Doan Bao Chau

An Asian bird influenza has infected and killed at least three people in Vietnam, sharpening fears of a future pandemic.

Hong Kong's National Influenza Center confirmed on Tuesday that samples taken from three people in Hanoi - two children and one adult - who had died from severe respiratory disease were positive for the avian influenza virus strain A(H5N1). Tests are continuing on samples from a further 9 children who died after suffering from respiratory disease, and 2 more who are now sick in hospital. Japan's National Institute of Infectious Diseases has also received some samples for testing and will look for the virus and antibodies to it over the next week.

Since late last year, the flu has struck hundreds of thousands of birds and prompted widespread poultry culls in South Korea, Thailand, Vietnam and Japan. In Vietnam, the problem began last October.

Spates of bird flu are viewed with alarm because they are a probable source of the next human pandemic. The first known case of a bird flu transferring to humans was in 1997, when the H5N1 strain killed six people in Hong Kong. Last year, an H7N7 strain infected more than 80 people and killed one person in the Netherlands, while another virus was held responsible for a death in Hong Kong.

Person to person

Fortunately, these viruses lacked the ability to hop easily between people, which helped to contain the problem. But a future strain might pick up this ability, either by mutating, or by mixing genes with a human flu virus. A virus capable of human-to-human transmission could rip through a population with little natural immunity, experts warn. "The ensuing virus would then be highly pathogenic and transmissible. This is why the World Health Organization [WHO] and the Vietnamese authorities are treating this matter seriously," says Shigeru Omi, WHO's regional director for the western Pacific.

In the cases reported since October, there is as yet no evidence of human-to-human transmission. Most of the families involved recalled chickens dying in their villages, adding weight to the idea that the virus came directly from sick poultry.

Researchers will need to trace the contacts of those infected in Hanoi to check that the virus has not started to spread, suggests poultry veterinarian Carol Cardona at the University of California, Davis.

Experts are unsure whether the flurry of reported bird-to-human influenza cases in the past year represents a real increase in the number of such events, or whether heightened surveillance, such as that performed by the WHO, means that more cases are being reported. "It's hard to say we're not simply detecting it more often," says Cardona.

Pandemic influenza strains are thought to start in wild aquatic birds, such as ducks, from where they leap directly to humans or spread to chickens.

Researchers will try to trace the path of the recent virus by sequencing viral DNA sequences from sick birds in different countries along with those from the Hanoi patients, to determine if and how they are related. Researchers expect to have these sequences in the next few days.