Chickens sit caged in a poultry market Sunday in Shanghai, where health officials have quarantined a farm over a suspected outbreak of bird flu.
CREDIT: Greg Baker, Associated Press
Two sisters in Vietnam have died after contracting bird flu in what may have been the first human-to-human transmission of the disease.
The World Health Organization warned Sunday that the women may have caught the disease at the wedding of their brother, who has also died.
If so, it will be the first known case of human transmission of the disease that has so far infected 13 people in Asia and killed 10 of them.
Until now, all the cases had been found in people in close contact with birds.
Bob Dietz, a WHO spokesman in Hanoi, said that laboratory tests in Hong Kong confirmed the sisters, aged 30 and 23, had been infected with the H5N1 avian flu subtype.
The Vietnamese health authorities do not know if the same disease killed their brother, aged 31, because he was cremated on Jan. 12 without any samples being kept. His bride also caught the disease but has since recovered.
Human-to-human transmission of avian flu has been feared by health authorities because it could herald the start of the disease spreading on a large scale. Two previous pandemics of flu, in 1957 and 1968, occurred after avian flu combined with human strains of the disease to produce a subtype to which human populations had little resistance.
So far, there is no evidence a reassortment of avian and human flu genes has taken place.
The tests show that the two women were carrying an H5N1 subtype indistinguishable from that infecting millions of chickens throughout Asia.
It is possible all four caught the disease from birds, but the WHO said that there was no evidence that they had been in contact with infected poultry.
"The investigation has not been able to conclusively identify the source of infection for the two sisters," it said in a statement.
"However, WHO considers that limited human-to-human transmission, from the brother to his sisters, is one possible explanation."
Reports from Hanoi said the two sisters became ill after attending their brother's wedding reception.
WHO added that similar instances of limited transmission between people of the H5N1 virus were seen in Hong Kong in the 1997 avian flu outbreak, but that this had never developed into a significant public health threat.
An investigation of the cases was undertaken with the National Institute of Hygiene and Epidemiology in Vietnam and local health officials to find out how the virus was transmitted.
Meanwhile, China's government has reported five more suspected bird flu cases.
With the new report, China now has three confirmed cases and eight suspected cases in a total of 10 regions spanning the country.