HANOI, Vietnam (AP) – Two Vietnamese sisters who died of bird flu may have caught the disease from their brother, the World Health Organization said Sunday. If confirmed, it would be the first known case of human-to-human transmission of the virus during the current outbreak sweeping Asia.

The source of the two sisters’ infection has not yet been conclusively identified, said Bob Dietz, a WHO spokesman in Hanoi. “However, WHO considers that limited human-to-human transmission from the brother to his sisters is one possible explanation,” he said.

Laboratory tests in Hong Kong verified that the sisters, ages 23 and 30, had been infected by the H5N1 bird flu virus, he said.

The sisters were among eight Vietnamese whose deaths were confirmed to be from the bird flu that has infected poultry, mostly chickens, in at least 10 countries. Thailand has confirmed two human deaths from the disease.

Most cases have been linked to contact with sick birds, and until now no evidence of human-to-human transmission has been found.

Limited human-to-human transmission of the virus is not considered a serious danger.

What experts fear is that the virus might mutate into a form that passes easily between people.

There is no evidence that a new strain has emerged, WHO spokeswoman Maria Cheng said. Such evidence would come from tests comparing the genetic makeup of the virus found in the two sisters with that seen in other people.

“This may be an isolated incident. These were very close contacts, family members,” she said.

Vietnam now has a total of 10 confirmed cases of bird flu – including the eight deaths. An 8-year-old girl remains in a Ho Chi Minh City hospital while a 4-year-old boy has recovered and been discharged.

The two sisters became sick after attending their brother’s wedding reception. Their 31-year-old brother died Jan. 14 but was cremated so no samples were available to determine whether he also had bird flu.

The women, whose identities have not been released, were admitted to the Institute of Clinical Research for Tropical Medicine on Jan. 13 and died Jan. 23.

“The investigation failed to reveal a specific event, such as contact with sick poultry, or an environmental source to explain these cases,” WHO said of the sisters’ deaths. “At the same time, such exposures cannot be discounted.”

The bird flu now hitting Asia spread between humans during an outbreak in Hong Kong in 1997, which killed six people.

In that outbreak, the virus passed from infected people to health workers but then lost its punch and failed to transmit further. Symptoms were very mild or nonexistent in those who caught it from patients rather than birds.

Experts believe Hong Kong may have averted a global pandemic that year by cracking down hard. Once it discovered it had the virus, it slaughtered its entire chicken population in three days.

AP-ES-02-01-04 0638EST

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