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 30 and 23, 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.

China closed poultry markets and processing factories in bird flu-affected areas, shortly after WHO warned that Beijing’s chances to contain the disease may be dwindling.

WHO on Saturday called on China to share more information about the disease, step up monitoring for possible human cases and take precautions so that workers engaged in the mass slaughter of birds are not accidentally infected.

The U.N. Food and Agriculture Organization appealed for international aid for farmers across Asia, saying the farmers may otherwise resist slaughtering their flocks, a crucial measure in stamping out the disease and preventing a human outbreak.

“We are ... concerned that mass culling is not taking place at a speed we consider absolutely necessary to contain the virus,” said Hans Wagner, an FAO animal production and health officer.

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 hospitalized in Ho Chi Minh City while a 4-year-old boy has recovered.

The two sisters from northern Thai Binh province 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. Their sister-in-law also was hospitalized with a respiratory illness, but she has recovered.

“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 current bird flu outbreak has spread to 47 of Vietnam’s 64 provinces — more than two-thirds of the country.

Vietnam has vowed tougher measures to control the epidemic, including a nationwide ban on transporting poultry. Bird flu has now killed or forced the slaughter of more than 7 million chickens and ducks.

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