A Vietnamese man who died over the weekend may have been killed by the bird flu virus that has claimed the lives of 46 people in Asia since 2003, a doctor said Tuesday.

The man died Sunday in the southern province of Kien Giang, three days after he was transferred from the Chau Thanh district health center, where he had been on a respirator, Dr. Tran Thanh Tung said.

“We have sent the patient’s samples for bird flu tests,” said Tung, who heads the anti-bird flu team at Kien Giang’s General hospital. He gave no further details.

Since December, when the H5N1 virus broke out anew, 35 of Vietnam’s 64 provinces have been affected, most of them in the Mekong Delta in the south. Thirteen Vietnamese and one Cambodian have died of it.

However, a laboratory researcher in Hanoi said test results on a 41-year-old female nurse from the northern province of Thai Binh who tended a bird flu patient showed she did not have the virus that infected a male nurse who cared for the same man.

“She tested negative for bird flu,” the researcher at the National Institute for Hygiene and Epidemiology told Reuters.

The nurse provided care for a 21-year-old man who caught the virus last month after drinking raw duck blood. His younger sister and his grandfather also tested positive for the H5N1 virus.

The Thai Binh bird flu cluster has prompted an investigation into whether relatives and health workers caught the virus directly from infected patients.

However, the World Health Organization said Monday it has seen no evidence so far to suggest the virus was changing into a form that could be transmitted easily from one human to another, the greatest fear of experts.

There have been several suspected cases of human-to-human transmission of the H5N1 virus since it spread across much of Asia at the end of 2003 and one probable case, that of a Thai woman who cradled her dying daughter for hours.

If it did acquire the ability to pass easily from person to person, the virus could set off a pandemic in a world population with no immunity to it and millions could die, the WHO says.

It has killed 34 Vietnamese, 12 Thais and a Cambodian and has recurred several times despite the slaughter of millions of poultry.

South Korean officials are investigating a report by Yonghap news agency of a bird flu outbreak in Pyongyang last month that killed several thousand chickens in secretive North Korea.

Yonghap said it was not immediately clear if the outbreak was restricted to the farm or if the strain of virus involved was H5N1.