Authorities in Vietnam are investigating the possibility of limited human-to-human transmission of avian influenza in the case of two brothers, one of whom was confirmed Friday as the country´s seventh death from the H5N1 virus in the past three weeks.

The head of the World Health Organization´s global influenza program said he believes the investigation will show the older brother infected the younger one. The younger man nursed his older brother and only fell ill nine days after his brother, suggesting they did not become infected from the same source.

"I would think it´s rather likely that the second one got infected from the first one, because of these nine days´ difference," Dr. Klaus Stohr said from Geneva.

The older man, 46, died Jan. 10 in Hanoi. Vietnamese authorities only confirmed him as a case of H5N1 on Friday, after a retesting of samples showed proof of infection. He had originally tested negative for the virus.

His brother, 42, remains in hospital but appears to be on the mend.

To date, there is no evidence the virus spread beyond the brothers, Stohr said, adding Vietnamese disease investigators looked for but did not find signs of more extensive human transmission.

"There has been no significant increase in respiratory disease in these villages. And that´s reassuring."

Sustained transmission over several generations would indicate that the virus had undergone genetic changes that allowed it to more easily infect humans - a development that could trigger the pandemic the world´s influenza experts have been warning is imminent.

There have been at least four previous cases dating back to 1997 where limited person-to-person transmission of H5N1 is believed to have occurred.

Experts worry naysayers may be tempted to point to these cases of limited transmission to argue the H5N1 virus does not have the potential to trigger a pandemic. That would be an unsafe assumption, they warned Friday.

"I still am absolutely convinced that it is still just a matter of time in Southeast Asia before this thing blows," said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

"One day we´re likely to wake up and find a number of people in a given household, in a particular work area, in a given village where we´ve got evidence of widespread transmission - and we no longer can account for that through bird-to-human transmission."

Stohr said he understands the temptation to try to downplay the pandemic potential.

"I would like to believe that it´s not going to happen. I would like to believe we will continue to see individual cases which are not going to be transmitted. I would like to believe this virus is not reassorting," he said.

"Unfortunately all the data we have would tell us otherwise.... There is too much at stake just to hope for luck."

There have been 52 confirmed human cases of H5N1 influenza in the past year, all in Thailand and Vietnam; 39 of the people died.

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Human transmission ruled out in bird flu cases
A man whose brothers contracted bird flu has tested positive for the deadly virus, Vietnam’s state-controlled media reported today, as the World Health Organisation said it was investigating the possibility that one of the siblings passed the disease to another.

Vietnamese officials, however, said there was no evidence of human-to-human transmission in the cases.

Health officials fear that the bird flu virus could mutate into a form that can easily spread among people, sparking a global pandemic that could kill millions. There is no evidence that has occurred, however.

A 36-year-old man from northern Thai Binh province tested positive for the H5N1 strain of bird flu, the Pioneer newspaper quoted Vice Minister of Health Tran Chi Liem as saying.

His 47-year-old brother died on January 10 from bird flu, while another brother, 42, was recovering in a hospital in Hanoi after also testing positive for the virus.

Since December 30, a total of nine people have died of bird flu in Vietnam.

Health officials have said the family had eaten raw duck blood pudding in late December, linking their infections to poultry.

“From the H5N1 virus infected cases in Thai Binh recently, there is no evidence of human-to-human transmission,” Liem was quoted as saying.

However, the WHO has not ruled out the possibility, because the middle brother cared for the elder sibling before he died.

But that alone would not raise the threat of a flu pandemic, said Peter Cordingley, spokesman for WHO’s Western Pacific Regional Office in Manila. The UN agency has not yet confirmed reports of the third brother’s infection.

Similar isolated cases of possible human-to-human transmission occurred in Hong Kong in 1997 and in Vietnam and Thailand last year, he said.

“WHO would not be unduly concerned about the public health implications of limited transmission of the virus from one person to another,” he said. “What would worry us is evidence of the virus being able to pass effectively between humans, thus setting off a chain of transmission. We have not seen this with the present cases, nor with previous ones.”

So far most human cases have been linked to contact with infected poultry.

“There is no indication that something is spreading quickly,” said Hans Troedsson, WHO representative in Vietnam. “It’s not that it has passed the threshold and opened the flood gates.”

Also in Vietnam, a 22-year-old woman in critical condition was being tested for the disease in Ho Chi Minh City after her younger brother died of bird flu, said Phan Van Tu, chief virologist at the city’s Pasteur Institute.

Bird flu outbreaks among poultry have been reported nationwide in Vietnam so far this year, killing or forcing the cull of more than 500,000 birds.

Vietnamese authorities have ordered quarantines, stricter border controls and a ban on poultry from neighbouring countries in an effort to battle the disease, fearing a repeat of last year.

Last year the virus surfaced in 10 Asian countries, killing or forcing the slaughter of more than 100 million birds. The virus jumped to humans in Vietnam and Thailand, killing 29 and 12, respectively.