Vietnamese health officials said Saturday they suspect a second nurse who cared for a bird flu patient has contracted the disease that´s killed 46 people across the region.

Dao Trong Bich, deputy director of the medical center in Thai Thuy District in northern Thai Binh province said the 41-year-old woman had cared for a 21-year-old man who tested positive for the H5N1 virus and remains in critical condition.

The nurse was admitted to Hanoi´s Bach Mai Hospital Thursday with a high fever, coughing and a lung infection - typical bird flu symptoms, a doctor there said on condition of anonymity. Test results to confirm if she has bird flu are expected next week, the doctor said.

The doctor refused to speculate on how the nurse may have contracted the suspected case of bird flu.

Earlier this week, Vietnam reported that a 26-year-old male nurse who cared for the same patient had contracted the virus and is in stable condition. Officials have said they don´t believe the male nurse had contracted the disease from the patient but said they couldn´t rule out that possibility.[why the officials rule out the human-to-human transmission?]

Experts have warned that if the bird flu virus mutates into a form that allows for easy transmission between humans, it could spark a global pandemic that kills millions.

So far there has been no evidence it has acquired that ability, with most bird flu infections apparently stemming from contact with sick poultry. A case of limited human-to-human transmission, between a mother and daughter, was recorded in Thailand but the virus had not changed its form.

Bich said health authorities are closely monitoring the health of two doctors and two other nurses at the center who had contact with the 21-year-old man. None of them have shown any symptoms, he added.

The 21-year-old man is at the center of a cluster of bird flu cases that include his 14-year-old sister and 80-year-old grandfather, who has the virus without showing any symptoms.

Bird flu has killed 33 people in Vietnam, 13 of them in the latest outbreak that began December 2004. A Cambodian woman died earlier this year; Thailand has had 12 deaths.

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HANOI (Reuters) - A cluster of human bird flu cases among relatives and possibly health workers in Vietnam may show the virus is changing into a form that can be passed on by humans, the World Health Organization said.

The WHO is worried that bird flu, which has killed 47 people in Asia, could mutate into an easily spread form that sparks the next influenza pandemic, killing millions.

"Such cases can provide the first signal that the virus is altering its behavior in human populations and thus alert authorities to the need to intervene quickly," the WHO said in a statement seen on Monday.

The main concern of the WHO was a series of cases of the deadly H5N1 bird flu virus in a family in the northern Vietnam province of Thai Binh and the possible infection of two nurses who cared for one of the patients.

It also said it had received confirmation of an additional 10 cases of human infections from Vietnam´s Health Ministry.

"Full information on new cases, including those that may be closely related in time and place, is critical to ongoing assessment of the pandemic risk posed by the H5N1 virus," the U.N health agency said in a statement.

The new cases were detected in early March or through re-examination of older cases, some of which dated back to late January and three of which had been fatal, the WHO said.

Since the H5N1 virus, which spread across much of Asia in late 2003, erupted again in Vietnam in December, 24 people have been confirmed to have contracted it and 13 have died.

Earlier, there had only been one probable case of human-to-human transmission of the virus, that of a Thai woman who cradled her infected and dying daughter in her arms for hours.

Now, medical experts are investigating whether two nurses who treated a bird flu victim in Thai Binh caught it from their patient.

In the Thai Binh cases, one male nurse tested positive for bird flu after tending a patient who had drunk raw duck blood.

At the weekend, Thai Binh health officials said a second nurse who had tended the patient was in hospital with symptoms of the disease and they were awaiting the results of tests being conducted in Hanoi.

The patient, whose 14-year-old sister and grandfather were also infected, remained in a critical condition, but his sister was recovering and his grandfather had shown no signs of illness despite testing positive.

The H5N1 virus has killed 34 Vietnamese, 12 Thais and a Cambodian and has recurred several times despite the slaughter of millions of poultry.

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commentary by H.L. Niman

The acknowledgement by WHO of the importance of time and place in analyzing H5N1 is important.  It is precisely the bimodal distribution of disease onset dates in cases within familial clusters that clearly show human-to-human transmission.

The number of familial clusters has been increasing recently and they account for over one third of reported cases in Vietnam, Thailand, and Cambodia.  Recently, the familial clusters have been enhance by geographic cluster in Thai Binh, raising additional concerns.

Adding one or two health care workers to the familial cluster extends the cluster in time, and also signals a more efficient human-to-human transmission.

A changing demographic in cases in the northern and southern positions of Vietnam was noted in January.  As patients in the north began to recover, the two populations became less similar.  As the case fatality rate began to drop in the north, it remained at 100% in the south.

Notifications to WHO from Vietnam had stopped in late January, and recent updates provided additional information for the WHO update.  However, this news blackout remains in place for new human infections in the south.  This lack of information is of concern because of reports of false negatives in the south as well as antigenic drift in one of nine sequenced isolates.  There have been no specific reports on sequences from the north, other than general comments about antigenic drift and cases becoming complex.

Thus, at a time when H5N1 appears to be changing the most, crucial data are being withheld, increasing the likelihood of a major pandemic that is not detected and monitored until it is clearly beyond control