GENEVA (AFP) - The World Health Organisation confirmed that it was examining a "probable" first full case of human-to-human transmission of bird flu in Thailand, raising fears of a deadly global flu pandemic.
WHO officials emphasised that the outbreak among four people from a Thai village appeared to have been confined to the immediate environment of the victims, a mother and her daughter.
But they could not rule out a mutation of the H5N1 bird flu virus into a more infectious form for humans until the results of tests due by the end of this week.
"There is still a possibility that the virus has mutated, but we will only know this before the end of the week when the virus has been analysed," WHO influenza and disease control expert Klaus Stohr told journalists.
Pranee Krongkaew, 26, died eight days ago and probably caught the disease while caring for her sick 11-year-old daughter, officials in Thailand said on Tuesday. The girl also died this month from suspected bird flu.
Two other people with close ties were also ill in hospital, the WHO said as health experts tried to piece together "circumstantial evidence".
Until now, humans were thought to have caught bird flu mainly from infected poultry.
"What we're seeing in Thailand could possibly be something which is not unexpected, it could be again one of these non-sustained, inefficient, dead-end street human-to-human transmission," Stohr said.
"We are concerned however, because the occurence of such a cluster could also indicate the beginning of the more widespread, sustained, transmission of an influenza virus in humans which could lead to the global spread," he added.
Although the preliminary evidence indicated that a chain of human transmission had been broken, Stohr said the fears would remain heightened until genetic tests ruled out a mutation.
Tests on genes from the Thai samples were being carried out at the US Center for Disease Control in Atlanta, WHO said.
"We cannot rule out that more cases can come from this village because H5N1 has been rampant in poultry there," Stohr said.
Health experts also raised other potential explanations for the cluster, and pointed out that there was evidence that humans had passed on bird flu elsewhere in the past.
WHO spokesman Dick Thompson said that in a close-knit family, the mother might also have been exposed to the same sources of the disease as her daughter.
A recent study had found that health care workers in Hong Kong had been infected with the most virulent strain of bird flu, H5N1, from patients they were caring for during an outbreak there in 1997.
But they had not reported outward symptoms of the illness, WHO said.
Stohr also highlighted suspicions that a small cluster of recent cases in Vietnam might also have produced a similar "dead-end" cluster of human-to-human infections.
Thompson praised Thai authorities for openly exchanging information as a curtain of surveillance was said to have been drawn around the region to track any signs of respiratory illness.
"We would expect to see human to human to human transmission, but it's important that clusters, like these family clusters, be identified earlier," Thompson said.
Bird flu has killed at least nine people in Thailand and 19 in Vietnam this year, after mainly being caught from infected poultry.
Stohr reiterated that stark choices might have to be made in the event of a mass international outbreak because of the lack of anti-viral drugs or adequate flu vaccines.
"We feel there's too little done on vaccines. We have a window of opportunity now to work on it," Stohr said.
Only two pharmaceutical companies have gone ahead with developing a vaccine in the United States, while others felt there would be no market, he added.