A bird flu virus may mutate to a human form that becomes as deadly as the ones that killed millions during three influenza pandemics of the 20th century.

Dr Julie L Gerberding, head of the Centers for Disease Control and Prevention, said yesterday that scientists believe it is highly likely that the virus that has swept through bird populations in Asia will evolve into a pathogen deadly for humans.

"We are expecting more human cases over the next few weeks because this is high season for avian influenza in that part of the world," Gerberding said in remarks at the national meeting of the American Association for the Advancement of Science.

Although cases of human-to-human transmission have been rare, "our assessment is that this is a very high threat" based on the known history of the flu virus, she said.

In Asia, there have already been a number of deaths among people who caught the flu from chickens or ducks. The mortality rate is very high - about 72 per cent of identified patients, said Gerberding. There also have been documented cases of this strain of flu being transferred from person-to-person, but the outbreak was not sustained, she said.

The avian flu now spreading in Asia is part of what is called the P family of flu viruses. It is a pathogen that is notorious in human history.

In the Vietnamese capital of Ho Chi Minh city, Bird flu experts from around Asia were gathering yesterday to figure out how to kill off the virus.

The H5N1 virus, which experts in Asia say thrives best in the cool season now coming to an end, spread to half Vietnam's 64 provinces and cities, killed 13 people and forced the government to take drastic measures to contain it.

The new outbreak a year after the highly contagious virus arrived in Asia, probably brought by migrating wildfowl, has reinforced fears that it could mutate into a form which could spread through a world population with no immunity to it.

In Tokyo, Japanese researchers found flies infected with the bird flu virus after an outbreak among chickens in Japan last year. While there was no risk of humans catching the bird flu virus from flies, the possibility that flies could spread the virus among birds could not be ruled out and they should be exterminated in any future bird flu outbreaks.

PHOTO CAPTION

Ducks are seen at a farm in northern Ha Tay province, 20 km (12 miles) south of Hanoi, February 23, 2005. (REUTERS)