An avian flu sweeping Hong Kong poultry farms is mutating fast and officials are fighting to stop it from evolving into a strain that crossed to humans and killed six people in 1997, a top scientist said on Wednesday.

"At the moment, it does not present an immediate threat (to humans). But that threat is if it does happen to go to the right type of reassortment (mutation)," Ken Shortridge, who is studying the virus, said on Hong Kong RTHK radio.

"It will need one little chance ... and this could give rise to a serious virus."

Earlier, Shortridge told the daily South China Morning Post that the bird flu outbreak came from the same family of viruses that mutated into the deadly human strain in 1997.

That outbreak forced authorities to slaughter all domestic fowl, more than a million birds. The latest scare hit as Hong Kong prepares for Chinese New Year next week, when fresh chicken is a main dish.

Hong Kong health officials have tried to move quickly to stop the virus before it mutates into a rogue strain that threatens humans.

Some 169,000 birds from two farms and four markets have been killed since the weekend and 24 more poultry farms were shut on Tuesday.

Imports of live chickens, mostly from Chinese mainland are halted on Wednesday and Thursday and chicken sales will be halted on Friday so "all poultry stalls and fresh provision shops can be thoroughly cleansed and disinfected", a government statement said.

About 20 percent of some 100,000 live chickens sold daily are raised in the territory. The remainder comes largely from the mainland.

Shortridge said the danger came from the speed with which the virus was mutating in Hong Kong's aquatic bird population. It could become a risk to humans but so far this was not the case.

The government has said the virus had not been identified as yet, but Shortridge said it was from the H5N1 goose family from which the 1997 strain that killed humans emerged.

"It is still the Guangdong goose family (of H5N1) but it is moving away from the Guangdong virus of 1996," the parent strain of the 1997 flu, he said.

"The way the virus is behaving now, we are seeing it undergo many, many of what we call reassortment, swapping genes with many other viruses quickly," he told RTHK on radio.

Hong Kong had to order a mass cull of 1.2 million birds last May after another virus scare.

Agriculture officials in neighbouring Guangdong province and Shenzhen city in southern China said they had seen no sign of the bird flu outbreak.

(China Daily February 06, 2002)