BEIJING - The bird flu virus, also known as H5N1, seems to exist widely in the poultry markets, particularly in South China, posing a tough challenge for the nation's epidemic intervention work, senior experts warned.

Worse, a mutant strain of the virus called H5N1-2.3.2.1 was found spreading among live poultry, which the vaccine now in use cannot fully protect against, Guo Fusheng, technical adviser in animal health of the Food and Agriculture Organization (FAO) of the United Nation, told China Daily on Tuesday, citing information and statistics from China's Ministry of Agriculture.

"With the arrival of autumn and winter, the country is facing an escalating risk of bird flu outbreaks among poultry as well as that of humans getting infected," he warned.

Yu Kangzhen, the national chief veterinary officer of the Ministry of Agriculture, said previously that it would be hard to avert regional bird flu cases during autumn and winter but "the chance of large-scale outbreaks is quite slim".

In late August, FAO warned of a possible resurgence of bird flu outbreaks around the world. According to the organization, since the virus was first detected in 2003, it has infected at least 560 people worldwide, killing 331 of them.

Sun Yan, an official of the Bureau of Veterinary under the Ministry of Agriculture, conceded that China is at even greater risk of avian influenza, given that it produced 25 percent chicken, 87 percent goose and 65 percent duck of the world's total yield.

In China, some 50 cases of bird flu involving poultry have been detected on the mainland since 2004, with 31 of these in 2005 alone, according to the Ministry of Agriculture.

Forty human cases were reported, including 26 deaths, statistics from the Ministry of Health showed.

The last human death happened in June in Hubei province and the infected woman tested positive for H5N1-2.3.2.1, said Shu Yuelong, director of the National Influenza Center of the Chinese Center for Disease Control and Prevention.

So far, going by all detected human bird flu cases worldwide, the primary risk factor for human infection appears to be direct or indirect exposure to infected live or dead poultry or contaminated environments, according to the World Health Organization.

"The poultry is safe to eat," said Guo, adding that the virus can be easily killed during cooking.

A new vaccine against the H5N1-2.3.2.1, developed mainly by the Ministry of Agriculture, has already entered clinical trials, he said.

But regional inspections showed that the inoculation rate among poultry that stayed in water, like ducks, was even lower than 50 percent.

Liu Baoyun, a duck raiser from Jiangsu province, told China Daily that he did not like his ducks vaccinated as "they produce less eggs after vaccination".