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.