A Chinese farmer has contracted the H5N1 strain of bird flu, the country´s first human case in months, leading to questions about whether the virus is circulating undetected among poultry.

The 37-year-old man surnamed Li from the eastern province of Anhui kept backyard birds, but as in other human bird flu cases in China there was no reported poultry outbreak in the area, raising questions as to how he contracted the virus.

"In China, the challenge is now to identify where this virus is hiding and how it is circulating," Henk Bekedam, the World Health Organization´s China representative, told Reuters.

The country has reported a total of 22 human cases of bird flu, including 14 fatalities, since 2003, and with the world´s largest poultry population and millions of backyard birds roaming free, it is seen as a center in the fight against the virus.

Bekedam said that as vaccination rates for birds improve in China, detecting avian influenza becomes harder and harder, offering a possible explanation for why there was no reported outbreak where Li lived.

"It´s not to say that the virus is not somehow still circulating, but that the detection of that circulating virus has become far much more difficult because of the active engagement of the government in avian influenza control," he said.

Li developed symptoms of fever and pneumonia on December 10 and was discharged from hospital on Saturday in Tunxi in Anhui province after a full recovery, the Health News, the Health Ministry-run newspaper, said.

The Chinese Center for Disease Control and Prevention confirmed on Monday that he tested positive for the deadly H5N1 strain, which scientists fear could mutate into a form that can be passed easily between people, leading to a potential pandemic.

People who have had close contact with Li were quarantined for medical observation until December 29, the Health News said, but there was no indication that others in the area had taken ill.

Bekedam said the WHO was still investigating whether any of Li´s birds had been sick, adding it was possible that they carried H5N1 but did not display any symptoms.

China has in the past been criticized for a lack of transparency in its handling of health threats, but Bekedam said the WHO was informed of Li´s case as soon as he tested positive and praised the fact that he was tested a second time after his first tests turned up negative.

"We think it´s very encouraging that there seems to be some routine mechanisms in place to determine whether someone is suffering from avian influenza or not, and it doesn´t stop after the first negative test," he said.

China last reported a human case of bird flu in July, when a farmer died of H5N1 in the northwestern region of Xinjiang. The most recent reported poultry outbreaks were in early October in the regions of Inner Mongolia and Ningxia.