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BEIJING, March 29 (Reuters) - A Chinese teenager has died from bird flu, state media reported on Thursday, marking the country's third human infection from the virus this year.

The 16-year-old boy from Bengbu in the rural eastern province of Anhui died late on Tuesday, Xinhua news agency reported, citing a provincial health official.

The boy, surnamed Wu, went into hospital on March 18 after falling ill with a fever and signs of pneumonia. Tests by the nation's Centre for Disease Control and Prevention later confirmed he had H5N1, which can spread from birds to humans through close contact.

But medical authorities appeared to have little idea how the boy encountered the virus, raising questions about how effectively China, with its huge population and threadbare rural healthcare, can track its spread through birds and people.

China, with the world's largest poultry population and a strained medical system, is seen as key to the fight against bird flu.

Initial investigations failed to reveal that the teenager had any "history of exposure to fowl that died because of illness", Xinhua reported. And animal health experts have also failed to find signs of the virus among birds in the area.

The World Health Organisation was notified of the death, as were Hong Kong and some other governments, the report said.

"My understanding is that at this stage there's no known contact with poultry and there'll have to be more work done," said Joanna Brent, a spokeswoman for the WHO office in Beijing, said of the death.

Late last year, Bengbu held a training drill to raise preparedness for a bird flu outbreak, local papers reported at the time. The city and its surrounding countries has a population of about 3.5 million, many of them farmers. The reports on Wu's death did not say whether he came from a farming area.

China has now reported a total of 24 human cases of H5N1, including 15 deaths, since 2003. It last reported a human death from bird flu in July last year, when the virus killed a farmer in the western region of Xinjiang.

The latest case brings total worldwide deaths from bird flu to 170, mostly in Vietnam and Indonesia, based on WHO figures.

The virus has now spread to more than 50 countries, raising experts' fears it could mutate into a form that can easily pass from person to person, risking a pandemic.

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