The December outbreak was the first discovered at a Hong Kong poultry farm in six years, and raised fears about the city's biosecurity measures and whether the deadly H5N1 virus had mutated.

"As with many epidemiological studies of this nature, it is difficult to determine the exact cause of the outbreak," said Thomas Sit, the head of the government's investigation team.

But he said the virus was "most likely to have been introduced to the farm by wild birds".

The dust and dirt near the entrance of one of the two affected chicken sheds could have been contaminated by droppings from infected wild birds and then blown into the shed area by a gust of wind, Sit said.

He added that they could not totally eliminate other sources, such as rodents or contaminated clothing of farm staff.

Sit said the farmer, who ran one of the city's major poultry farms, had been warned to improve biosecurity measures, but would not face further action.

Cheung Siu-hing, director of city's agriculture department, said authorities would strengthen biosecurity measures for all Hong Kong poultry farms by increasing the number of inspections and testing of blood samples.

"It is difficult, if not impossible, to say we can 100 percent prevent the spread of avian flu. That's why we have to learn from each incident. We can't afford to be complacent," she told reporters.

The outbreak was discovered after a dead chicken was found at the farm in the New Territories area of Hong Kong, near the border with China.

All chickens within a three-kilometre (1.9-mile) radius of the farm were slaughtered, local farms were barred from selling chickens and eggs for 21 days and imports of chickens were banned for the same period.

Hong Kong was the scene of the world's first reported major bird flu outbreak among humans in 1997, when six people died. Since then, H5N1 has killed more than 250 people worldwide.

Scientists fear the virus could mutate into a form which is much more easily transmissible between humans, triggering a global pandemic that could kill millions.

Sit said genetic analysis showed the virus was of the type commonly found in southern China, but added it could have come from anywhere in Asia.

Hong Kong's Health Secretary, York Chow, said the city needed to stay on full alert against avian influenza which posed an imminent worldwide threat.

"To help achieve such a task, the biosecurity measures of local chicken farms will be vital to minimise the risk of avian influenza," Chow said in a statement.

The role of wild birds in the spread of bird flu is not yet fully understood and many experts say they have been unfairly scapegoated as a distraction from the more pressing risks of intensive poultry farming.

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