The latest human bird flu infection on the Chinese mainland is worrying as it shows the H5N1 virus may have mutated and become as infectious in warm months as in cooler ones, Hong Kong's health chief said on Friday.

The virus thrives in lower temperatures and is usually most infectious in the cooler months between October and March.

But confirmation on Thursday that a 31-year-old truck driver in the southern city of Shenzhen has been infected has caused uneasiness.

"Is this because the virus has changed, so that it is highly infectious all year round? Or, if it is happening in summer, winter would be even worse?" said the Secretary for Health, Welfare and Food, York Chow.

He said the virus might have become "more virulent and spread wider than we've expected," though its mutation was not confirmed.

"If that is the case, the risk for humans to be infected in the future is higher," he added.

The truck driver was admitted to hospital and was critically ill on Friday. He had visited a market where live poultry was sold and eaten chicken before he fell ill. But he is not known to have had any other close contact with poultry.

University of Hong Kong microbiology head Yuen Kwok-yung said the Shenzhen case was abnormal and worried the disease would spread in winter.

"If there are human infections from June to August, it means the virus is extremely active. I am worried that a major outbreak will happen in winter," he said.

In neighbouring Shenzhen, authorities have stepped up virus prevention and surveillance efforts.

The local government said it will now report the situation relating to human bird flu cases every day.

The Shenzhen Centre for Disease Control and Prevention has been asked to enhance its surveillance of any pneumonia-like cases.

So far the city has not reported any poultry infections.

But vendors said they are required to disinfect shelves twice a day and stop on-the-spot slaughtering. Some supermarkets have stopped selling live chickens.

"Business is really bad. I didn't even sell one chicken today," said a vendor at a Xiangmei Road market.

(China Daily June 17, 2006)