It remains unclear whether there are truly distinctive seasonal patterns to outbreaks of the bird flu in poultry, said an official with the World Health Organization (WHO) on Friday.

"We do know that the bird flu virus can survive for a time in colder weather, but it's really not clear at this point whether the virus is changing in such a way that it can survive in warm weather for a longer period than it was previously able to," said Roy Wadia, spokesman of the WHO office in China.

Hong Kong's health chief, York Chow, said on Friday that the latest human case of bird flu on the Chinese mainland might indicate the virus has mutated and become as infectious in summer months as it is in cooler months.

"Much more research needs to be carried out on this," Wadia said.

Virus changes all the time and these changes could make the virus more transmittable between poultry, from poultry to humans, and from humans to humans, he told Xinhua in an interview.

"This is why it is so important that agricultural authorities everywhere -- including the Ministry of Agriculture of China -- share virus isolates from animal outbreaks with the international scientific community," he said.

Genetic sequences of H5N1 virus samples have been shared by the Chinese Ministry of Agriculture which is helpful, but isolates of virus are key to understanding how the virus is changing on the Chinese mainland, said Wadia.

A 31-year-old man in Shenzhen, South China's Guangdong Province, was confirmed on Thursday to have contracted bird flu, bringing the country's total human infections of the disease to 19.

He showed symptoms of fever and pneumonia on June 3 and is in critical condition, said the Ministry of Health in a report.

Epidemiological research found Jiang had been to a local market where live poultry are sold, several times before developing the symptoms.

The Hong Kong government on Thursday decided to stop poultry imports from the Chinese mainland and Macao announced on Wednesday it would halt the import of live poultry from Shenzhen.

"We are concerned if the virus has changed to become infectious all year round or even more virulent in winter," Hong Kong's Health, Food and Welfare Secretary, York Chow, told the media on Friday.

Source: Xinhua