Bird flu experts are warning the deadly H5N1 bird flu strain could re-emerge in Thailand even though there has been little sign of it for years.

Migratory birds and pet dogs and cats could possibly transmit avian influenza from animal to human without stringent disease surveillance, Pilaipan Puthavathana, a virologist at Siriraj Hospital's department of microbiology, said yesterday.

"Nobody dares to say the virus has been wiped out here as long as our neighbouring countries continue to report bird flu cases," she said in a summary of a three-year follow up on avian flu surveillance from animals to humans in Thailand.

The country has not had a bird flu case in humans since July 2006. A total of 27 cases were confirmed with 17 deaths reported during a series of bird flu outbreaks from 2004 to 2006.

In contrast, Vietnam has confirmed 119 bird flu cases in humans to date, with 56 fatalities. The latest two cases were reported in its northern province of Bac Kan in April.

The initial epidemiological investigations showed there were sick and dead poultry in surrounding areas. The patients' families slaughtered the sick poultry to eat.

Egypt was also still reporting bird flu outbreaks, although the mortality rate was less than in the Southeast Asian region, she said.

Ms Pilaipan said seagulls could contribute to the risk of H5N1 virus re-emerging in Thailand as flocks of the birds usually migrate from Qinghai and Xinjiang provinces in China, where bird flu outbreaks were still being reported. Samut Prakan's Bang Pu district contains the country's largest habitat for migratory gulls. But there have been no reports of the avian flu in the district so far.

The virologist said disease surveillance among pet dogs and cats was essential if bird flu virus re-emerged in the future.

Studies had found H5N1 antigens in cats and dogs. These animals could survive bird flu but transmit the virus to their owners.

"We should not only focus on disease surveillance among poultry but also pets if avian flu occurs in the future," she said.

Jacqueline Katz, chief of the immunology and pathogenesis branch of the US Centers for Disease Control and Prevention's influenza division, said there was no significant evidence showing a reassortment or antigenic change of seasonal flu and avian influenza or virus monitoring worldwide.

Also vaccination remained an effective prevention measure against the virus.