The two latest confirmed cases of human bird flu in Thailand might be human-to-human transmissions, a senior health official has said.

Dr Charoen Chuchottaworn, a bird-flu expert at the Public Health Ministry's Department of Medical Services, said doctors concluded after reviewing the history of the past two cases that both victims presented very mild symptoms of avian influenza and neither had any physical contact with chickens or birds.

One of the victims was a boy in Bangkok and the other was an 18-year-old man from Nonthaburi province, The Nation newspaper reported Friday.

This left doctors no clues as to where the patients became infected with the H5N1 virus and showed that the avian influenza had moved from causing severe human infection to milder cases.

Charoen, who is also a member of the national committee issuingguidelines for the treatment of avian influenza, made the remarks Thursday at the Joint International Tropical Medicine Meeting 2005 in Bangkok.

Dr Kamnuan Ungchusak, director of the Epidemiology Bureau, challenged Charoen's assertion about human-to-human transmissions.

He told The Nation that while neither of the patients had direct contact with chickens, they lived in an environment where the virus was prevalent.

"Chickens were dying near their homes and chicken droppings were everywhere around their neighborhood," he said. "They might have contracted the virus through contaminated soil."

Dr Charoen said the milder the symptoms, the harder it is for doctors to diagnose. This means that a lot more advanced laboratory facilities are needed with a testing technique called RT-PCR to confirm cases and decide if patients should be treated with antiviral Tamiflu (oseltamivir phosphate).

He said this meant that avian influenza could become asymptomatic now.

The only tool available in Thailand to fight H5N1 infections atthe moment is insufficient, he said. At present, Thailand has about one million capsules for 100,000 treatments of Tamiflu, but it is estimated that about 120 million capsules of the drug will be needed.

In the past, only severe cases of human bird flu have been detected in Thailand simply because patients went to hospital for treatment. But doctors believe that there have been many cases with mild symptoms of the disease.

"We believe that this is the tip of the iceberg," he said.

Signs of possible human-to-human transmission were closely observed in Vietnam, where 10 clusters of probable human transmissions were detected in which the victims had no contact with infected poultry, Charoen said.

Thailand and Indonesia had one official cluster, he said, but the Indonesian cluster showed clear-cut evidence because a child contracted H5N1 without going to an infected area, as her father had.