(Reuters)
Updated: 2005-11-16 17:14

Vietnamese scientists have found more variations of bird flu viruses in poultry, adding to the health risk to people if these strains are allowed to keep circulating, a health official said on Wednesday.

Dong Manh Ha, director of Ho Chi Minh City Regional Animal Health Center, told Reuters on Wednesday a study conducted by the center found new avian influenza strains of the H3 and H4 subtypes in poultry.

Scientists say strains of the H3 and H4 subtypes were capable of causing death in birds but are generally less virulent and spread more slowly than the H5 subtype they fear could trigger a human pandemic.

The H5N1 virus is endemic in poultry in several countries in Asia and has killed 64 people, including 42 in Vietnam, since late 2003. Experts fear that H5N1 could mutate into a form that passes easily among people, triggering a pandemic.

"The presence of more subtypes of the flu virus in poultry make the virus all the more dangerous," Ha said, adding that samples had been sent to a World Organization for Animal Health (OIE) laboratory in Australia for further investigation.

State media reports said the new virus strains are H3N4 and H4N5.

Vietnam is tackling more than a dozen H5N1 outbreaks in poultry and has stepped up culling in its two main cities. The virus is highly virulent and can kill poultry in a matter of hours. It has killed about half the people it has infected.

Hong Kong infectious disease expert Lo Wing-lok said that with heightened surveillance, it was not surprising for them to find other subtypes, such as H3 and H4, though these are not generally thought of as being highly pathogenic.

But Lo, a member of scientific committee on zoonotic and emerging diseases in Hong Kong, said if the strains are allowed to spread in poultry, people who are in close contact with these birds would be at risk.

There was also the risk of these subtypes mixing and exchanging genetic material with the H5N1 virus. There would then be the possibility of the H3N4 and H4N5 strains becoming more pathogenic.

Influenza type A viruses, which include human and avian flu viruses, are categorized by H and N components, which denote specific types of proteins on their surface.

The H component governs the ability of the virus to bind to and enter cells, which become virus-making factories. The N component governs the release of the newly made viruses from the animal host cell.

There are 16 H subtypes and nine N subtypes. Human influenza viruses are usually strains of H1N1, H2N2 and H3N2 but avian flu subtypes H5 and H7 can also infect people.

Thirteen of Vietnam's 64 provinces have now had outbreaks of H5N1 since the virus reappeared last month.

Media reports said two more people in Hanoi, a 15-year-old girl and a 71-year-old woman, were in hospital with symptoms of bird flu and were being tested. The latest cases bring to five the number of suspected infections in Vietnam.

The Agriculture Ministry said the H5N1 virus had hit 3 more provinces -- the central province of Quang Ngai and northern provinces of Vinh Phuc and Bac Ninh.