By Will Dunham

WASHINGTON, March 26 (Reuters) - Ducks, people and rice paddies are the primary forces driving outbreaks of avian influenza in Thailand and Vietnam, and the number of chickens is less pivotal, scientists said on Wednesday.

U.N. Food and Agriculture Organization experts and others looked at three waves of H5N1 bird flu in Thailand and Vietnam in 2004 and 2005. The virus has killed 236 people in 12 countries since 2003.

They used computer modeling to study how various factors were involved in the spread of the virus, including the numbers of ducks, geese and chickens, human population size, rice cultivation and local geography.

Even though Thailand and Vietnam addressed the outbreaks in different ways, the researchers found that the numbers of ducks and people, and the extent of rice cultivation were the most important contributing factors underpinning the outbreaks.

"This provides better insight on where and when the H5N1 risk is highest, so it's possible to better pinpoint where to look for the virus or where to expect flare-up of disease and also when to expect it," Jan Slingenbergh, senior veterinary officer for the Food and Agriculture Organization, said in a telephone interview.

"It helps to better target the interventions," he added.

Monitoring duck populations for the H5N1 virus and tracking rice farming by satellite are the optimal ways to predict an outbreak's distribution, the researchers said. They added that their model also can be extended to Laos and Cambodia, where there are similar land use patterns.

Avian influenza has been closely linked to chickens in the past, but the study found the number of chickens to be less important as a predictor.

The findings were published this week in the journal Proceedings of the National Academy of Sciences.

"In the past in Vietnam, there have been major (bird) vaccination exercises countrywide, which is an enormous effort in terms of logistics and time and effort and staff requirements," Slingenbergh said.

"And there is fatigue, also, among the farmers and veterinarians. And if it's now possible to better time and localize the efforts, that is a major efficiency achievement."

The researchers said there are close ties between duck grazing patterns and rice cropping intensity. They said ducks feed mainly on leftover rice grains in harvested paddy fields, so free-ranging ducks may go to many different sites following rice harvest patterns.

The H5N1 avian flu virus has swept through flocks of poultry in Asia and sometimes in Africa and Europe. It has infected 373 people in 14 countries and killed 236 of them since 2003. Experts fear the virus may change just enough to pass easily from person to person, sparking a deadly pandemic. (Editing by Mohammad Zargham)

AlertNet news is provided by