WASHINGTON (Reuters) - Ducks, people and rice paddies arethe primary forces driving outbreaks of avian influenza inThailand and Vietnam, and the number of chickens is lesspivotal, scientists said on Wednesday.

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

They used computer modeling to study how various factorswere involved in the spread of the virus, including the numbersof ducks, geese and chickens, human population size, ricecultivation and local geography.

Even though Thailand and Vietnam addressed the outbreaks indifferent ways, the researchers found that the numbers of ducksand people, and the extent of rice cultivation were the mostimportant contributing factors underpinning the outbreaks.

"This provides better insight on where and when the H5N1risk is highest, so it´s possible to better pinpoint where tolook for the virus or where to expect flare-up of disease andalso when to expect it," Jan Slingenbergh, senior veterinaryofficer for the Food and Agriculture Organization, said in atelephone interview.

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

Monitoring duck populations for the H5N1 virus and trackingrice farming by satellite are the optimal ways to predict anoutbreak´s distribution, the researchers said. They added thattheir model also can be extended to Laos and Cambodia, wherethere are similar land use patterns.

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

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

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

"And there is fatigue, also, among the farmers andveterinarians. And if it´s now possible to better time andlocalize the efforts, that is a major efficiency achievement."

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

The H5N1 avian flu virus has swept through flocks ofpoultry in Asia and sometimes in Africa and Europe. It hasinfected 373 people in 14 countries and killed 236 of themsince 2003. Experts fear the virus may change just enough topass easily from person to person, sparking a deadly pandemic.

(Editing by Mohammad Zargham)