WASHINGTON - Intensive rice farming and large duck populations — not the number of chickens raised — may be the best predictors of where bird flu might develop in Southeast Asia, according to researchers reviewing outbreaks in Vietnam and Thailand.

About 140 million birds in Southeast Asia have been killed in recent years to prevent the H5N1 virus from spreading. Researchers are trying to understand what factors have contributed to continued outbreaks despite significant control efforts.

By isolating those factors, policymakers can better target efforts to stem or prevent future outbreaks. For example, they could limit the movement of ducks into the rice paddies at certain times of the year, which would reduce the prospects of the virus being exchanged between domestic ducks and wild birds.

Researchers reviewed three outbreaks in early 2004 through late 2005. They looked at five variables: duck abundance, human population, chicken numbers, elevation and rice cropping intensity.

The researchers concluded that monitoring duck populations for H5N1 and tracking rice paddies by satellite were the best ways to predict where outbreaks were most likely to occur. They said that chickens are no longer a "highly significant predictor" of the presence of the H5N1 virus for Vietnam and Thailand.

"Essentially, (the virus) is so pathogenic in chickens that it kills them before they can spread it," said Marius Gilbert of the Free University of Brussels, Belgium.

The outbreaks were most concentrated in regions where rice is cultivated two or three times a year.

"Rice paddy fields are an important habitat of free-ranging ducks, but also for wild waterfowl exploiting the same food resource in the wintering season," the researchers said. "Thus, they may form a critical risk factor in ... virus introduction, persistence and spread."

The researchers described the predictive power of their models as "moderate." They also said that their work appeared to warrant development of maps in other Southeast Asian countries identifying those areas most susceptible to future bird flu outbreaks.

Since 2003, bird flu has killed at least 236 people. Although it has been hard for people to catch, experts worry the virus could mutate into a form that passes easily among humans, sparking a pandemic. So far, most human cases have been linked to contact with infected birds.

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