Ducks, rice and human beings  -- and not chickens are the "most significant factors" behind persistent outbreaks of bird flu, a new study has claimed.

The study released by United Nations Food and Agriculture Organization (FAO) found ducks, rice and people  not chickens  to be the "most significant factors" in the spread of the bird flue in Thailand and Vietnam.

"Mapping H5N1 highly pathogenic avian influenza risk in Southeast Asia: ducks, rice and people" also concludes that these factors are probably behind persistent outbreaks in other countries such as Cambodia and Laos.

The study, which examined a series of waves of H5N1 highly pathogenic avian influenza in Thailand and Vietnam between early 2004 and late 2005, was initiated and coordinated by FAO senior veterinary officer Jan Slingenbergh and is published in the latest issue of the Proceedings of the National Academy of Sciences of the United States.

Through the use of satellite mapping, researchers looked at a number of different factors, including the numbers of ducks, geese and chickens, human population size, rice cultivation and geography, and found a strong link between duck grazing patterns and rice cropping intensity.  

In Thailand, for example, the proportion of young ducks in flocks was found to peak in September-October. These rapidly growing young ducks can therefore benefit from the peak of the rice harvest in November-December.

"These peaks in congregation of ducks indicate periods in which there is an increase in the chances for virus release and exposure, and rice paddies often become a temporary habitat for wild bird species," the agency said.

"We now know much better where and when to expect H5N1 flare-ups, and this helps to target prevention and control," said Slingenbergh. "In addition, with virus persistence becoming increasingly confined to areas with intensive rice- duck agriculture in eastern and south-eastern Asia, evolution of the H5N1 virus may become easier to predict."

The findings, he said, can help better target control efforts and replace indiscriminate mass vaccination.

FAO estimates that approximately 90 per cent of the world's more than 1 billion domestic ducks are in Asia, with about 75 per cent of that in China and Viet Nam. Thailand has about 11 million ducks.