A Vietnamese man feeds ducks (file photo)

The combination of ducks, people and rice paddies may be the biggest reason for the spread of avian influenza in Southeast Asia.

Researchers working with the United Nations Food and Agriculture Organization studied a series of avian flu outbreaks in Thailand and Vietnam between 2004 and 2005. They say almost all the outbreaks can be traced to ducks that fed near rice paddies that had just been harvested.

They say infected ducks then spread the virus to wild birds that also came to the rice paddies to feed.

Researchers say limiting the movement of domestic ducks and vaccinating ducks helped to curb the spread of the virus.

The findings are published in the Proceedings of the National Academy of Sciences of the United States (PNAS).

Avian flu is often associated with chickens, but the research found chickens die from the virus too quickly to effectively spread the disease.

Avian flu has killed nearly 240 people across Asia since it was first detected in 2003. It also is blamed for the deaths of more than 140 million domestic birds in Southeast Asia, costing the region about $10 billion.