Chickens may play less of a role in the spread of H5N1 highly pathogenic avian influenza in Thailand and Viet Nam than ducks, people and rice paddies. The same may also be true in Cambodia and Laos. 
No ducking the issue: Ducks in paddy fields, not chickens, could be responsible for the bird flu epidemic in Southeast Asia
Photo: Jeff Jones
These findings come from scientists at the UN Food and Agriculture Organization (FAO) and associated research centers and are published in a report entitled Mapping H5N1 highly pathogenic avian influenza risk in Southeast Asia: ducks, rice and people.
Many countries that have experienced outbreaks of H5N1 have successfully controlled the virus, while others face periodic recurrence, despite significant control efforts. The study found that the abundance of ducks, particularly free-grazing ducks in areas of intensive rice cultivation, was an important risk factor for H5N1.
The researchers modeled how different factors contributed to the spread of the highly pathogenic virus in Thailand and Viet Nam. The numbers of ducks and people, along with the extent of rice cultivation, were the most significant factors, even though each country had pursued its own disease control strategy.
Ducks graze on leftover rice grains in harvested paddy fields. In Thailand, the number of young ducks in flocks peaks in September to October, when rice paddies become a haven for wild birds. Ducks are slaughtered in late winter at the time of Chinese New Year, when sales-related duck movement is at its highest. The peaks in duck congregation increase the likelihood of virus transmission.
Also in 2005, Viet Nam began a nationwide vaccination program, repeated in 2007-2008. The program included the Mekong delta where 50 million birds live. At first, human H5N1 infections dropped and poultry infections fell. However, the virus has slowly re-appeared, mainly in the Mekong delta and among unvaccinated ducks.
The study was initiated and coordinated by Jan Slingenbergh, an FAO senior veterinary officer. He commented: "We now know much better where and when to expect H5N1 flare-ups and this helps to target prevention and control. In addition, with virus persistence becoming increasingly confined to areas with intensive rice-duck agriculture in eastern and southeastern Asia, evolution of the H5N1 virus may become easier to predict." He also felt that interventions based on knowledge of hotspots and local rice-duck calendars is vital to target disease control measures and replace mass vaccination.
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