Ducks, rice, people add up to risk of H5N1 outbreaks

Mar 26, 2008 (CIDRAP News) – High concentrations of ducks, rice fields, and human populations—rather than chickens—pose the highest risk of sparking deadly H5N1 avian influenza outbreaks in Southeast Asia, according to a recent study.

Researchers affiliated with the United Nations Food and Agriculture Organization (FAO) collected data from three waves of hundreds of avian flu outbreaks in Vietnam and Thailand in 2004 and 2005, and then used statistical modeling to determine what factors most contributed to infection and spread of the disease.

They looked at known H5N1 risk factors that provide "relatively robust risk prediction": human population size, rice cultivation intensity, local elevation, and numbers of ducks, geese, and chickens. Using satellite mapping of rice paddies, they were able to compare rice production and poultry and human concentrations to outbreak timelines.

They found that human population concentration and high rice production were both strong risk factors for H5N1 outbreaks, as were numbers of both free-ranging and farm ducks.

"Chicken numbers," the study found, "failed to show as a significant predictor of disease presence, except in wave 3 in Vietnam." Researchers attributed that exception to the fact that the third Vietnamese wave, in late 2005, primarily affected the chicken-dense Red River delta area of southern Vietnam.

The study's authors, writing this week in Proceedings of the National Academy of Sciences, explained that free-range duck farming in these two countries typically takes place in areas with high rice yield "because the rice grain left in the field after harvest provides a low-cost source of feed for duck production."

In an FAO news release today, study coordinator and coauthor Jan Slingenbergh, an FAO senior veterinary officer, described the study's import. "We now know much better where and when to expect flare-ups, and this helps to target prevention and control," he said. "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."

Ninety percent of the world's 1 billion domestic ducks live in Asia, according to the FAO release.

Applying findings to other countries
The authors of the study wrote that the sound predictive power of their statistical model "supports its application to other countries with similar agro-ecological conditions such as Laos or Cambodia."

The researchers explained that their model applied equally well to both Thailand and Vietnam, despite the countries' different approaches to disease containment. Thailand focused on early disease detection and on restricting the movement of ducks, whereas Vietnam initiated aggressive nationwide vaccination of all poultry. The differing approaches succeeded at the time, but both countries have experienced fairly recent recurrences of bird outbreaks, and Vietnam has had recent human cases.

"It is thus quite remarkable," the authors wrote, "that this model based on Thailand data loses so little predictive power when validated in Vietnam, which gives confidence to extrapolating the model to adjacent areas."

They encourage researchers to apply their technique to other areas with high duck concentrations as well, such as the Nile Delta region in Egypt and parts of Nigeria. They also explored applying their model to Indonesia but urged caution with that country because of the large differences in climate, agriculture, and avian flu situation between Indonesia and mainland Southeast Asia.

Gilbert M, Xiao X, Pfeiffer DU, et al. Mapping H5N1 highly pathogenic avian influenza risk in Southeast Asia. Proc Natl Acad Sci 2008 Mar 25;105(12):479-74 [Full text]

See also:

Mar 26 FAO news release


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