A1A South

WASHINGTON (AP) — 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.

Source: Associated Press

In a different story but covering similar material see the following story from Bloomberg:

March 24 (Bloomberg) — Scientists in the Netherlands tracking the spread of bird flu in wild ducks say mallards may be the best long-distance carrier of the deadly H5N1 virus.

Researchers at the Erasmus Medical Center in Rotterdam experimentally infected six wild-duck species with H5N1 to determine which were capable of excreting the virus without succumbing to the disease. Pochards and tufted ducks shed the most virus, though tend to become ill or die earlier, they said.

“Of the six wild duck species studied, the mallard is the prime candidate for being a long distance vector,” the researchers wrote in a study published in the April edition of Emerging Infectious Diseases. “It was the only species to show abundant virus excretion without clinical or pathologic evidence of debilitating disease.”

The study suggests mallards should be given priority in any surveillance for the H5N1 virus in wild ducks, the authors wrote. The H5N1 strain has spread to more than 60 countries since 2003 through trade in poultry and the movement of wild birds. It has infected people in 14 countries and world health officials say it may spark a pandemic if it becomes capable of spreading among humans through coughing and sneezing.

While the transport of infected poultry, contaminated equipment and people associated with the poultry industry has helped spread H5N1 among poultry flocks, wild birds are “suspected of playing a major role as long-distance” carriers of the virus, the researchers said.

Besides ducks, other water bird species, such as geese, swans and gulls also play a role in the epidemiology of avian flu, they said. There about 9 million mallards in Western Europe, making it the most abundant species in the family that includes ducks and duck-like waterfowl, according to the study.

Disease Link

“Part of the population migrates long distances northeast to southwest between breeding and wintering areas,” wrote the researchers, led by Juthatip Keawcharoen. “It is found on nearly every type of wetland and is very tolerant of human presence, thus forming a potential link between wild waterfowl, domestic animals and humans.”

Pochards and tufted ducks, along with mute swans, are more likely to act as sentinels for H5N1 in wild bird populations, the study found. Some of the pochards and tufted ducks studied developed neurologic disease that caused them to compulsively swim around in circles, the researchers said.

The virus’s ability to invade multiple tissues, including the brain, suggests tests on live birds should include swabs of the throat as well as the end of the digestive tract or cloaca, they said. Tests on wild duck carcasses should also probe internal organs such as brain, pancreas, liver, kidney and spleen, the authors found.

This is not really new information; that some ducks can carry the disease with little sign of illness has been known for several years now.  However, most (though not all) of the people that have been infected have had contact with sick or dying chickens.  Some of those that contracted the disease and subsequently died have had no known contact with poultry of any kind.

Keeping domestic ducks separated from wild ducks is quite a chore; I have gone outside to find my ducks happily swimming with migratory Canadian geese, and herons and other wading birds are frequent visitors as well.