December 3, 2008

Mallards Affected By Avian Flu

In a broad series of tests on thousands of ducks migrating through Sweden, researchers found that avian flu makes mallard ducks thinner than other duck breeds.

Reporting in the Proceedings of the Royal Society B: Biological Sciences, Jonas Waldenstrom of Sweden's Kalmar University, Albert Osterhaus of Erasmus Medical Center in Rotterdam and colleagues found that the viruses do affect the birds.

Their findings contradict previous notions among experts that mallards were immune to avian flu.

Additionally, researchers found that the birds only "shed," or release, the virus for a few days.

"Mallard ducks are a main reservoir for low-pathogenic avian influenza virus in nature, yet surprisingly little is known about how infection affects these birds," said Waldenstrom.

"We analyzed 10,000 samples from migratory mallards in Sweden for presence of influenza virus and were able to demonstrate that infected birds were leaner than uninfected birds, and that weight loss was related to the amount of virus shed in their feces," he added.

"Although many mallard populations are migratory, the short virus shedding times (often less than a week) imply that individual birds are not long-distance dispersers of the virus on a continental scale."

Low-pathogenic avian influenza strains generally have little effect, although the highly pathogenic forms can wipe out flocks in a matter of days.

H5N1 avian influenza is currently being spread across Asia into parts of Europe and Africa. Scientists' worst nightmare is that H5N1 could mutate into a form that humans would be more susceptible to, resulting in the deaths of millions.

Since 2003, non-mutated H5N1 has infected 387 people and killed 245.

Waldenstrom's team found that infection did not affect how fast or far the birds migrated. On average, the ducks were infected eight days and spread the virus for just three of them in their droppings.

"The short virus shedding time suggests that individual mallards are less likely to spread the virus at continental or intercontinental scales," they wrote.


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