WASHINGTON, Dec 2 (Reuters) - Avian flu viruses make mallard ducks thinner than other ducks, a finding that implies they do not spread the germs over long distances, researchers reported on Tuesday. Their tests of thousands of ducks migrating through Sweden showed the viruses do affect the birds, contrary to conventional wisdom that the pathogens have no effect on them. And, to their surprise, they found the birds only "shed", or release, virus for a few days, the researchers reported in the Proceedings of the Royal Society B: Biological Sciences. "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," Jonas Waldenstrom of Sweden's Kalmar University, Albert Osterhaus of Erasmus Medical Center in Rotterdam and colleagues wrote. A reservoir is a species that hosts a virus without becoming ill, and thus serves to spread it. Avian flu viruses have most often been found in migratory waterfowl, especially mallard ducks. "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," Waldenstrom's team 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." There are hundreds of kinds of bird flu, and evidence suggests that human forms of influenza originate in birds. Low-pathogenic avian influenza strains generally have little effect, although the highly pathogenic forms can wipe out flocks in a matter of days. Highly pathogenic H5N1 avian influenza is currently affecting flocks in Asia, parts of Europe and Africa and experts fear it may mutate into a form that humans can catch and transmit easily. If it does, it could kill millions. Even in its current hard-to-catch form, H5N1 has infected 387 people and killed 245 since 2003. Researchers do not know precisely how it spreads, although migrating birds are prime suspects, as is the poultry trade. 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. But they may stay longer in one place when they are infected -- something that needs to be studied, they added. (Reporting by Maggie Fox, Editing by Anthony Boadle)
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