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WASHINGTON (Reuters) - Wild ducks that are immune to the effects of H5N1 avian influenza could be spreading the virus far and wide, U.S. government researchers said on Monday.Satellite tracking of migrating northern pintail ducks showed they flew from a bird flu-infected marsh in Japan to nesting areas in Russia, said the scientists from the U.S. Geological Survey and the University of Tokyo said.The study does not prove the pintails carried the virus, but the species can be infected with H5N1 with no ill effects.H5N1 bird flu has been circulating in Asia and the Middle East, with occasional outbreaks in Europe, since 2003. It rarely infects people but when it does it is deadly: the World Health Organization has documented 493 cases and 292 deaths.It wipes out chickens, who have no immunity, and some other bird species and can seriously damage poultry farms. Experts fear it has the potential to cause a human flu pandemic that would be much worse than the H1N1 swine flu pandemic.Experts have argued about whether wild birds, spread the virus, or the poultry trade, or both.Writing in the journal Ibis, the researchers described how they attached satellite transmitters to 92 northern pintail ducks several months before the H5N1 virus was discovered in dead and dying whooper swans in a wetlands in Japan.Twelve percent of marked pintails used the same wetlands as infected swans. Then some of them migrated more than 2,000 miles to nesting areas in eastern Russia.Birds can spread flu viruses orally and in their droppings."Consequently, infected wild birds that do not become ill, or birds that shed the virus before they become ill, may contribute to the spread of H5N1," said Jerry Hupp of the USGS.USGS scientists have been testing birds in Alaska, considered a potential place where H5N1 could enter the Americas from Asia. So far, no case of highly pathogenic H5N1 has been found in either birds or people in the Americas.