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Waterfowl are natural reservoir hosts for influenza viruses, and migratory birds can carry viruses into new regions. Two recent studies address these issues.
To investigate the ecology of influenza A virus in birds in Europe, Wallensten and colleagues caught wild waterfowl on a Swedish island from 2002 through 2005. They obtained cloacal-swab or fecal samples from 4800 birds of 16 species, screened them for influenza A viruses, and characterized the viruses found. Most samples (85.5%) were from mallards. Overall, the prevalence of influenza A virus in the sampled birds was 12.5%, with 95.8% of the positive samples coming from mallards. Prevalence was highest in October 2005 (25.7%) and lowest in spring 2005 (0%). Juvenile mallards were more …