Ecology and epidemiology of avian influenza in North and South America.

Wild waterfowl and shorebirds are known to be the natural reservoir for influenza A viruses. Surveillance studies in waterfowl and shorebirds in North America show that influenza A viruses are repeatedly recovered from these birds. However, the virus recovery is influenced by geography, season, age and species of birds. In addition to the natural reservoir, the live-bird marketing system (LBMS) in certain regions of the United States has been recognized as a man-made reservoir of influenza viruses and has been linked to several outbreaks of low pathogenicity avian influenza (LPAI) in poultry. Outbreaks of LPAI in commercial poultry is attributed to movement of infected birds, dirty or improperly cleaned crates, and contaminated vehicles from the LBMS to poultry farms. However, in the majority of outbreaks in poultry, the source of infection is suspected to be wild aquatic birds or the source is unknown. Since 2002, three outbreaks of highly pathogenic avian influenza (HPAI) have occurred in the Americas; one each in Chile (H7N3), United States (H5N2), and Canada (H7N3). In each of these outbreaks, a precursor virus of low pathogenicity mutated to become highly pathogenic after circulating in poultry. The HPAI viruses recovered from the three outbreaks had unique molecular and phenotypic characteristics that do not conform to other known HPAI viruses. These findings emphasize the need for monitoring wild and domestic bird species for presence of influenza A viruses.