Ecology and evolution of influenza viruses in wild and domestic birds

Aquatic birds host the widest range of influenza viruses, with hemagglutinin subtypes H1-H16 and neuraminidases N1-N9 resident in these animals. Aquatic birds are the natural reservoirs of influenza A viruses and, directly and indirectly, are the source of influenza viruses in other animals. Worldwide surveillance since the 1970s confirms that influenza viruses are perpetuated in resident, migratory, and domestic aquatic birds. Large-scale poultry farming has altered the influenza ecosystem, with vast numbers of host birds kept at high population densities. This has led to multiple lineages of H9N2 and H5N1 viruses becoming endemic in domestic birds, increasing the importance of monitoring the interactions of influenza viruses in wild and domestic birds. Sporadic human infections with the H9N2 and highly pathogenic H5N1 viruses from domestic or wild birds create a persistent pandemic threat.