Evolution of Influenza a Viruses in Wild Birds

The emergence of highly pathogenic (HP) H5 influenza A viruses in Asia and the increase of HP H7 viruses in Europe and the Americas focused greater attention on the ecology of influenza in wild birds. Influenza virus surveillance studies in wild bird populations in the Americas, Europe, and Asia confirmed that wild aquatic birds are the reservoir for all known influenza A viruses. Phylogenetic analysis groups the influenza viruses in wild aquatic birds into two distinct superfamilies—one in the Americas and one in Eurasia. The separation of viruses into American and Eurasian clades implies that transmission of HP H5 into the Americas by wild birds is likely to be a rare event. The rapid evolution of the Eurasian H5N1 viruses makes them a continued threat to poultry and humans worldwide.