Avian influenza virus surveillance and wild birds: past and present.

Influenza surveillance in wild birds has established that the aquatic birds of the world are the source of influenza A viruses, which occasionally spread to domestic avian species and to mammals, including humans, and cause mild to severe disease. With the realization that the pandemics of influenza in poultry and people originate from inapparent infections of aquatic birds, including the highly pathogenic H5N1 virus, much more attention has been given to understanding the ecology of influenza in wild aquatic birds. This article deals with the major events establishing the role of wild birds in the natural history of influenza and with some of the unresolved issues. These include 1) whether all H5 and H7 influenza viruses have high pandemic potential, 2) whether avian influenza (AI) is exchanged between Eurasia and the Americas, and 3) whether the highly pathogenic H5N1 AI virus is now being perpetuated in wild birds, one of the most important unresolved issues. Continued surveillance of wild birds for influenza is essential to resolve the many unanswered questions concerning the zoonotic spread of influenza and pandemicity.