Since 2003, the global panzootic of highly pathogenic avian influenza A (H5N1) among domestic poultry and wild birds has resulted in rare, sporadic, human H5N1 cases of severe respiratory disease with high mortality in Asia, Europe, the Middle East, and Africa. Family clusters of H5N1 cases have been documented, and though most transmission of H5N1 viruses to humans is believed to be directly from sick or dead birds, limited human-to-human transmission of H5N1 viruses has been reported. As H5N1 viruses continue to evolve, the concern for a global influenza pandemic rises.
Highly pathogenic avian influenza A (H5N1) viruses are single-stranded negative-sense RNA viruses of the Orthomyxoviridae family, whose natural reservoir is in wild aquatic ducks and geese. Influenza A viruses are subtyped on the basis of the two major surface glycoproteins, hemagglutinin (HA), and neuraminidase (NA). Avian influenza A viruses include all 16 known HA and nine known NA subtypes.
Avian influenza is a disease of birds caused by infection with avian influenza A viruses that infect the respiratory and gastrointestinal tracts. Birds excrete avian influenza A viruses in feces, and the virus can remain viable for prolonged periods in the setting of low temperatures, low humidity, and abundant fecal protein matter. H5N1 virus infections of other animals, including pigs, cats, dogs, civet cats, a stone marten, tigers, and leopards have also been reported.