Influenza A viruses belong to the family Orthomyxoviridae with the majority of viruses maintained in wild aquatic bird reservoirs,predominantly those of the anseriform species.
Classification of these viruses is determined by the two surface glycoproteins, the hemagglutinin (HA) and neuraminidase (NA) of which 16 HA and 9 NA subtypes have been isolated from aquatic birds
The migratory nature of the aquatic fowl reservoir resultsin the wide geographic spread and distribution of most circulat-ing subtypes.