ABSTRACT
Inuenza viruses are classied as members of the family Orthomyxoviridae, which comprises ve genera: Inuenzavirus A, Inuenzavirus B, Inuenzavirus C, Thogotovirus, and Isavirus. Thogotovirus includes Thogoto virus and Dhori virus, whereas Isavirus includes infectious salmon anemia virus (ISAV). Three types of human inuenza viruses have been recognized (types A, B, and C), on the basis of their type-specic nucleoprotein and matrix protein antigens. Type A inuenza viruses are further classied into subtypes according to the antigenic properties of the hemagglutinin (HA or H) and neuraminidase (NA or N) glycoproteins expressed on the surface of the virus. The present nomenclature system of inuenza viruses encompasses the type of virus, the species from which the virus was isolated (except if human), the location of isolate, the number of the isolate and the year of isolation; in the case of inuenza A viruses, in addition to the hemagglutinin and neuraminidase subtypes. For example, the 15th isolate of a H1N1 subtype of inuenza A virus isolated from pigs in Iowa in 1930 is designated: inuenza A/swine/ Iowa/15/30(H1N1). Currently, 16 HA subtypes (H1-H16) and 9 NA subtypes (N1-N9) are recognized in the nomenclature system for inuenza A viruses recommended by the World Health Organization [1-3]. All these subtypes have been found circulating in wild and domestic birds. Thereby, avian hosts are the major reservoirs for all subtypes, so far, only three types of HA (H1, H2, H3) and two types of NA
(N1, N2) have been widely prevalent in humans (H1N1, H2N2, H3N2). Only two of these viruses (H1N1 and H3N2) are currently circulating as seasonal inuenza. H2N2 has not circulated in humans since 1968.
