A brief introduction to the avian influenza virus.

The avian influenza (AI) virus is type A influenza isolated from and adapted to an avian host. Type A influenza belongs to the orthomyxovirdae virus family, is enveloped, and is pleiomorphic with a size ranging from 80-120 nm (reviewed in [1]). Type A influenza strains are classified by the serological subtypes of the primary viral surface proteins, the hemagglutinin (HA) and neuraminidase (NA). The HA has 16 subtypes (H1-H16) and contains neutralizing epitopes. Antibodies against the NA are not neutralizing, and there are nine neuraminidase or "N" subtypes. The "H" and N subtypes seem to be able to assort into any combination, and many of the 144 possible combinations have been found in natural reservoir species, although some combinations are more common than others. All 16 subtypes have been found in ducks, gulls, or shorebirds, the natural reservoir host species of the virus. However, in these species certain subtypes are more common than others; for example, H3, H4, and H6 are most common in ducks in North America [2, 3] and although there is no clear association between host range or host restriction based on HA subtype, some subtypes are more common in some species than others, i.e., H1 and H3 in swine, H3 in horses, and H5 and H7 in chickens.