Three types of influenza viruses exist (A, B, and C), but influenza A and B viruses are the two that most commonly infect and cause disease in humans. Of these, influenza A viruses are the most important type of influenza, in terms of both medical and economic relevance. In addition to humans, influenza A viruses infect a wide range of species, including pigs, chickens, turkeys, horses, marine animals, dogs, cats, and migratory waterfowl.
Influenza A viruses are further divided into different subtypes, designated by the forms of the two surface glycoproteins they contain: the hemagglutinin (HA) and the neuraminidase (NA). (see Figure 1 for a schematic drawing of an influenza A virus). The HA is responsible for binding to specific receptors on cells, initiating the infection cycle. The NA functions primarily to release newly formed virions from the surface of infected cells in the last step of the infection cycle. There are 16 subtypes of HA and 9 subtypes of NA, and the combination present in any virus strain defines its subtype. For example, the two most common human influenza A viruses are H1N1 and H3N2.
The mammalian hosts of influenza virus infection have historically been regularly infected only with certain subtypes of influenza. For instance, only H1N1, H2N2 and H3N2 viruses have commonly circulated among human beings, only H7N7 and H3N8 viruses have circulated widely among horses (and recently dogs for H3N8 viruses), and only H1N1, H1N2, and H3N2 viruses have regularly infected pigs. In contrast, all 16 HA and all 9 NA subtypes can be found in various combinations among viruses in the wild bird population. Wild waterfowl are thus the global reservoir for influenza viruses.
For more general information regarding influenza viruses, see References 1 and 2 below.
1. Wright, P.F., Webster, R.G. 2001. Orthomyxoviruses, p. 1533-1579. In Field's Virology. Lippincott-Raven Publishers, Philadelphia.
2. Webster, R. G., Bean, W.J., Gorman, O.T., Chambers, T.M., and Kawaoka, Y. 1992. Evolution and ecology of influenza A viruses. Microbiological Reviews 56: 152-79Back to Top