Influenza A virus causes influenza in birds and some mammals, and is the only species of the genus Alpha Influenza virus of the virus family Orthomyxoviridae. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon. Some isolates of influenza A virus cause severe disease both in domestic poultry and, rarely, in humans. Occasionally, viruses are transmitted from wild aquatic birds to domestic poultry, and this may cause an outbreak or give rise to human influenza pandemics.

Influenza A viruses are negative-sense, single-stranded, segmented RNA viruses. There are 18 different known H antigens (H1 to H18) and 11 different known N antigens (N1 to N11). H17N10 was isolated from fruit bats in 2012. H18N11 was discovered in a Peruvian bat in 2013.

Each virus subtype has mutated into a variety of strains with differing pathogenic profiles; some are pathogenic to one species but not others, some are pathogenic to multiple species.

A filtered and purified influenza A vaccine for humans has been developed and many countries have stockpiled it to allow a quick administration to the population in the event of an avian influenza pandemic. Avian influenza is sometimes called avian flu, and colloquially, bird flu. In 2011, researchers reported the discovery of an antibody effective against all types of the influenza A virus.

Variants and subtypes

Influenza type A viruses are RNA viruses categorized into subtypes based on the type of two proteins on the surface of the viral envelope:

• H = hemagglutinin, a protein that causes red blood cells to agglutinate.
• N = neuraminidase, an enzyme that cleaves the glycosidic bonds of the monosaccharide sialic acid (previously called neuraminic acid).

The hemagglutinin is central to the virus's recognizing and binding to target cells, and also to its then infecting the cell with its RNA. The neuraminidase, on the other hand, is critical for the subsequent release of the daughter virus particles created within the infected cell so they can spread to other cells.

Different influenza viruses encode for different hemagglutinin and neuraminidase proteins. For example, the H5N1 virus designates influenza A subtype that has a type 5 hemagglutinin (H) protein and a type 1 neuraminidase (N) protein. There are 18 known types of hemagglutinin and 11 known types of neuraminidase, so, in theory, 198 different combinations of these proteins are possible.

Structure and genetics

Influenza type A viruses are very similar in structure to influenza viruses types B, C, and D. The virus particle (also called the virion) is 80–120 nanometers in diameter such that the smallest virions adopt an elliptical shape. The length of each particle varies considerably, owing to the fact that influenza is pleomorphic, and can be in excess of many tens of micrometers, producing filamentous virions. Confusion about the nature of influenza virus pleomorphy stems from the observation that lab adapted strains typically lose the ability to form filaments and that these lab adapted strains were the first to be visualized by electron microscopy. Despite these varied shapes, the virions of all influenza type A viruses are similar in composition. They are all made up of a viral envelope containing two main types of proteins, wrapped around a central core.

The two large proteins found on the outside of viral particles are hemagglutinin (HA) and neuraminidase (NA). HA is a protein that mediates binding of the virion to target cells and entry of the viral genome into the target cell. NA is involved in release from the abundant non-productive attachment sites present in mucus as well as the release of progeny virions from infected cells. These proteins are usually the targets for antiviral drugs. Furthermore, they are also the antigen proteins to which a host's antibodies can bind and trigger an immune response. Influenza type A viruses are categorized into subtypes based on the type of these two proteins on the surface of the viral envelope. There are 16 subtypes of HA and 9 subtypes of NA known, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.

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