Influenza A virus

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Influenza A virus subtype H5N9
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Influenza A virus
Structure of influenza A virus
Transmission electron micrograph of influenza A viruses (light objects on a dark background).
TEM
micrograph of influenza A viruses
Virus classification Edit this classification
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum:
Negarnaviricota
Class: Insthoviricetes
Order: Articulavirales
Family: Orthomyxoviridae
Genus: Alphainfluenzavirus
Species:
Influenza A virus
Subtypes

See text

Influenza A virus (IAV) is a pathogen that causes the flu in birds and some mammals, including humans.[1] It is an RNA virus whose subtypes have been isolated from wild birds. Occasionally, it is transmitted from wild to domestic birds, and this may cause severe disease, outbreaks, or human influenza pandemics.[2][3][4]

Each virus subtype includes a wide variety of strains with differing pathogenic profiles; some may cause disease only in one species but others to multiple ones. Because the viral genome is segmented, subtypes are neither strains nor lineages, as the subtype designation refers to proteins encoded by only two of the eight genome segments.

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. In 2011, researchers reported the discovery of an antibody effective against all types of the influenza A virus.[5]

Variants and subtypes

Diagram of influenza nomenclature

Influenza A virus is the only

proteins on the surface of the viral envelope:[citation needed
]

H = , a feature that is physiologically irrelevant to the virus in vivo.
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.[citation needed]

Different influenza virus genomes encode different hemagglutinin and neuraminidase proteins. For example, the H5N1 virus designates an 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.[7][8]

Some variants are identified and named according to the isolate they resemble, thus are presumed to share lineage (example

human flu virus); according to their subtype (example H3N2); and according to their deadliness (example LP, low pathogenic). So a flu from a virus similar to the isolate A/Fujian/411/2002(H3N2) is called Fujian flu, human flu, and H3N2 flu.[citation needed
]

Variants are sometimes named according to the species (host) in which the strain is endemic or to which it is adapted. The main variants named using this convention are:[citation needed]

Variants have also sometimes been named according to their deadliness in poultry, especially chickens:[citation needed]

  • Low pathogenic avian influenza (LPAI)
  • Highly pathogenic avian influenza (HPAI), also called deadly flu or death flu

Most known strains are extinct strains. For example, the annual flu subtype H3N2 no longer contains the strain that caused the Hong Kong flu.[citation needed]

Annual flu

The annual flu (also called "seasonal flu" or "human flu") in the US "results in approximately 36,000 deaths and more than 200,000 hospitalizations each year. In addition to this human toll, influenza is annually responsible for a total cost of over $10 billion in the U.S."[9] Globally the toll of influenza virus is estimated at 290,000–645,000 deaths annually, exceeding previous estimates.[10]

The annually updated, trivalent

B influenza viruses.[11]

Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005.[citation needed]

"Contemporary human H3N2 influenza viruses are now endemic in pigs in southern China and can reassort with avian H5N1 viruses in this intermediate host."[12]

FI6 antibody

FI6, an antibody that targets the hemagglutinin protein, was discovered in 2011. FI6 is the only known antibody effective against all 16 subtypes of the influenza A virus.[13][14][15]

Structure and genetics