influenza A virus

Other Scientific Names

• fowl plague virus ICTV
• highly pathogenic avian influenza virus ICTV

International Common Names

• English: avian influenza A virus; avian influenza virus; avian influenzavirus; bird flu virus; HPAI virus; swine flu virus; swine influenza virus; swine influenzavirus

English acronym

• AIV
• FLUAV
• SIV

• Domain: Virus
•     Group: "Positive sense ssRNA viruses"
•         Group: "RNA viruses"
•             Order: Mononegavirales
•                 Family: Orthomyxoviridae
•                     Genus: Influenzavirus A
•                         Species: influenza A virus

Editor's note: this datasheet is being reviewed and updated. 

Influenza A viruses are enveloped, single-stranded RNA viruses. The two most important proteins of the virus, haemagglutinin (HA) and neuraminidase (NA) project from the surface of the virus envelope. Matrix (M) proteins line the underside of the envelope and surround the core, inside of which is the viral genetic information surrounded by nucleoprotein. The nucleoprotein and matrix protein are type-specific antigens which are used to classify influenza viruses as type A, B or C. The "segmented" genome consists of 8 single-stranded RNA segments that encode 10 viral proteins: the surface glycoproteins HA and NA, matrix proteins (2), nucleoprotein, nonstructural proteins (2) and polymerase proteins (3).

To date, (at least) 16 different HAs (H1 to H16) and 9 different NAs (N1 to N9) have been recognised among all influenza A viruses. Virologists classify influenza viruses into subtypes by referring to their combination of HA and NA as HxNy (where x=1 to 16, and y=1 to 9). Three different subtypes, H1N1, H3N2 and H1N2 currently circulate in pigs. The international nomenclature of influenza viruses designates the type, host, geographic origin, strain number, year of isolation and antigenic subtype (WHO, 1980). For example, a H1N1 SIV isolated in Belgium in 1983 is designated A/Sw/Belgium/1/83 (H1N1).

Antibodies against the HA play the dominant role in protection against influenza infection, and there is generally no cross-immunity between viruses of different HA subtypes. A characteristic of influenza viruses is that they can change their envelope glycoproteins by the mechanisms of antigenic "shift" and "drift". Antigenic shift refers to the introduction of a new influenza virus subtype into the population, which can lead to widespread epizootics. If a host is infected with two different influenza viruses at the same time, they can exchange genome segments. This phenomenon is called "genetic reassortment" and contributes to the introduction of new subtypes.

Antigenic drift is the occurrence of relatively minor changes in the HA and/or NA as a result of immune pressure.

HPAI (fowl plague) is caused by infection with an influenza A virus. Many species of birds have been shown to be susceptible to infection with avian influenza A viruses, but most isolates have been of low pathogenicity for chickens and turkeys, the main birds of economic importance to be affected.

The HPAI A viruses that cause acute clinical disease in chickens and turkeys have been associated only with the H5 and H7 subtypes. However, many H5 and H7 subtype isolates from birds have been seen to be of low virulence (Alexander, 1993).

Different subtypes of SIV circulating in pigs include H1N1, H3N2 and H1N2.

Avian influenza notifiable to OIE is defined as an infection of poultry caused by any influenza A virus with high pathogenicity (HPAI) and by H5 and H7 subtypes with low pathogenicity (H5/H7 LPAI). Influenza A viruses with high pathogenicity in birds other than poultry, including wild birds, are also notifiable.