influenza A virus

Pictures

Picture	Title	Caption	Copyright
Title Structural features of SIV Caption Copyright Kristien Van Reeth	Structural features of SIV		Kristien Van Reeth
Title Immunostained SIV in lung tissue Caption Immunostained SIV in a cryostat section of lung from an experimentally infected pig. Note that approximately 100% of bronchial epithelial cells are virus-positive (green fluorescence). Copyright Kristien Van Reeth	Immunostained SIV in lung tissue	Immunostained SIV in a cryostat section of lung from an experimentally infected pig. Note that approximately 100% of bronchial epithelial cells are virus-positive (green fluorescence).	Kristien Van Reeth
Title Histopathology of healthy pig lungs Caption Copyright Kristien Van Reeth	Histopathology of healthy pig lungs		Kristien Van Reeth
Title Histopathology of SIV-infected pig lungs Caption Note the necrosis of lung epithelia and obstruction of the airways with inflammatory cells. Copyright Kristien Van Reeth	Histopathology of SIV-infected pig lungs	Note the necrosis of lung epithelia and obstruction of the airways with inflammatory cells.	Kristien Van Reeth
Title Histopathology of SIV-infected pig lungs Caption Note the necrosis of lung epithelia and obstruction of the airways with inflammatory cells. Copyright Kristien Van Reeth	Histopathology of SIV-infected pig lungs	Note the necrosis of lung epithelia and obstruction of the airways with inflammatory cells.	Kristien Van Reeth

Identity

Preferred Scientific Name

• influenza A virus (ICTV)

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

Taxonomic Tree

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

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Pathogen Characteristics

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.

Vectors and Intermediate Hosts

References

Alexander DJ, 1993. Orthomyxovirus infection. Virus infections of birds., 287-316; 122 ref.

OIE Handistatus, 2002. World Animal Health Publication and Handistatus II (dataset for 2001). Paris, France: Office International des Epizooties.

OIE Handistatus, 2003. World Animal Health Publication and Handistatus II (dataset for 2002). Paris, France: Office International des Epizooties.

OIE Handistatus, 2004. World Animal Health Publication and Handistatus II (data set for 2003). Paris, France: Office International des Epizooties.

OIE Handistatus, 2005. World Animal Health Publication and Handistatus II (data set for 2004). Paris, France: Office International des Epizooties.

WHO, 1980. Memorandum: A revision of the system of nomenclature for influenza virus. Bull. WHO, 58:585-591.

Zhou NanNan; Shortridge KF; Claas ECJ; Krauss SL; Webster RG, 1999. Rapid evolution of H5N1 influenza viruses in chickens in Hong Kong. Journal of Virology, 73(4):3366-3374; 42 ref.

Distribution Maps