Swine influenza (illness in pigs) is caused by influenza A viruses. Influenza A viruses of swine origin can cause influenza in humans. General information about influenza A viruses is presented in the bullet points below.
- Descriptive information
- Influenza A viruses are negative-sense single-stranded RNA viruses and belong to the family Orthomyxoviridae and the genus Influenzavirus A.
- Enveloped virions are 80 to 120 nm in diameter, are 200 to 300 nm long, and may be filamentous. They consist of spike-shaped surface proteins, a partially host-derived lipid-rich envelope, and matrix (M) proteins surrounding a helical segmented nucleocapsid (6 to 8 segments).
- The virus envelope glycoproteins (hemagglutinin [HA] and neuraminidase [NA]) are distributed evenly over the virion surface, forming characteristic spike-shaped structures; antigenic variations in these proteins form the basis of the classification system for influenza A virus subtypes.
- Influenza A virus subtypes
- There are 16 different HA antigens (H1 to H16) and nine different NA antigens (N1 to N9) for influenza A.
- Human disease historically has been caused by three subtypes of HA (H1, H2, and H3) and two subtypes of NA (N1 and N2). More recently, human disease has been recognized to be caused by additional HA subtypes, including H5, H7, and H9 (all from avian origin).
- All of these subtypes have been found in birds, and birds are the primordial reservoir for influenza A viruses.
- Several subtypes have been found in pigs
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