The natural reservoir for Type A influenza viruses is wild water birds such as ducks and geese. New influenza A subtypes are continually emerging in the waterfowl population due to the constant mutation of the virus. While avian influenza is caused by Type A viruses, seasonal influenza outbreaks in people, which occur almost every winter, are caused by either Type A and Type B influenza viruses. Influenza Type C viruses cause mild respiratory illness in humans but are not usually responsible for outbreaks of the flu. Type A viruses are found in both people and animals, whereas Type B viruses are normally only found in humans.

Influenza Subtypes

Influenza subtypes are named for two antigens present on the surface of the virus. These are:

  • H (hemagglutinin)
  • N (neuraminidase)

There are 16 possible H antigens and 9 possible N antigens. Virus subtypes are named H9N2, H5N1, etc, depending on their combination of antigens.

Avian influenza viruses are classified in two ways. One makeup, which determines how it elicits the host’s immune response (see more detailed description in box at right). The other is by the severity of the disease they cause in domestic poultry, which is designated as:

  • Low pathogenicity avian influenza (LPAI)
  • High pathogenicity avian influenza (HPAI)

While it is common practice to equate the term “avian influenza” directly with the highly pathogenic form of H5N1 virus in birds (i.e. HPAI H5N1), this is not accurate. There are many different H and N antigenic combinations (see box) that can occur in Type A influenza viruses that cause avian influenza, and the H and N antigens do not, in and of themselves, determine pathogenicity. In other words, there are both HPAI H5N1 and LPH5N1 avian influenza viruses.

 

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