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Background Information on Influenza in Animals


  • Influenza viruses are RNA viruses in the family Orthomyxoviridae. The influenza viruses are categorized into three main types on the basis of their antigenic properties: A, B, and C.
    • Type A influenza viruses are the most virulent and are maintained in humans, lower mammals, and birds.
    • Type B and C influenza viruses are only maintained in humans.


  • Influenza A viruses are divided into subtypes based on two glycoproteins on the surface of the virus:
    • Hemagglutinin (HA): 16 different subtypes (plus 2 more found in bats)
    • Neuraminidase (NA): 9 different subtypes (plus 1 more found in bats)


  • Influenza A subtype combinations are further classified by strains (using genetic sequencing).
    • Avian influenza virus strains are further classified as either low pathogenic (LPAI) or highly pathogenic (HPAI) based on specific molecular genetic and pathogenesis criteria that require additional specific testing using chickens.
      • Classification as low or high pathogenicity (severity of disease) is specific to poultry, and not necessarily to other animal species that can be susceptible to avian influenza viruses including humans.
      • Only H5 and H7 subtypes have shown the capability of being HPAI.
  • Antigenic drifts can result in minor antigenic changes in the HA and NA creating a new strain (same subtypes).
  • Antigenic shifts can result in a sudden appearance of a new strain possessing a distinctly different HA and/or NA subtype.


  • Influenza type/host of origin/geographic source/isolate number/year of isolation (HA subtype NA subtype)
    • Example: Influenza A/long-tailed duck/WI/10OS3912/2010 (H14N6)

HA NA Combination Found by Host

  • Humans: H1N1, H2N2, H3N2; Since 1997: H5N1, H7N2, H7N3, H7N7, H7N9, H9N2
  • Swine: H1N1, H3N2, H1N2; (H3N1, H4N6, H5N1, H9N2)
  • Equine: H7N7, H3N8
  • Poultry: Many HA NA combinations recovered; not all HA NA subtypes recovered.
  • Wild birds: Many more HA NA combinations recovered; all HA NA subtypes recovered (except newly discovered subtypes which have only been found in bats).
    • Wild waterfowl are the primary natural reservoir for all subtypes of influenza A viruses and are believed to be the original source of influenza A viruses in all other animals.
    • In wild birds, most influenza viruses cause asymptomatic (subclinical) or mild infection; however, the range of symptoms varies greatly depending on the strain of virus.
    • Influenza viruses typically infect the avian gastrointestinal tract rather than the respiratory tract; thus, virus is shed through feces.

"Who's giving what to whom?" (quote by: Dr. B.C. Easterday, circa 1971)

Influenza Cycle