Section 1. Avian Influenza Facts

Virus Characteristics

There are many different avian influenza viruses. The virus can be identified in two ways: the subtype and the category. 1,6


Subtypes are identified in a lab based on two proteins found on the outside of the virus virus called hemagglutinin (H) and neuraminidase (N).
  • Hemagglutinin (H) proteins are responsible for binding the virus to the cell that is being infected. There are 16 H subtypes. (H1-H16) H5 and H7 commonly infect poultry.
  • Neuraminidase (N) proteins help the virus get into a host cell. There are 9 N subtypes. (N1-N9)

Birds can carry 144 possible combinations of influenza A subtypes. One very commonly occurring subtype is H5N1.

Avian Influenza VirusAI structure CDC
Adapted from CDC Image Library: Dan Higgins

AI is classified into two categories, Low Pathogenic Avian Influenza (LPAI) and Highly Pathogenic Avian Influenza (HPAI) based on how sick the bird looks.

  • LPAI - Is the most common form of AI. Signs of disease range from none ,to ruffled feathers and a decrease in egg production. Subtypes LPAI H5 and H7 are carefully watched since these virus subtypes can mutate into a high pathogenic form.
  • HPAI - Is less common, but spreads rapidly in poultry flocks, causing severe illness, and can kill 90 - 100% of infected birds within 48 hours of exposure.