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

Subtypes: 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 Virus Adapted from CDC Image Library: Dan Higgins

Categories:
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.