Section 1. Avian Influenza Facts

  1. Source and Transmission
  2. Agent Characteristics
  3. Virus Survival
  4. Clinical Signs in Birds
  5. HPAI H5N1
  6. Human Concerns

Virus Characteristics

There are many different avian influenza (AI) viruses and they are characterized into subtypes based on two proteins found on the virus called hemagglutinin (H) and neuraminidase (N). 1,6
  • Hemagglutinin (H) proteins are responsible for binding the virus to the cell that is being infected. There are 16 H subtypes. (H1 to H16)
  • Neuraminidase (N) proteins help the virus get into a host cell. There are 9 N subtypes. (N1 to N9)
Birds can carry 144 possible combinations of influenza A subtypes. One very commonly occurring subtype is H5N1.

AI structure CDC
Adapted from CDC Image Library: Dan Higgins


AI is also classified into two categories, Low Pathogenic Avian Influenza (LPAI) and Highly Pathogenic Avian Influenza (HPAI) based on the severity of the disease they cause.

  • 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 monitored as they have been known to mutate into HPAI.
  • 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.