Avian Influenza - Classifications

Agricultural Disaster Preparedness and Recovery August 11, 2011 Print Friendly and PDF

Avian influenza viruses in poultry occur in two main forms.

  • The low pathogenic form (LPAI) may go undetected because it usually causes only mild illness such as ruffled feathers and a drop in egg production. Most avian influenza viruses are LPAI viruses.
  • The highly pathogenic form (HPAI) spreads rapidly through poultry flocks and can cause severe illness and high mortality. Only some viruses of the H5 and H7 subtypes have been known to mutate into a highly pathogenic form of the disease.

Current research has shown that H5 and H7 viruses are introduced to poultry flocks in their low pathogenic form. If allowed to circulate in the flocks, the viruses can mutate, usually within a few months, into the highly pathogenic form. That is why the presence of an H5 or H7 virus in poultry is always cause for concern even when initial signs of infection are mild.

In rare cases, a highly pathogenic strain of H5N1 which kills poultry has infected and killed some humans. This type of HPAI has been referred to as bird flu.


Avian Influenza Homepage

Connect with us

  • Twitter
  • Facebook
  • YouTube
  • Pinterest
  • Google+


This is where you can find research-based information from America's land-grant universities enabled by eXtension.org



This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.