Avian Influenza (AI) type A viruses are divided into subtypes based on two proteins on the surface of the virus:
- Hemagglutinin (HA), of which there are 16 subtypes (H1-H16)
- Neuraminidase (NA), of which there are 9 subtypes (N1-N9)
- Many combinations of HA and NA proteins are possible (i.e., H5N1, H5N2, H7N2, H7N8, etc).
AI viruses are also classified into two groups based on their ability to produce disease in chickens: highly pathogenic avian influenza (HPAI) or low pathogenic avian influenza (LPAI).
- HPAI viruses cause high mortality in poultry and occasionally high death rates in certain species of wild birds.
- LPAI viruses can cause a variety of outcomes in poultry ranging from no apparent clinical signs to moderate death rates. LPAI viruses usually cause little to no signs in wild birds.
H5 and H7 LPAI viruses have the potential to mutate or evolve into HPAI viruses and are closely monitored by animal health officials.
Learn more: USGS Avian Influenza