What are the different types of avian influenza? What do the numbers next to H and N mean?
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, for example H5N1, H5N2, H7N2, and H7N8.
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 death in 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 change into HPAI viruses and are closely monitored by animal and public health officials. The first HPAI outbreak in the U.S. occurred in 2015 and the second outbreak started in 2022. The U.S. Department of Agriculture is the lead federal agency for avian influenza in animals, but they work closely with many partner agencies, including the USGS.
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