Highly Pathogenic Avian Influenza (HPAI) is a category of influenza A viruses that infect birds, causing severe illness and mortality in both poultry and wild birds. These strains are named based on two proteins on the virus’s surface: hemagglutinin (HA) and neuraminidase (NA). There are 16 subtypes of HA (H1-H16) and 9 subtypes of NA (N1-N9). The HPAI know as H5N1 is therefore made of a virus with hemagglutinin type 5 and neuraminidase type 1. Other examples of HPAI have included H5N2, H5N8, and H7N9
HPAI is distinct from low pathogenic avian influenza (LPAI), which can cause various outcomes in poultry that range from no apparent clinical signs to moderate death rates. LPAI viruses do not typically cause signs in wild birds, or else the symptoms are mild. However, H5 and H7 LPAI have the potential to develop into HPAI. Two previous outbreaks of LPAI include one with H7N2 and one with H7N8.