What are the HPAI (pathogenic avian influenza) viruses?
There are many different subtypes of type A influenza viruses. These subtypes differ because of certain proteins on the surface of the influenza A virus (hemmaglutinin HA and neuraminidinase NA proteins). There are 16 different HA subtypes and nine different NA subtypes of influenza A viruses. Each combination is a different subtype. All known subtypes of influenza A viruses can be found in birds.
To date only the H5 and H7 subtypes are known to cause the highly pathogenic form of the disease. However, not all viruses of the H5 and H7 subtypes are highly pathogenic, causing severe disease in poultry. It appears that the H5 and H7 viruses are introduced to poultry flocks in their low-pathogenic form. When allowed to circulate, these viruses mutate within a few months into the highly-pathogenic form. This is the reason for concern with the presence of the H5 or H7 virus in poultry even when initial signs of infection are mild.
Understanding Avian Influenza