Influenza viruses are broadly divided into three types: A, B, and C. Type A influenza includes most human and all avian influenza viruses.
Avian influenza viruses are divided into subtypes based on a combination of two proteins found on the virus’s surface: hemagglutinin (H), which has 16 recognized types (H1–H16), and neuraminidase (N), which has nine (N1–N9).
Various strains of subtypes have been identified, and these strains are grouped into two broad categories based on their ability to cause disease in poultry: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI).
The H5 and H7 strains are the most common HPAI viruses found in nature. This said, not all H5 and H7 influenza viruses are highly pathogenic. Likewise, other strains that are not H5 or H7 can be considered highly pathogenic in certain circumstances.
Epidemiology of avian influenza
Avian influenza appears periodically all over the world, including in the United States. The virus spreads easily among wild, migratory aquatic birds, with waterfowl and shorebirds considered natural hosts (reservoirs). Certain virus strains also can infect domesticated birds (including chickens, turkeys, ducks, and—rarely—pet birds), humans (rarely), and a variety of other mammals, both wild and domestic. The majority of human cases of avian influenza have resulted from direct contact with infected poultry.
In the United States, most outbreaks of avian influenza have been associated with LPAI strains. These strains may cause mild disease in poultry.
However, outbreaks also have occurred involving HPAI strains. These strains, many of which are harmless in wild birds, can cause severe disease and death in poultry. HPAI can rapidly devastate entire flocks, result in major economic losses, and potentially infect people and other animal species.
APHIS keeps a geographical tally on HPAI detection in wild birds, commercial and backyard flocks, and mammals. Use this resource to find out whether and when HPAI has been found in your county.