Avian influenza (AI) is caused by an influenza type A virus which can infect poultry (such as chickens, turkeys, pheasants, quail, domestic ducks, geese, and guinea fowl) and some wild bird species (such as wild ducks and swans).
AI viruses are classified by a combination of two groups of proteins: hemagglutinin or “H” proteins, of which there are 16 (H1–H16), and neuraminidase or “N” proteins, of which there are 9 (N1–N9). AI viruses are further classified by their pathogenicity—the ability of a particular virus strain to produce disease in domestic chickens.
Highly pathogenic avian influenza (HPAI) virus strains are extremely infectious, often fatal to domestic poultry, and can spread rapidly from flock-to-flock.
Low pathogenicity avian influenza (LPAI) virus strains occur naturally in wild migratory waterfowl and shorebirds without causing illness. LPAI can occur in domestic poultry, with little or no signs of illness.
The U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) works closely with States and the poultry industry to prevent AI from becoming established in the U.S. poultry population.