Worldwide, there are many strains of avian influenza (AI) virus that can cause varying amounts of clinical illness in poultry. AI viruses can infect chickens, turkeys, pheasants, quail, ducks, geese, and guinea fowl as well as wide variety of other bird. Migratory waterfowl have proved to be a natural reservoir for AI viruses.
The viruses can be classified into low-pathogenicity (LPAI) and high-pathogenicity (HPAI) forms based on the severity of the illness they cause . Most AI virus strains are classified as LPAI and typically cause few or no clinical signs in infected birds. However, some LPAI virus strains are capable of mutating under field conditions into HPAI viruses.
HPAI is an extremely infectious and fatal form of the disease in poultry. The U.S. Deparatment of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) works to keep HPAI from becoming established in the U.S. poultry population.
