"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 a wide variety of other birds. Migratory waterfowl have proved to be the natural reservoir for this disease. AI viruses can be classified into low pathogenic (LPAI) and highly pathogenic (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. The U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) works to keep HPAI from becoming established in the U.S. poultry population. HPAI can strike poultry quickly without any warning signs of infection. Once established, the disease can spread rapidly from flock to flock. It is essential for the U.S. poultry industry to be alert to this disease threat."
Program Aid No. 1704
United States Department of Agriculture: http://www.usda.gov/wps/portal/usda/usdahome