There are many strains of avian influenza virus that for thousands of years have been carried by birds. Migratory waterfowl carry the low pathogenic strain of avian influenza without showing clinical signs. However, domestic poultry may become infected by the low pathogenic strain of the virus which may then mutate into a highly pathogenic strain. The highly pathogenic strain causes severe clinical signs and death in birds, especially in domestic poultry.
The highly pathogenic H5N1 virus was first isolated from a domestic goose in Guangdong, China in 1996. This form of highly pathogenic avian influenza led to a poultry epidemic in Hong Kong and resulted in the first documented cases of human death. In most cases since then, the wild birds appear to have contracted the highly pathogenic H5N1 virus from infected poultry. The virus has been found in Europe, the Near East, and Africa in a variety of birds including mute swans, bar-headed geese, tufted ducks, several birds of prey, and house sparrows.
It is possible, but considered unlikely, that H5N1 could enter the United States via migratory birds. In order for H5N1 to enter the United States, infected wild birds from Asia or West Africa would have to survive the disease and carry it to breeding grounds in the Pacific Islands, Alaska, or Canada. The virus would then have to pass to another species that migrates to the continental United States.
Ninety-nine percent of North American migratory waterfowl do not come from Europe, Asia, or Africa where the virus has been found. Scientists continue to test a representative sample of birds in Alaska and Canada and along major flyways in the continental United States in an effort to detect whether avian influenza has arrived via these routes.