Avian Influenza: Overview


 What is Avian Influenza

Avian influenza is usually an inapparent or nonclinical viral infection of wild birds that is caused by a group of viruses known as type A influenzas. These viruses are maintained in wild birds by fecal-oral routes of transmission. This virus changes rapidly in nature by mixing of its genetic components to form slightly different virus subtypes. Avian influenza is caused by this collection of slightly different viruses rather than by a single virus type. The virus subtypes are identified and classified on the basis of two broad types of antigens, hemagglutinin (H) and neuraminidase (N); 16 H and 9 N antigens have been identified among all of the known type A influenzas. Avian influenza viruses have been found in many bird species, but are most often found in migratory waterfowl, especially the mallard duck.

Source: National Wildlife Health Center - Field Manual of Wildlife Diseases


Avian influenza A viruses can infect a variety of domestic and wild avian species (including chickens, turkeys, ducks, domestic geese, quail, pheasants, partridge, psittacines, gulls, shorebirds, seabirds, emu, and others). The clinical manifestation of infection in birds ranges from asymptomatic infection to rapidly fatal disease.

Aquatic birds, particularly ducks, shore birds, and gulls, are considered the natural reservoirs for avian influenza viruses. These waterfowl generally do not develop disease when infected with avian influenza viruses; however, an outbreak of H5N1 among migratory geese and other wild birds in Qinghai province, China, was reported in May 2005.

Recently, investigators in Asia have shown that asymptomatically infected domestic ducks are shedding more H5N1 virus for longer periods in 2004 when compared with 2003, which may be amplifying the spread of H5N1 to domestic poultry. Another report demonstrated the presence of H5N1 influenza virus in asymptomatic eagles that were smuggled from Thailand into Belgium in 2004.

Influenza A viruses (including those of avian origin) also can cause illness in humans and other animal hosts (pigs, horses, sea mammals, and mustelids). The H5N1 avian influenza virus circulating in Asia has been found in pigs in China and Indonesia and has also expanded its host range to include cats, leopards, and tigers, which generally have not been considered susceptible to influenza A.

Source: CIDRAP - Avian Influenza: Agricultural and Wildlife Considerations

This site was developed and is maintained by the Wildlife Data Integration Network with support from:
U. S. Geological Survey's National Wildlife Health Center and the
University of Wisconsin-Madison's Nelson Institute for Environmental Studies

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University of Wisconsin-Madison's Nelson Institute of Environmental Studies