More than 60 species of domestic, wild, captive and experimental birds may be affected by avian influenza. Migratory waterfowl are considered natural reservoirs of the avian influenza virus, which means that they may harbor the virus without developing clinical signs. Wild birds have historically carried low pathogenic forms of influenza viruses. Domestic poultry are most susceptible to H5 and H7 types of avian influenza viruses, which can sometimes mutate into a highly pathogenic form.
Infected birds shed the influenza virus in their saliva, nasal secretions, and feces. Domestic poultry may become infected with avian influenza virus through direct contact with infected waterfowl or other infected poultry, or through contact with contaminated surfaces such as dirt or cages or materials such as water or feed.
The highly pathogenic H5N1 avian influenza virus, also referred to as bird flu, has been shown to reinfect wild birds. For more information on the wild birds most susceptible to H5N1 visit the USGS National Wildlife Center's website.