Birds have probably carried avian influenza viruses for a very long time—thousands of years or more.

Low pathogenicity avian influenza

Many avian influenza virus subtypes circulate among birds worldwide. Most of them are low pathogenicity subtypes. Wild birds can carry these low pathogenicity viruses in their intestines without becoming sick. When they shed the virus in their saliva, nasal secretions, and feces, they may pass the infection on to other birds.

Most avian influenza viruses have been found in waterfowl and shorebirds. In wild ducks, the infection rate is usually below one percent but is higher in summer and early fall. Shorebird infection rates tend to peak during spring migration, but the infection rates are generally much lower than in waterfowl.

High pathogenicity avian influenza

To date, the high pathogenicity H5N1 virus has been found in wild or domestic birds in 50 countries in Africa, Asia, Europe, and the Near East.

Poultry have low resistance to H5N1 viruses, which can spread rapidly through poultry flocks. In poultry, the mortality rate can be 90–100 percent, often within 48 hours.

The H5N1 virus has been found in a variety of birds in Europe and the Near East, including mute swans, Tufted Ducks, several birds of prey, and House Sparrows. As of May 22, 2006, the USGS National Wildlife Health Center reports that the H5N1 virus has been found in 91 bird species, both wild and domestic. For a complete list, visit the USGS “List of Species Affected by H5N1.”

Waterfowl and shorebirds have been most visibly affected by outbreaks in wild birds so far. In spring 2005 at Lake Qinghai, China, some 6,000 birds died. Ninety percent of them were Bar-headed Geese; the dead birds are estimated to have been about 5 percent to 10 percent of the world’s population of Bar-headed Geese. Brown-headed Gulls and Black-headed Gulls also died in the outbreak.
     
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