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The avian flu virus that causes avian influenza (or bird flu) is generally a low pathogenic virus associated with mild disease. However, highly pathogenic strains do occur and can cause severe illness and high mortality in poultry. In general, direct human infection with the virus occurs rarely, and transmission primarily occurs through direct contact with infected sick or dead birds.

What Is the Avian Flu Virus?

The influenza virus that infects birds is called the avian flu virus or bird flu. Only influenza A viruses, including all known subtypes, can infect birds. However, there are substantial genetic differences between the subtypes that typically infect both people and birds.


Avian flu virus has three subtypes, which include H5, H7, and H9. Each of these three avian flu viruses theoretically can be partnered with any one of nine neuraminidase surface proteins. Thus, there are potentially nine different strains of each subtype. (For example: H5N1, H5N2, H5N3, and H5N9.)

Low Pathogenic Versus Highly Pathogenic

Strains of the avian flu virus are classified as low pathogenic (LPAI) or highly pathogenic (HPAI) on the basis of specific molecular genetic and pathogenesis criteria that require specific testing. Most avian flu viruses are low pathogenic viruses that are usually associated with mild disease in poultry. In contrast, highly pathogenic avian flu viruses can cause severe illness and high mortality in poultry. More recently, some highly pathogenic avian flu viruses (for example, H5N1) have been found to cause no illness in certain poultry, such as ducks. Low pathogenic avian flu viruses have the potential to evolve into highly pathogenic avian flu viruses, and this has been documented in some poultry outbreaks.
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Last reviewed by: Arthur Schoenstadt, MD
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