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.

 

Subtypes

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.

 

Avian Flu Virus and Human Infections

Avian influenza A viruses of the subtypes H5 and H7, including H5N1, H7N7, and H7N3, have been associated with highly pathogenic viruses. Human infection with these viruses has ranged from mild (H7N3, H7N7) to severe and fatal (H7N7, H5N1).

 

Human illness due to infection with low pathogenic influenza viruses has been documented, including mild symptoms to influenza-like illness. Examples of low pathogenic influenza viruses that have infected humans include H7N7, H9N2, and H7N2.

 

In general, direct human infection with the avian flu virus occurs infrequently, and has been associated with direct contact with infected sick birds or infected dead birds (domestic poultry).

 

Three important avian flu viruses include:

 

• Influenza A H5
• Influenza A H7
• Influenza A H9.

 

Key information about influenza A H5 is as follows:

 

• There are potentially nine different subtypes of this avian flu virus
• This virus can be highly pathogenic or low pathogenic
• H5 infections have been documented among humans, sometimes causing severe illness and death.

 

Key information about influenza A H7 is as follows:

 

• There are potentially nine different subtypes of this avian flu virus
• This virus can be highly pathogenic or low pathogenic
• H7 infection in humans is rare, but can occur among people who have direct contact with infected birds; symptoms may include conjunctivitis and/or upper respiratory symptoms.

 

Key information about influenza A H9 is as follows:

 

• There are potentially nine different subtypes of this avian flu virus
• This virus has been documented only in low pathogenic form
• At least three H9 infections in humans have been confirmed.