Avian influenza classification

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For more information about seasonal human influenza virus that is not associated with animal exposure, see Influenza

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Yazan Daaboul, M.D.

Overview

To date, only influenza type A has been associated with avian influenza. Neither influenza B nor influenza C is associated with avian influenza. Classification of avian influenza may be based on either the pathogenicity of the virus (low pathogenicity vs. high pathogenicity) or the viral genetic subtypes (H5 vs. H7 vs. H9).

Classification

Classification of avian influenza may be based on either the pathogenicity of the virus or the viral genetic subtypes. To date, only influenza type A has been associated with avian influenza.

Pathogenecity of the Virus

Avian influenza may be classified based on the pathogenicity of the virus:

• Low pathogenic avian influenza (LPAI)

Mild/no clinical manifestations among humans

May convert to highly pathogenic avian influenza

• Highly pathogenic avian influenza (HPAI)

Moderate/severe clinical manifestations among humans

Viral subtypes H5 and H7 are associated with HPAI

Viral Genetic Subtypes

• To date, only influenza type A has been associated with avian influenza. Neither influenza B nor influenza C is associated with avian influenza.
• Although avian influenza has many subtypes of haemagglutinin (HA) and neuraminidase (NA), only 3 subtypes have been associated with human infections.
• The table below lists the subtypes of influenza A that have been associated with avian influenza in humans:

Influenza Subtype	Number of Subtypes	Pathogenicity	Clinical Manifestations
Influenza A H5	At least 9	May be either HPAI or LPAI	May cause either mild or severe/fatal clinical manifestations
Influenza A H7	At least 9	May be either HPAI or LPAI	May cause either mild or severe/fatal clinical manifestations
Influenza A H9	At least 9	Only reported as LPAI	Mild clinical manifestations in humans