Avian influenza classification

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.

Classification Based on 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

Classification Based on 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:^[1]^[2]^[3]


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

References

↑ Gao R, Cao B, Hu Y, Feng Z, Wang D, Hu W, Chen J, Jie Z, Qiu H, Xu K, Xu X, Lu H, Zhu W, Gao Z, Xiang N, Shen Y, He Z, Gu Y, Zhang Z, Yang Y, Zhao X, Zhou L, Li X, Zou S, Zhang Y, Li X, Yang L, Guo J, Dong J, Li Q, Dong L, Zhu Y, Bai T, Wang S, Hao P, Yang W, Zhang Y, Han J, Yu H, Li D, Gao GF, Wu G, Wang Y, Yuan Z, Shu Y (2013). “Human infection with a novel avian-origin influenza A (H7N9) virus”. N. Engl. J. Med. 368 (20): 1888–97. doi:10.1056/NEJMoa1304459. PMID 23577628.
↑ Liu D, Shi W, Shi Y, Wang D, Xiao H, Li W, Bi Y, Wu Y, Li X, Yan J, Liu W, Zhao G, Yang W, Wang Y, Ma J, Shu Y, Lei F, Gao GF (2013). “Origin and diversity of novel avian influenza A H7N9 viruses causing human infection: phylogenetic, structural, and coalescent analyses”. Lancet. 381 (9881): 1926–32. doi:10.1016/S0140-6736(13)60938-1. PMID 23643111.
↑ Xiong C, Zhang Z, Jiang Q, Chen Y (2013). “Evolutionary characteristics of A/Hangzhou/1/2013 and source of avian influenza virus H7N9 subtype in China”. Clin. Infect. Dis. 57 (4): 622–4. doi:10.1093/cid/cit294. PMID 23650290.

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[pmid23577628-1] Gao R, Cao B, Hu Y, Feng Z, Wang D, Hu W, Chen J, Jie Z, Qiu H, Xu K, Xu X, Lu H, Zhu W, Gao Z, Xiang N, Shen Y, He Z, Gu Y, Zhang Z, Yang Y, Zhao X, Zhou L, Li X, Zou S, Zhang Y, Li X, Yang L, Guo J, Dong J, Li Q, Dong L, Zhu Y, Bai T, Wang S, Hao P, Yang W, Zhang Y, Han J, Yu H, Li D, Gao GF, Wu G, Wang Y, Yuan Z, Shu Y (2013). “Human infection with a novel avian-origin influenza A (H7N9) virus”. N. Engl. J. Med. 368 (20): 1888–97. doi:10.1056/NEJMoa1304459. PMID 23577628.

[pmid23643111-2] Liu D, Shi W, Shi Y, Wang D, Xiao H, Li W, Bi Y, Wu Y, Li X, Yan J, Liu W, Zhao G, Yang W, Wang Y, Ma J, Shu Y, Lei F, Gao GF (2013). “Origin and diversity of novel avian influenza A H7N9 viruses causing human infection: phylogenetic, structural, and coalescent analyses”. Lancet. 381 (9881): 1926–32. doi:10.1016/S0140-6736(13)60938-1. PMID 23643111.

[pmid23650290-3] Xiong C, Zhang Z, Jiang Q, Chen Y (2013). “Evolutionary characteristics of A/Hangzhou/1/2013 and source of avian influenza virus H7N9 subtype in China”. Clin. Infect. Dis. 57 (4): 622–4. doi:10.1093/cid/cit294. PMID 23650290.

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