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H7N9 Virology

Influenza A viruses in animals and humans


Influenza A viruses are common worldwide. They infect many different species: poultry, wild birds, pigs, horses, dogs, humans and even sea mammals (e.g. whales). Some influenza viruses infect preferentially only a few species, while others can infect many species. Occasionally, infection of humans with animal influenza viruses is reported.

Influenza A viruses are complex. Interactions of influenza A viruses with different host species, their reassortment with each other and their evolution is not always fully understood.

The H and the N in influenza virus names come from two major virus proteins: Hemagglutinin (HA) and Neuraminidase (NA). Influenza A contains 17 HA types and 10 NA types. Wild birds (particularly wild ducks, geese and shorebirds) maintain 16 HA types and 9 NA types of influenza A viruses.

In poultry, influenza A viruses are divided into two types based on their pathogenicity: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI). LPAI causes no or little signs of disease (e.g. decreased egg production). HPAI causes very high mortality rates in chickens.

Poultry does not generally maintain LPAI viruses, because the virus is not well adapted to poultry as a host species. Wild birds can transmit LPAI viruses to poultry, but the virus usually circulates briefly and dies. However, sometimes LPAI viruses can become established in poultry (e.g. H9N2 LPAI viruses in chickens is endemic in a large number of countries).

Viruses of the LPAI H5 and H7 variety are notifiable to the World Organisation for Animal Health (OIE). Wild birds have the potential to transmit these viruses to poultry, where they can circulate, mutate into HPAI and eventually cause disease in chickens.


Novel avian influenza A(H7N9) virus


On 31 March 2013 the Chinese authorities released the full genome sequences of the virus strain through the Global Initiative on Sharing Avian Influenza Data (GISAID1). This has allowed FAO and the international community to conduct an in-depth analysis of the viral genome.

Avian influenza A(H7N9) is an influenza virus generated through the reassortment of three low pathogenic avian influenza (LPAI) viruses. All of these avian influenza viruses donated genetic material to make up this novel H7N9 strain are LPAI viruses in birds.


  1. A low pathogenic H7 virus found in wild and domestic birds, particularly waterfowl, donated the HA gene segment.
  2. The N9 gene has been donated by a low pathogenic virus in wild birds.
  3. A low pathogenic H9N2 virus that is commonly found in chickens donated the six internal genes.

Edited from: New England Journal of Medicine

See figure ::: Generation of H7N9 via reassortment of three avian influenza viruses


Adaptation of H7N9 virus to mammals


The genomes of avian influenza A(H7N9) virus contain several features indicating adaptation to mammals as previously identified in other influenza A viruses. Viral adaptations to mammals include increased:


  • capacity to bind to cells in the upper respiratory tract of mammals (avian influenza viruses preferentially bind to cells in the intestinal tract of birds); and
  • ability to grow at the lower temperature of the upper respiratory tract of mammals (compared to the  higher temperature of the intestinal tract of birds).

How the H7N9 virus acquired these adaptations is unknown. How these adaptations will affect virus growth and transmission in various bird species is also unknown.

The avian influenza A(H7N9) virus has been detected in several bird species, the current degree of virus spread in poultry is unknown. The potential for future spread in poultry or even in wild bird species is currently under assessment.


Antiviral treatment of H7N9 in humans


For information on avian influenza A(H7N9) in humans please consult the WHO and CDC websites.


1 The Global Initiative on Sharing All Influenza Data (GISAID), 2008.