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H7N9 Background

On 31 March 2013, the authorities of the People's Republic of China reported the first human cases of infection with the novel avian influenza A(H7N9) to the World Health Organization in line with the International Health Regulations of 2007. The disease situation in humans continues to evolve, with new cases reported regularly by the Chinese authorities (see Disease Situation).


General facts on avian influenza ecology


  • Aquatic wild birds represent the reservoir for avian influenza viruses, generally without showing any disease signs.
  • Domestic poultry become infected occasionally when in close contact with wild birds.
  • Some of influenza virus subtypes from wild birds are able to adapt to poultry and cause disease.
  • Avian influenza viruses in poultry can be low pathogenic (i.e. causing little or no signs of disease), or high pathogenic (i.e. resulting in severe disease and high mortality).
  • Low pathogenic viruses can mutate into high pathogenic viruses.

Origin of H7N9


Genetic analysis suggests the avian influenza (A)H7N9 virus likely originated from the re-assortment of three different avian origin influenza viruses that are currently circulating in Asia.


Avian infection with H7N9


The virus does not cause disease in domestic poultry. This implies: i) poultry flocks can be infected without showing clinical signs of disease; and ii) laboratory testing represents the only way to confirm infection.

The People’s Republic of China continues to report virus detections in poultry and the environment to the World Organisation for Animal Health (OIE).

So far, ongoing animal experiments by researchers worldwide have revealed that: i) infected chickens and quail shed high amounts of virus; while ii) the shedding from ducks is limited. Oropharyngeal swabs are recommended for sampling poultry.


Human infection with H7N9


The current situation represents the first time influenza A viruses of the H7N9 subtype have been identified in humans.

Current reports suggest some human cases had contact with poultry prior to the onset of disease. Reported methods of human-avian contact include:


  • live poultry brought into the home;
  • live bird market visits;
  • slaughtering of poultry; and
  • transportation of poultry.

Animal surveillance activities conducted in areas reporting human cases identified individual birds infected with the virus. These birds showed no clinical signs. The most likely source for human infection seems to be infected domestic poultry (e.g. chickens, ducks, captive pigeons and quails). Investigations in China are ongoing. Although the avian influenza A(H7N9) virus is able to infect humans and cause severe disease or death, WHO reports there is no evidence suggesting sustained human-to-human transmission.


Previous zoonotic influenza outbreaks


Zoonotic influenza viruses can infect animals and humans. Recent examples include H5N1 highly pathogenic avian influenza (HPAI) and pandemic influenza A(H1N1) 2009.

Facts about H5N1 HPAI:


  • Main outbreak started in birds in Southeast Asia in late 2003, although the origin of the virus was first documented in 1996.
  • Infects numerous avian species and humans, and has also been detected in some other animals such as cats and tigers.
  • Transmitted from birds to birds and birds to humans.
  • Spread throughout East Asia, Southeast Asia, Central Asia, Europe, parts of Middle East and some countries in Africa in 2005–2006.

Facts about pandemic influenza A(H1N1) 2009:


  • First detected in humans in North America in early 2009.
  • Infects humans and swine species.
  • Spread worldwide among humans in 2009.
  • WHO declared the beginning of the post-pandemic period.

Global response to both H5N1 HPAI and H1N1 resulted in improved:


  • sensitization and vigilance of the international community;
  • pandemic preparedness at national, regional and international levels; and
  • collaboration across disciplines (e.g. human health, animal health and wildlife).