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French Agency for Food, Environmental and Occupational Health & Safety

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Updated on 04/08/2016

Avian Influenza

Keywords : H5N1 (Bird), Avian influenza, Animal diseases, Animal health, Zoonosis, Bird flu

Avian influenza is a highly contagious viral infection which affects wild and domestic birds. The disease, which has a very high mortality rate in birds and can cause great financial losses, can also under certain conditions be transmitted to humans. Here we present information on the disease and on the pathogens which cause it. 

Identified for the first time in Italy in the 1900s, avian influenza is a highly contagious viral infection affecting both wild and captive birds. It can cause extremely high mortality rates due to respiratory and/or digestive and/or nervous disorders (especially among poultry, including turkeys, hens and chickens) and lead to huge financial losses. It also threatens biodiversity and, under certain conditions, these viruses may cause human fatalities.

Depending on the virulence of the viral strains involved (strains being classified into a plethora of sub-types such as H5N2, H7N7 and H6N2) and the bird species infected, avian influenza can also occur without any clinical signs, in which case it can only be detected by laboratory analyses. It may be revealed through attenuated to moderate signs such as loss of appetite, a drop in egg production or respiratory disorders of greater or lesser severity. 


For the sake of clarity

Avian influenza has its very own vocabulary, sometimes misused.

Avian influenza: Viral disease affecting birds, occasionally transmitted to other species. Also known as avian flu.

Bird flu: Generic term used by the media which includes avian influenza and its transmission to humans or other species. This scientifically inaccurate name has been popularly accepted to mean avian influenza. 



The virus is mainly transmitted by direct contamination, such as contact with respiratory secretions, faeces or the organs of infected animals. Indirect contamination is also possible through exposure to contaminated elements such as food, water, equipment or clothing.

The incubation period can vary from one to 21 days, depending on the kind of exposure and the virulence of the viral strain. This maximum period is applied in outbreak mitigation and when tracking epidemiological relationships (e.g. other farms likely to have been exposed to the virus).


The virus

Influenza viruses are able to mutate and change constantly. They can swap genes—an event known as reassortment—or acquire mutations on different genes, enabling them to infect new species, to get around the immune system defences of an animal or human host, or to become more virulent, for example.