Bird flu (or avian influenza, AI) is a highly-contagious viral disease, that affects several species of birds used for food production, as well as ornamental birds and wild birds.

Flu viruses are classed in three (3) genera: influenza virus A, B and C. Only Influenza A viruses are currently thought to cause natural infections in birds.
Type A flue viruses are further divided into subtypes. There are currently 16 subtypes H (H1 to H16) and nine (9) subtypes N (N1 to N9) have been recognized. Each virus shows an H antigen and an N antigen, apparently in an arbitrary combination; all the subtypes and the majority of the possible combinations have been isolated from wild bird species.

The avian influenza viruses are known for their virulence in poultry (in particular in chickens and turkeys), where they cause two distinct pathologies:

• highly pathogenic avian influenza (HPAI): is the result of extremely virulent viruses that cause a high mortality rate
• low pathogenic avian influenza (LPAI): is caused by viruses associated with a minor respiratory disease, reduced egg-production and a moderate increase in mortality.

Avian flu therefore represents a considerable menace for the poultry industry, with an enormous economic impact. Only the viruses of subtypes H5 and H7 are capable of evolving into a highly-pathogenic virus.

Certain AI viruses are zoonotic, which means they can infect humans. In most cases, infections are mild, but certain viruses can be fatal for humans (for example the H5N1 avian influenza and more recently H7N9).

The virus is transmitted between animals, directly by oral-fecal contamination via the fecal matter of infected animals, or indirectly, through exposure to contaminated substrates (through food, water, agricultural equipment, etc.). Wild birds (mainly water fowl) are said to be asymptomatic, i.e. they are carriers of the AI virus without ever showing signs of the disease. Trade, transport of poultry, migrations of wild birds are equally means of longer-distance transmission.

Avian flu is highly contagious in chickens and turkeys and is likely to cause increased mortality for these species.

Aavian flu viruses evolve by mutation and are, for the most part, unpredictable in terms of the risks they pose to animal and human health. They cannot be eradicated from wild bird reservoirs and, once introduced in domestic birds, their control is often difficult. For all these reasons, they represent a considerable challenge to scientists, decision-makers and risk-managers.
Collaboration between the animal health and public health authorities is therefore vital to prevent and control this disease.

The type A avian flu virus can, in certain cases, be transmitted from animals to humans. Contamination essentially occurs during close, prolonged and repeated contact with infected animals or their feces, either by a direct or indirect route (surfaces and/or hands soiled with feces). Consequently, these cases remain exceptional and mainly concern people who work or operate in a contaminated area: breeders, technicians for cooperatives, vets, cleaning and disinfection teams etc.

Avian influenza is a notifiable disease.

To limit its spread, a range of very strict health measures are taken, such as putting warning systems in place, the quarantine or destruction of contaminated or potentially exposed flocks, stopping the transport of poultry or equipment that comes into contact with it.