Avian influenza in 11 questions

Avian influenza is a highly contagious animal disease caused by influenza A viruses, which can affect many species of wild and domestic birds, as well as birds kept in zoos.

There are two categories of viruses, distinguished according to their virulence characteristics for birds:

• low pathogenic viruses (LPAI),
• highly pathogenic viruses (HPAI), all of which are of the H5 or H7 subtypes (see box).

In its highly pathogenic form, the disease spreads very quickly in birds and can have major consequences for both farms and wildlife. It can cause very high mortality in some species.

Avian influenza has a global reach, with different viral strains occurring more frequently in some parts of the world than in others. Since the 1950s, several outbreaks of varying severity caused by highly pathogenic avian influenza viruses have been reported on poultry farms around the world or in wild birds.

In France, since 2015, highly pathogenic viruses have caused animal epidemics of increasing magnitude, particularly in duck and goose farms in south-west France until the introduction of vaccination in October 2023. These episodes, most of which also affected other European countries, led to poultry culls on a scale never before seen in France or Europe.

To prevent such outbreaks from recurring and limit the number of infected flocks needing to be culled, France decided to make vaccination of ducks in flocks of more than 250 birds mandatory from October 2023.

Since 1997, there has been much talk about the highly pathogenic avian influenza A virus (H5N1) strain, due to major outbreaks in Asia in both domestic and wild birds and because of its zoonotic potential, i.e. its ability to infect humans under certain conditions.

Since 2003, this strain has spread worldwide and is still circulating today. H5N6, H5N5 and H5N8 viruses derived from it have also been spreading in recent years, albeit to a lesser extent than the H5N1 virus. All of these viruses are a cause for concern, due to their degree of virulence and their wide distribution, not only in domestic poultry but also in wild birds.

Some strains of the virus can infect mammals such as pigs, foxes, minks, ferrets and cats. They can cause respiratory or neurological symptoms in these species.  

When a bird virus infects a mammal (human or animal), certain mutations may be selected in the virus that promote its multiplication or transmission in this new host species.

Avian influenza is a disease with zoonotic potential; the virus can therefore, under certain conditions, be transmitted to humans. This transmission can occur, in the absence of personal protective equipment, through frequent and/or prolonged contact with animals infected with the virus, whether or not these animals are symptomatic.

The virus then causes symptoms in humans that most often resemble a common cold or flu. In rare cases, this can worsen rapidly due to severe respiratory problems. Human-to-human transmission is extremely rare. To date, no human cases have been observed in France. However, people who have been in contact with infected poultry and subsequently develop respiratory symptoms are advised to mention this to their doctor. A specific active surveillance protocol for bird flu (SAGA) has been put in place by the Directorate General for Health, to offer medical monitoring to anyone who has been exposed.

Lastly, simultaneous infection of a human or a pig with an avian influenza virus and a human flu virus could promote reassortment. This is the mixing of genetic material between the two viruses, which can lead to the emergence of a new virus that is highly contagious to humans. For this reason, people likely to come into contact with infected birds or pigs are advised to get vaccinated against seasonal flu.

Farmed poultry may be contaminated by infected wild birds or, in the event of an animal epidemic, by other domestic birds, whether farmed or kept for recreational purposes (backyard flocks, ornamental birds, decoys used when hunting waterfowl). Many different bird species can be infected.

Numerous factors can contribute to the spread of avian influenza viruses: 

• migratory movements of wild birds,
• breeding practices,
• the flow of people and materials within production sectors or between pet birds and farms.

Transmission between birds can be direct due to close contact between individuals: respiratory secretions, faecal matter; or indirect via exposure to contaminated materials or surfaces: feed, water, equipment, feathers, dust or clothing. The virus enters the bodies of poultry through the respiratory or digestive tract.

Depending on the exposure conditions and the virulence of the viral strain, the disease’s incubation period can range from one to three days within an individual; within a poultry flock, it can be as long as 14 days. The disease causes a variety of symptoms in birds, including nervous disorders (paralysis, convulsions, loss of balance), respiratory disorders, digestive disorders, swelling of the head, a decrease in the number of eggs laid and mortality, which in the most severe cases can occur almost without any prior symptoms.

The disease can also be asymptomatic, in which case it can only be detected via laboratory analyses. There are also more moderate forms presenting as a decrease in appetite, reduced laying performance, more or less inconspicuous respiratory signs, etc.

The vaccination programme for farmed ducks implemented in France helps to reduce clinical signs but does not necessarily prevent infection, depending on the age of the birds.

The European regulations were amended in 2023 to allow Member States to decide whether to launch a vaccination campaign if warranted by the health situation. They authorise the vaccination of poultry subject to certain conditions, such as introducing the monitoring of vaccinated flocks. This ensures that these birds are free from latent infection, because the available vaccines reduce but cannot completely stop shedding of the virus by birds that could still be infected, despite being vaccinated. Specific, validated tests are used to identify infected birds among vaccinated poultry.

New generations of vaccines and new detection techniques are already accessible or are being developed to make it easier to meet these requirements. It is also important to have vaccines tailored to the avian influenza viruses circulating in each domestic bird species at a given time. Several vaccines that have demonstrated their protective effect against the circulating viral strains have therefore been granted a temporary authorisation for use in France.

The fight against the disease today is essentially based on:

• reinforcement of biosecurity during risk periods, with birds kept indoors and reinforced hygiene measures at the entrance to farms (cleaning of equipment, change of work clothes, washing of hands, etc.), to avoid contamination from wild birds or transmission between farms;
• monitoring of poultry farms, with the aim of early detection and elimination of infected poultry as quickly as possible;
• during an outbreak, depopulation measures in the most affected areas, to limit the number of susceptible poultry that could potentially contribute to the spread of the disease.

Monitoring of this disease is regulated at the international level. Avian influenza is classified under the European Regulation on transmissible animal diseases as a Category A+D+E health hazard, i.e. one that should remain absent from the European Union. It is a notifiable disease subject to emergency measures to be taken in the event of an outbreak. There is a permanent monitoring and diagnostic network in France. It calls on the National Reference Laboratory (NRL, run by ANSES), farmers, state-mandated veterinarians, farm technicians, Departmental Directorates for the Protection of Populations (DDPPs), employees of the French Biodiversity Agency (OFB), ornithologists and veterinary diagnostic laboratories.

• As the NRL, ANSES is tasked with carrying out all the confirmatory analyses and methodological development work needed to diagnose avian influenza. The NRL leads the network of veterinary laboratories approved or recognised for first-line diagnosis, and regularly verifies their skills. When the virus is detected by laboratories, the NRL supplements this diagnosis with specialised "second-line" tests, which can, for example, determine the genetic characteristics of the influenza viruses detected.
• ANSES assists the health authorities in managing animal epidemics, mainly through epidemiological investigations to identify the causes of influenza outbreaks and study the risks of spread. It also conducts trials to assess the effectiveness of certain management approaches: experimental assessment of the time needed to sanitise slurry, contribution to a study of the effectiveness of vaccine candidates for waterfowl, and monitoring of viral persistence in landfill sites.
• In September 2026, ANSES published a report on a pilot study, developed as part of a consortium led by the Directorate General for Food, to support the roll-out of a "heterologous" vaccination protocol in ducks (i.e. one in which the first and second injections consist of different vaccines, as opposed to "homologous" protocols, where both injections are administered with the same vaccine). The aim of this study was to assess the practical implementation of such a vaccination protocol in the field, the quality of the post-vaccination antibody response (humoral immune response) obtained under these conditions, particularly at the end of the rearing period for the ducks, and how this compared with the immunity acquired through conventionally used equivalent protocols. Based on the encouraging results obtained, the option of implementing such a heterologous vaccination protocol was offered by the DGAL in the summer of 2026, on a voluntary basis.
• The Agency is conducting research on ducks as a potential relay for transmission to other poultry species, and on interspecies transmission of avian influenza viruses. This work ranges from the development of methods for detecting the virus on farms and in the environment, or studies of the mutations enabling the virus to adapt to a new host, through to seeking solutions to reduce the risks of virus introduction and spread.
• When infections are observed in areas near France, or on French poultry farms, or when wild birds die from avian influenza in our country, ANSES assesses the risk of introduction and spread of the disease on French poultry farms. It publishes scientific opinions to support the health authorities in managing the disease. It works in coordination with the national reference centres tasked with characterising human influenza viruses, while interacting with Santé Publique France and the regional health agencies, wich are the bodies responsible for monitoring and managing possible transmission to humans, in order to analyse the risks that influenza viruses detected in animals may pose to humans.