Avian influenza: the current health risks for animals and humans

What is avian influenza?

Avian influenza is a highly contagious animal disease caused by type A influenza viruses, which can affect numerous species of birds, both wild and domestic. Certain viral strains can also be transmitted to terrestrial and marine mammals, and to humans.

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

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

In its highly pathogenic form, the disease spreads very quickly in birds and can have major consequences for both farms and wildlife. In some species it can cause very high mortality (> 90%).

What are the symptoms of avian influenza in birds?

Béatrice Grasland: Depending on the exposure conditions and the virulence of the viral strain, the disease’s incubation period can range from one to seven days within an individual, according to the species, and can be as long as 14 days within a poultry flock. The disease causes a variety of symptoms in birds, including nervous disorders (paralysis, convulsions, loss of balance), respiratory or 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.

How is avian influenza transmitted among animals?

Eric Cardinale: 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). Birds may also become infected through contact with a contaminated environment. Transmission between birds can be direct due to close contact: 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.

Is there a seasonal pattern to episodes of avian influenza?

Nicolas Eterradossi: We are seeing recurring winter peaks of avian influenza in birds due to the influx of migratory birds, but also more sporadic cases in spring and summer, linked to the virus's persistence in local non-migratory birds and in the farm environment.

Is the current avian influenza virus of greater concern than the one that circulated during previous episodes?

Nicolas Eterradossi: Since 2003, a highly pathogenic strain of the influenza A virus (H5N1) has been spreading around the world. Numerous viruses have evolved from this strain, through mutations and a genetic phenomenon known as "reassortment". The H5N1 viruses currently circulating in France belong to one of the genetic lineages derived from these viruses (clade 2.3.4.4b). Since 2016, it has been responsible for several large-scale animal epidemics around the world and in France, particularly on waterfowl farms in the South-West, but also in wild birds.

Subtype H5 viruses of clade 2.3.4.4b are of concern due to their extensive worldwide distribution in domestic and wild birds, which is having a major ecological and economic impact, including in previously unaffected areas (South America, Antarctica) since 2023.

In addition, these viruses have great potential for genetic evolution. They occasionally cross the "species barrier" to infect mammals (domestic or wild mammals exposed to infected wild birds, or humans exposed to infected poultry or wild animals). These infections in mammals can be serious.

Under certain conditions the virus can even be transmitted between mammals, as has been observed on dairy farms in the United States and in colonies of marine mammals. This raises fears, in the event of uncontrolled circulation, of the emergence of a strain of H5 virus able to be transmitted between humans. Controlling the circulation of these viruses therefore helps protect both animal and public health.

What explains the persistence of major crises of highly pathogenic avian influenza in France and other countries around the world in recent years?

Nicolas Eterradossi: The extensive worldwide distribution of the virus explains why migratory birds exchange influenza viruses every year, when they gather in Arctic regions to breed or just before their migration. These viruses can then be carried to new parts of the world during migration. Moreover, the host bird species of the currently circulating H5 viruses of clade 2.3.4.4b include certain non-migratory species, which explains the year-long persistence of the virus in certain areas. In the last few years, France has been confronted by both phenomena. 

Is there a risk of the virus being transmitted to cattle in France, as has been seen in the United States? Does this phenomenon pose a health risk to cattle or humans?

Eric Cardinale: There is a risk of the virus being transmitted to cattle in France, as it has been shown experimentally that the viruses circulating in Europe can infect ruminants. However, no cases have yet been reported. We are monitoring the possible risk of introduction of the virus currently circulating in the United States, which could enter Europe during the migratory movements of wild birds in spring and autumn, or through an infected human or animal in a herd .   

How can the circulation of avian influenza on farms be contained?

Nicolas Eterradossi: Control of the disease continues to be based first and foremost on biosecurity measures: keeping birds indoors during high-risk periods and reinforcing hygiene measures on entering farms (cleaning equipment, complete change of working clothes, washing hands, etc.). These measures are effective in preventing both contamination from wild birds and transmission between farms. We monitor cases of avian influenza on farms, with the aim of early detection and elimination of infected poultry as quickly as possible. Similarly, together with our partners we are monitoring viruses associated with wildlife mortality. In addition to these biosecurity and rapid outbreak elimination measures, since October 2023, production duck farms with more than 250 birds have been required to vaccinate them against subtype H5 highly pathogenic avian influenza.

Can vaccinating farmed poultry be a long-term solution to preventing avian influenza crises?

Nicolas Eterradossi: Since 2015, France has managed several large-scale outbreaks – which have also affected other European countries – using a combination of biosecurity, eradication and large-scale preventive depopulation, particularly during the winter of 2021-2022 and, to a lesser extent, the winter of 2022-2023.

Following changes in European regulations and preparatory research by ANSES into the effectiveness of duck vaccines, the French health authorities, with the support of the European Commission, authorised vaccination on duck farms. A policy combining biosecurity, vaccination and reinforced surveillance of farms, with the aim of continuing to eliminate any infected flocks as soon as possible, even if they have been vaccinated, was adopted with the professional organisations.

This policy was one of the determining factors behind the sharp reduction in the number of cases of avian influenza observed on farms in France since the winter of 2023-2024. However, it is difficult to quantify the contribution of each control method. Vaccination has very likely played a part in this observed reduction in the number of cases, but it is no substitute for compliance with the other prevention and control measures. In addition, other factors, such as the low infectious pressure in wild birds and the characteristics of the viruses circulating during the winter of 2023-2024, may also have contributed to the improvement in the situation.

Does vaccinating ducks pose a health risk for consumers?

Franck Fourès: Consuming meat or products from vaccinated animals is not dangerous for humans. The safety of vaccines   for consumers is verified prior to granting marketing authorisation or temporary authorisation for use.

How can transmission of the virus from birds to humans be prevented?

This transmission can be prevented by using suitable personal protective equipment (PPE) when handling or coming into contact with dead domestic or wild birds. This PPE must at least prevent the virus from coming into direct contact with the mucous membranes of the respiratory tract (respiratory masks) and eyes (safety goggles), and should protect the user from indirect transmission to these mucous membranes via the hands (gloves).

What is the risk of the currently circulating avian influenza virus adapting to humans and causing a pandemic?

Eric Cardinale: Influenza viruses have a considerable ability to exchange genetic material and adapt. Mutations have occasionally been observed that enable these viruses to establish themselves more easily in mammals, and particularly in humans. However, the viruses that have infected dozens of people in the United States in recent months have not yet accumulated enough mutations to enable transmission between humans. It is important to keep monitoring viruses circulating in animal species, in order to identify any changes in a virus that could make it more adaptable to mammals, and enable us to plan ahead.

What is being done to monitor the spread of the disease/epidemic?

Eric Cardinale: Surveillance remains an essential key in our fight against these influenza viruses. We currently monitor not only domestic poultry and wild birds, but also mammals, including marine mammals, in mainland France and the French overseas territories and Southern and Antarctic territories. We are also working with our human health colleagues to detect any viruses that may have potentially evolved.

Is this the end of free-range farming? What about animal welfare?

The active circulation of these avian influenza viruses in wild and domestic birds does not signal the end of free-range farming. It does nevertheless call for the utmost vigilance and for animals to be kept indoors during the periods of greatest risk, i.e. during the southerly migration (late autumn and early winter), while adapting farming methods to protect animals from this contamination (reinforced biosecurity, vaccination where appropriate for the most sensitive species), particularly during these high-risk periods, and maintaining rearing conditions that protect the welfare of these animals.

This article was produced by interviewing Béatrice Grasland, head of the National Reference Laboratory (NRL) for avian influenza (Ploufragan-Plouzané-Niort Laboratory, ANSES); Éric Cardinale, Scientific Director for Animal Health and Welfare, ANSES; Nicolas Eterradossi, Director of the Ploufragan-Plouzané-Niort Laboratory, ANSES; and Franck Fourès, Director of the French Agency for Veterinary Medicinal Products (ANMV), ANSES.

Further information:

• Dossier on avian Influenza (in French)
• Zoonoses, when animals contaminate humans
• Swine flu: a challenge for livestock and human health

Zootopique podcast "Influenza, a disease of the past that has a future" (in French)