Why is West Nile virus circulating so extensively in France this year?

What is the current situation concerning West Nile virus in France?

Gaëlle Gonzalez: West Nile virus is transmitted by mosquitoes. First detected in the Mediterranean region in 1960, it’s gradually spread to new geographical areas. The year 2026 marks a turning point, with the virus circulating at particularly high levels this year and cases being reported in new parts of France, including the Ariège, Drôme, Ille-et-Vilaine, Haut-Rhin, Meurthe-et-Moselle, Dordogne and Loire-Atlantique administrative départements.

Since the beginning of the 2026 transmission season, 161 areas affected by West Nile virus have been identified in 15 countries across Europe.

As of 18 September 2026, we’d confirmed 282 symptomatic and 24 asymptomatic cases in horses, as well as 12 cases in birds. In humans, just over 92 cases had been recorded by Santé Publique France as of 14 September 2026 – the first time France has seen numbers on this scale. Across Europe, meanwhile, no fewer than 1,285 locally acquired human cases of West Nile virus infection were reported over the same period.

This year, the virus therefore began circulating earlier – from the beginning of August – and more intensely than we’d observed up to now. 

Why are there so many cases this year?

Gaëlle Gonzalez: At this point, we don’t have an answer to that question. There are, however, several possible explanations.

One possibility is that surveillance has improved. Since 2022, ANSES has been working with equine veterinarians and other stakeholders – including the Epidemiological Monitoring Network for Equine Diseases (RESPE), the French Association of Veterinarians and the Departmental Directorates for the Protection of Populations – to raise awareness of West Nile virus and encourage further testing as part of the diagnostic process. These efforts may have resulted in more cases being identified.

We’re also investigating whether the virus itself may have evolved. Has it become established in populations of semi-migratory or resident birds? Could a new, more virulent strain be circulating? By characterising the strains that are currently circulating, our laboratory hopes to gain a better understanding of what’s happening. 

It’s also important to take account of the very particular circumstances of 2026. We’ve experienced unprecedented weather conditions (very high temperatures, localised storms in the Aude département, etc.). Such conditions can affect the abundance and behaviour of mosquito vectors, among other things, and can also alter the routes taken by migratory birds. 

Is West Nile virus seasonal?

Gaëlle Gonzalez: Yes, but its seasonality depends on the geographical area because it’s linked to the activity of the mosquitoes transmitting the virus. In metropolitan France, the main vectors are mosquito species of the genus Culex, which are generally active from April or May until late November. For transmission to occur, however, the mosquito vectors, the birds that serve as the main reservoirs of the virus, and other hosts, including horses and humans, must all be present in the same place at the same time.

Climate change might alter this seasonal pattern. For example, warmer winters could extend the period during which mosquitoes are active, as they only die when temperatures fall below 14°C. 

The current seasonal pattern therefore isn’t set in stone. This is one of the reasons why surveillance also needs to be reinforced.

How is the ANSES laboratory working to address the topic?

Gaëlle Gonzalez: ANSES is playing a central role. Through its Laboratory for Animal Health, it has national and European reference mandates for West Nile virus. We’re therefore involved in diagnosing and confirming cases in horses and birds and characterising the viral strains circulating in France and Europe.

Our National Reference Laboratory relies on a network of five authorised laboratories, which perform first-line serological tests. The samples are then sent to ANSES for confirmation.

As part of the surveillance scheme, ANSES has been mandated by the Directorate General for Food (DGAL) to confirm cases of West Nile virus in birds as well as in horses showing suggestive clinical signs. Asymptomatic cases are also sometimes detected, in particular following testing due to the discovery of a symptomatic case.

But diagnosing the virus is only part of our role. Data collected in the field can also raise new research questions. When we observe unusual changes in the circulation of the virus, for example, we can carry out further studies, such as seroprevalence surveys, to better understand the extent of this circulation and the underlying mechanisms.

We're also working at European level to develop effective, affordable diagnostic methods to facilitate field surveillance of the virus in animals and mosquitoes in various EU Member States.

Why is a “One Health” approach essential to control West Nile virus? 

Gaëlle Gonzalez: Surveillance of West Nile virus can only be effective when animal health, human health and the environment are considered as a whole.

For West Nile virus, we work closely with equine veterinarians and zoos, as well as with human health stakeholders – the National Reference Centre at Aix-Marseille University, the French Blood Agency and Regional Health Agencies – and various authorities at national, departmental and local level.

Surveillance data from animals are now early warning signals for human health. Detecting cases in horses or birds helps us identify the areas where the virus is circulating and implement appropriate measures to protect the population. These may include improving the safety of blood and organ donations, for example by testing all blood bags for the virus.

Is this what the “One Health” approach looks like in practice?

Gaëlle Gonzalez:  Yes. West Nile virus shows that “One Health” isn’t just a theoretical concept: it's a very specific way of working.

The aim is to have an agile and adaptable surveillance system capable of taking the specific characteristics of each geographical area into account and rapidly mobilising the right stakeholders.

Situations may differ from one region to another. The stakeholders involved, the environmental conditions, the animal populations concerned and the possible courses of action may vary. By staying closely connected to what's happening in the field, we can adapt our response.

We also tailor our exchanges and messages to different audiences, such as researchers, healthcare professionals, veterinarians, farmers, public decision-makers, local stakeholders and citizens. This can take the form of coordination meetings with government departments, discussions with professionals or awareness-raising initiatives for the general public.

The work being done on West Nile virus clearly demonstrates the value of breaking down barriers between different areas of expertise and sources of data. By combining information on animal health, human health and the environment, we can better understand how the virus is circulating and act more quickly to limit the consequences.