West Nile Virus: An Update on This Rapidly Spreading Virus in France
In early fall 2026, France is reporting a record number of cases ofWest Nile virus infection, both in humans and in horses—both of which are “accidental” hosts of this virus, which primarily infects birds.
Yannick Simonin, University of Montpellier
THE ESSENTIALS
- After a record-breaking 2025 in France, the number of West Nile virus infections reached a new, exceptional high in 2026.
- Although it is very often asymptomatic, this virus can cause serious neurological complications, some of which can be fatal.
- The spread of this mosquito-borne virus is likely to increase further due to climate change, which extends the active season of these insects, increases their lifespan, and alters their metabolism.
As of September 21, 2026, more than 250 equine cases and 117 human cases had already been reported, far exceeding the 62 cases recorded throughout all of last year—which was already a record year! Our European neighbors have not been spared, as more than 1,500 cases have already been identified on the continent, where, unfortunately, nearly 80 deaths have been reported.
The number of infections caused by this virus, which is transmitted by the common mosquito, is expected to rise further in the coming weeks. Which regions are affected, what factors account for this unusual situation, and should we be concerned? Here’s an update on this virus, which is considered an emerging threat in Europe.
From Africa to Europe
The West Nile virus gets its name from the place where it was first isolated in 1937: the West Nile District in northern Uganda.
We do not yet fully understand all the details of how this virus was introduced into Europe, but we do know that it likely first arrived on the continent via migratory birds from Africa, before spreading to resident birds.
The first European outbreak was identified in France, in 1962–1963, in the Camargue. However, serological surveys (which involve testing blood for virus-specific antibodies, indicating past exposure) revealed that the West Nile virus had been circulating on the continent as early as the 1950s. It then remained largely unnoticed until 1996, when a major outbreak occurred in Romania: approximately 400 cases were reported. Others followed, in southern, eastern, and then western Europe.
In 1999, the virus made its way into the United States. There, it caused massive deaths among wild birds as well as animals at the Bronx Zoo and infected 62 people in New York, 7 of whom died. Within a few years, it had spread to nearly the entire United States, affecting birds, horses, and humans.
But for Europe, 2018 was the turning point. The virus then spread widely across the western part of the continent, causing the largest outbreak ever recorded, according to the European Centre for Disease Prevention and Control (ECDC): 2,083 cases and 181 deaths in a dozen countries, including France—more cases in a single year than in the previous ten years combined.
Since then, the virus’s range has continued to expand every year. Germany, the Netherlands, and then Belgium reported their first cases. The virus has become endemic in certain regions of southern Europe, particularly in southern France.
Île-de-France: The Virus’s New Playground
The countries hardest hit this year are Italy and Greece, which together accounted for more than half of the cases identified as of early September 2026.
Once confined to the Mediterranean region, the West Nile virus continues to spread across Europe.
In our country, the Santé publique France bulletin dated September 21, 2026, leaves no room for doubt: 117 locally acquired cases (i.e., contracted within the country) of West Nile virus infection have been detected. The record set in 2025 (62 cases and the first five deaths ever recorded in our country) has therefore already been significantly surpassed.
In addition, more than 250 cases in horses have been reported, more than double the previous record. No fewer than 16 departments are affected, including—for the first time—Aude, Seine-et-Marne, Drôme, Rhône, Isère, Loire-Atlantique, Maine-et-Loire, and Loiret, which have thus recorded their very first cases… The PACA region is the hardest hit, with 39 human cases reported.
Even more concerning is that Nouvelle-Aquitaine, which was first affected in 2023, is seeing new cases this year. And Île-de-France, which—to the surprise of experts—reported its first cases last year, is once again facing the spread of the virus. Essonne, Seine-Saint-Denis, Val-d’Oise, Val-de-Marne, Seine-et-Marne: the list of affected departments is growing…
This recurrence of cases indicates that the virus is likely establishing itself in new regions. And it’s not over yet: the situation is expected to continue to worsen in the coming weeks. Indeed, the virus is known to cause infections through November, and current weather conditions are conducive to mosquito activity.
A complex transmission cycle
The West Nile virus is an arbovirus (from the English “arthropod-borne virus,” meaning a virus transmitted by arthropods, specifically “hematophagous” ones—that is, those that feed on blood). It belongs to the genus Orthoflavivirus, which comprises more than 70 members, including the dengue virus, the Zika virus, and the yellow fever virus. At least seven lineages are known, but only lineages 1 and 2 have, to date, been associated with epidemics in humans.
One of the distinctive features of this virus compared to others (such as those that cause dengue, chikungunya, or Zika) is that it is not transmitted by the tiger mosquito, but by the common mosquito, known as Culex pipiens. This mosquito is found throughout mainland France and is the most widespread mosquito in Europe.
Birds—particularly passerines (such as sparrows, chickadees, swallows, etc.) and strigiformes (owls)—are considered the primary reservoir of the West Nile virus. Mosquitoes become infected after feeding on the blood of birds carrying the virus.
When a mosquito bites an infected bird, the virus can multiply within its body before infecting a new host—such as a human or a horse—during its next blood meal.
Thus, unlike most other arboviruses known to circulate in France, the West Nile virus is transmitted from animals to humans, making it a classic example of a zoonosis (a zoonosis is a disease transmitted from an animal species to humans).
However, unlike most other arboviruses circulating in France—such as dengue or chikungunya, for example—the West Nile virus cannot be transmitted from one person to another via mosquitoes. This is true even when a person develops symptoms. The hypothesis is that the viral load in the blood (the number of viral particles circulating in the blood) is too low to cause infection.
Numerous studies have also shown that several species of wild mammals may be naturally exposed to the virus. This is the case, for example, with bats, rodents, and wild boars.
However, their ability to spread the virus remains much lower than that of birds. These mammals could nevertheless play a role in maintaining the virus in certain ecosystems and indirectly contribute to human exposure.
No human-to-human transmission
Although the virus is not transmitted from person to person through mosquito bites, other routes of transmission may exist. In particular, the virus can be transmitted through blood transfusions or organ transplants.
In 2025, in France, two people were infected following kidney transplants. The donor, a man in his fifties who died of a cerebral hemorrhage, was found to have been infected with the virus before his death, but had not developed any clinical symptoms (except for a few small, diffuse lesions on the skin of his abdomen and legs).
The existence of such alternative routes of transmission therefore requires us to reevaluate our blood donation and organ transplantation strategies during periods when the virus is circulating.
In France, a blood safety protocol is triggered as soon as the virus is detected in a human. When the first case is confirmed in a department, blood donations there are systematically screened.
In addition, travelers returning from a high-risk area (such as the United States) are either tested or barred from donating for twenty-eight days during the vector-borne disease season. The same rule applies to organ donors, with an exception for vital organs that are transplanted without waiting for test results when an emergency requires it. France goes even further in the most at-risk departments by screening donations before the first human case appears.
Still, this system has its limitations: the two recipients in 2025 had received kidneys from a living donor in a department that was not yet on the list of high-risk areas. This is why it is important to monitor birds, mosquitoes, and horses early on, as they often signal the presence of the virus well before the first human cases appear.
A virus that is very often asymptomatic, but can sometimes be fatal
Originally, the West Nile virus was considered a benign virus. In fact, in the vast majority of cases, it causes no symptoms, and for a long time it was believed that, at worst, it caused only a brief fever.
That image was abruptly shattered in the late 1990s: a series of outbreaks—first in Romania, then in Russia, and later in the United States— revealed the existence of severe, sometimes fatal neurological forms of the disease.
It is currently estimated that, following infection, approximately 20% of infected individuals develop symptoms. Most often, these consist of nonspecific flu-like symptoms: fever, headache, generalized weakness, and muscle and joint pain, sometimes accompanied by skin rashes or eye symptoms.
In less than 1% of infected individuals, the virus crosses the physiological barrier that protects the central nervous system (the blood-brain barrier, which isolates the brain and spinal cord) and causes a form known as “neuroinvasive,” which can be fatal.
The latter manifests as encephalitis (inflammation of the brain), meningitis (inflammation of the membranes surrounding the brain), or acute flaccid paralysis, similar to that seen in poliomyelitis. These severe forms of the disease primarily affect the elderly, as well as patients with chronic conditions such as cardiovascular disease, diabetes, and cancer.
Furthermore, several studies have shown that 40 to 50 percent of these patients experience lasting cognitive, functional, or physical sequelae,including muscle weakness, persistent fatigue, pain, memory problems, kidney damage, and episodes of depression.
This is an often-overlooked aspect of the burden of this infection, which cannot be reduced to a simple tally of deaths.
The West Nile virus is currently the arbovirus responsible for the highest number of deaths in Europe, with an average of 80 deaths recorded per year over the past five years: in 2025, 97 deaths were recorded there out of just over 1,100 cases. This year, nearly 80 deaths have already been recorded, with Italy bearing the heaviest toll, having recorded more than 50 deaths as of early September. It is worth noting that last year, France unfortunately recorded its first five deaths linked to this virus.
A Deadly Virus for Horses From a veterinary perspective, horses are particularly susceptible to infection. As a result, they are used as indicators of the virus’s circulation, particularly in the Camargue, where sporadic cases of infection have been regularly detected since the early 2000s.
In these animals, infection can lead to severe clinical signs, particularly neurological ones, including the development of encephalitis. The mortality rate can reach 30 to 40 percent among infected animals exhibiting symptoms. This is cause for concern among owners and breeders in affected areas.
A Difficult Diagnosis
Diagnosing the disease is no easy task. In fact, its symptoms are not distinctive, and they are never sufficient on their own to determine the cause of the infection. For this reason, the number of identified cases is significantly underestimated, since, in our country, asymptomatic and febrile forms of the disease are rarely diagnosed.
The virus can be detected in the blood, urine, or cerebrospinal fluid, but the detection window is narrow, and it has often already closed by the time the patient seeks medical care. Diagnosis is therefore usually based on the detection of antibodies (IgM type), which appear early on in serum samples collected approximately five to eight days after the onset of clinical symptoms.
However, this approach has a major limitation: these antibodies can cross-react with those directed against closely related viruses, such as the Usutu virus or the dengue virus, which sometimes requires additional tests (known as “seroneutralization”) that are much longer and more complex.
In terms of treatment, the most severely affected patients are hospitalized to receive supportive care (hydration, respiratory support if necessary, etc.), while mild cases are treated with simple pain relievers.
The most frustrating aspect remains: there is an effective equine vaccine that provides good protection for horses, but no human vaccine. This situation can be explained in part by the difficulty in identifying sufficiently effective vaccine targets, but also by the complexity of clinical trials, given the unpredictable nature of the location and intensity of transmission clusters.
Furthermore, there is no specific antiviral treatment, as is the case with most arboviruses… Research programs are underway to try to address this gap, but results are not expected for several years.
Growth Amid Climate Change
The current increase in West Nile virus cases is likely a prelude to what lies ahead in the coming years with this virus and, more generally, with mosquito-borne viruses.
This rise, which began with the market breakdown in 2018 and has been confirmed ever since, is, in the view of many experts, inevitable.
There are many explanations. While the movement of migratory birds—which serve as reservoirs for the virus—facilitates the regular reintroduction of new viral strains onto the European continent, one of the most significant factors is undoubtedly climate change.
In fact, because mosquitoes cannot regulate their body temperature, they are extremely sensitive to climate fluctuations. However, the climate changes currently underway in certain regions are extending their active season. As a result, Culex pipiens mosquitoes are appearing earlier and earlier in the year, while they are disappearing later and later. This situation correspondingly extends the window during which the virus can circulate.
Furthermore, it is known that the lifespan of mosquitoes—which is two to four weeks during the summer— increases with rising temperatures (up to a certain temperature, around 35 °C). Under these conditions, their metabolism accelerates, which promotes the replication of the virus within their bodies.
This is a crucial point: in order to transmit the virus, a mosquito must first harbor it long enough for it to replicate, penetrate the wall of its intestine, and colonize its salivary glands. However, this period—which typically lasts one to two weeks— is significantly shortened in warm weather (up to a certain threshold). As a result, the mosquito becomes infectious more quickly and lives long enough to bite more people.
Added to this are heat waves and thunderstorms, which increase the number of warm, stagnant water sources where Culex mosquitoes readily lay their eggs. Furthermore, during periods of prolonged drought, mosquitoes and birds tend to congregate around the few available water sources, which encourages contact between birds and, consequently, the spread of the virus. Finally, milder winters allow mosquitoes to survive better and thus help maintain larger populations from one year to the next.
Given this situation, it is imperative to strengthen surveillance networks. This is the whole point of theOne Health approach: jointly monitoring birds, horses, and mosquitoes makes it possible to detect the virus before it causes the first human cases, and thus to take action before the first human cases occur rather than after.
Finally, while we wait for a human vaccine—which is long overdue—protection relies on simple measures: eliminating standing water around the home, where mosquitoes lay their eggs, and protecting yourself from bites using repellents, protective clothing, and mosquito nets—especially at dusk and at night, when Culex mosquitoes are most active.
Yannick Simonin, virologist specializing in the surveillance and study of emerging viral diseases. University Professor, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.