Chikungunya, dengue, West Nile virus: In 2025, France faces an unprecedented viral outbreak
Once numbering just a handful each year, then in the tens, there are now hundreds of locally acquired cases—that is, cases contracted within France—of infections caused by mosquito-borne viruses. More than 800 cases have been reported this year, an unprecedented number!
Yannick Simonin, University of Montpellier
The surge in the number of cases of mosquito-borne viral infections in France makes 2025 a truly exceptional year. This unfortunate record is mainly due to the widespread circulation of the chikungunya virus, but our country is also facing a circulation—admittedly more limited, but nonetheless significant—of the dengue and West Nile viruses.
We are therefore facing, for the first time, a three-way circulation of these viruses known as arboviruses (short for “arthropod-borne viruses,” meaning viruses transmitted by arthropods, in this case mosquitoes). This two-pronged challenge—a significant increase in the number of infections and a wide variety of circulating viruses—creates a particularly complex situation.
How should we describe the 2025 season? Exceptional? Unprecedented? Unexpected? While the first two adjectives are undoubtedly apt, the last one is inaccurate, given that experts in the field have been warning for several years about the foreseeable—even inevitable—risk of these viruses, once confined to tropical regions, becoming permanently established in our part of the world.
France is not the only country affected: Italy, Spain, and other European countries are also seeing a surge in cases, reflecting a broader trend, even though our country remains one of the hardest-hit in Europe.
The chikungunya virus accounts for the lion's share
In France, the chikungunya virus alone is responsible for nearly 800 cases spread across 80 different clusters—that is, groups of cases linked to the same location or event.
The chikungunya virus is transmitted by the now-famous tiger mosquito (Aedes albopictus). Introduced to France in 2004, this insect is now the primary vector for viral diseases in Europe. While the south of France, particularly the PACA region, remains the most affected area, cases of infection have also been identified further north, notably in Alsace and Île-de-France, where the tiger mosquito is now well established.
The main reason for the rise in chikungunya cases observed in mainland France is the widespread circulation of the virus this year in Réunion and Mayotte, which are facing their largest chikungunya outbreak in more than twenty years. Regular air travel between these overseas territories and mainland France—as well as, more broadly, with other areas of high viral circulation—has facilitated the introduction of the virus to the mainland.
The proof is in the fact that this year, nearly 80% of imported cases (where the disease was contracted while traveling) reported in mainland France originated in Réunion.
It is these imported cases that are responsible for the local cases, as the tiger mosquito can bite these infected individuals and transmit the disease to others who have not traveled. With numerous imported cases and the tiger mosquito’s increasingly widespread distribution across France, the reasons for this year’s surge are clear…
What are the consequences? The symptoms of chikungunya are often similar to those of the flu, including fever, headaches, and muscle aches. While most patients make a full recovery within a few days, some people may develop a chronic form of the disease characterized by severe, debilitating joint pain that can last for several years after infection!
Although it accounts for the vast majority of cases, chikungunya is not the only virus circulating in mainland France this year.
Numerous cases of dengue and West Nile virus infections
Two other viruses have made headlines this year. The first is the dengue virus (usually the most widespread arbovirus in the world), which is also transmitted by the tiger mosquito. About thirty locally acquired cases of dengue have been identified in France. Just a few years ago, that number would have been considered exceptional!
In addition, there are nearly 1,000 imported cases, mainly from Guadeloupe, French Polynesia, and Martinique, but also from many other parts of the world. The actual number of cases is likely significantly underestimated, as dengue is primarily asymptomatic, and in those who do develop symptoms, these can easily be confused with those of other illnesses, since they include fever, headaches, and muscle aches.
Fortunately, in the vast majority of cases, dengue fever causes only a mild infection. However, in about 1% of those infected, the disease can take a much more severe form—known as dengue hemorrhagic fever—which can be fatal.
The latest member of this unprecedented trio that has sent case numbers soaring this year is the West Nile virus, with nearly 60 cases reported, mainly in southern France. Unlike the two previous viruses, this one is spread by the common house mosquito (Culex pipiens), a native species that has been present in Europe for thousands of years and is widely distributed throughout our territory.
Furthermore, we are not talking about imported or indigenous cases here; all identified cases are local infections. Why? Because humans cannot transmit the virus to other humans via mosquitoes.
It is infected birds—which often come from distant countries during their migration—that transmit the West Nile virus to other “local” birds via common mosquitoes. It is these mosquitoes that can then transmit the virus to us. This is therefore a classic case of a zoonosis: the transmission of a disease from animals to humans.
This situation, which is more difficult to anticipate, is further complicated by the fact that West Nile virus can also be transmitted through blood donations or organ transplants. This year in France, two people were infected following kidney transplants due to infection with the virus from the donor’s organ. These alternative modes of transmission require us to reevaluate our blood and organ donation strategies during periods when this virus is circulating…
Key development: The West Nile virus, which was previously confined mainly to southern Europe, is now spreading further north. The Île-de-France region has been affected for the very first time, with about 20 cases identified, illustrating the growing threat.
While this virus is harmless for most people who contract it, in some individuals—particularly the elderly—it can target the brain and cause encephalitis or meningitis (inflammation of the brain or the meninges), which can be fatal. As a result, it is the arbovirus that causes the most deaths in Europe, with more than 60 deaths recorded in Europe in 2025, including, unfortunately, the first two in France this year…
Against the backdrop of climate change
The current rise in cases is likely a prelude to what lies ahead in the coming years… This increase, which began modestly in the early 2020s, is, in the view of many experts, inevitable.
There are many explanations, such as the increase in air travel, which facilitates the importation of cases in an increasingly interconnected world. But one of the most significant factors is undoubtedly global warming. Indeed, insects such as mosquitoes, which cannot regulate their body temperature, are extremely sensitive to climate fluctuations.
In some regions, ongoing climate change is extending their active season. As a result, mosquitoes are appearing earlier and earlier in the year and disappearing later and later. Furthermore, up to a certain temperature, mosquitoes’ lifespans increase with rising heat, and their accelerated metabolism promotes the replication of viruses within their bodies. A dangerous combination…
To address this new situation, it is imperative to strengthen surveillance networks for these emerging viruses. Furthermore, eliminating standing water (which promotes mosquito breeding) remains, along with personal protection against bites (mosquito nets, repellents), the best current strategy for combating these new threats. These threats are difficult to anticipate, but we know they will become a daily reality in the years to come…
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.