Chikungunya, dengue fever, West Nile virus: in 2025, France faces unprecedented viral circulation
Once counted on the fingers of one hand each year, then in the dozens, today there are hundreds of indigenous cases—that is, cases contracted within the country—of infections by mosquito-borne viruses in mainland France. More than 800 cases have been recorded this year, an unprecedented number!
Yannick Simonin, University of Montpellier
The explosion in the number of cases of mosquito-borne virus infections in France makes 2025 a truly exceptional year. This sad record is mainly due to the very active circulation of the chikungunya virus, but our country is also facing a circulation, albeit more limited but nevertheless significant, of the dengue and West Nile viruses.
We are therefore faced, for the first time, with a triple circulation of these viruses called arboviruses (from the English term arthropod-borne virus, meaning "viruses transmitted by arthropods," in this case mosquitoes). This dual problem—a significant increase in the number of infections and a diversity of circulating viruses—creates a particularly complex situation.
How can we describe the 2025 season? Exceptional? Unprecedented? Unexpected? While the first two adjectives are undoubtedly appropriate, the last one is inaccurate, given that experts in the field have been warning for several years about the predictable, even inevitable, risk of these viruses, once confined to tropical regions, becoming established in our latitudes.
France is not the only country affected: Italy, Spain, and other European countries are also seeing a resurgence of cases, reflecting a global trend, even though France remains one of the most affected countries in Europe.
The chikungunya virus takes the lion's share
In France, the chikungunya virus alone is responsible for nearly 800 cases spread across 80 different clusters, i.e., groups of cases linked to the same place or event.
The chikungunya virus is transmitted by the now infamous tiger mosquito (Aedes albopictus). Introduced to France in 2004, this insect is now the main vector for viral diseases in Europe. While the south of France, particularly the Provence-Alpes-Côte d'Azur region, remains the most affected area, cases of infection have also been identified further north, particularly in Alsace and Île-de-France, where the tiger mosquito is now well established.
The main explanation for the increase in chikungunya cases observed in mainland France lies in the high circulation of the virus this year in Réunion and Mayotte, which are facing the most significant chikungunya epidemic in over twenty years. Regular air travel between these overseas territories and mainland France, but also, more broadly, with other areas of high viral circulation, has facilitated the introduction of the virus to the continent.
Proof of this is that this year, nearly 80% of imported cases (where the disease was reported after travel) observed in mainland France originated in Réunion.
These imported cases are the source of the local cases, as the tiger mosquito can bite these infected individuals and transmit the disease to other people who have not traveled. With numerous imported cases and the tiger mosquito spreading increasingly widely across France, the reasons for this year's increase are clear...
What are the consequences? The symptoms of chikungunya are often similar to those of the flu, with fever, headache, and muscle pain. Most patients recover completely within a few days. However, in some people, a chronic form of the disease can develop with very debilitating joint pain that can last for several years after infection!
Although it is by far the most prevalent, 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 been in the news this year. The first is dengue fever (usually the most widespread arbovirus in the world), which is also transmitted by the tiger mosquito. Around 30 indigenous cases of dengue fever have been identified in France. Just a few years ago, this figure 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 regions around the world. The actual number of cases is probably greatly underestimated, as dengue is mainly asymptomatic and, in people who do develop symptoms, these can easily be confused with those of other diseases, as they include fever, headache, and muscle pain.
Fortunately, in the vast majority of cases, dengue causes a mild infection. However, in about 1% of infected individuals, the disease can take a much more serious, hemorrhagic form, which can be fatal.
The last member of this unusual trio that has caused a stir this year is the West Nile virus, with nearly 60 cases reported, mainly in the south of France. Unlike the two previous viruses, this one is spread by the common 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, as all identified cases are local infections. Why? Because humans cannot transmit the virus to other humans via mosquitoes.
It is infected birds, often arriving from distant countries during their migration, that transmit the West Nile virus to other "local" birds via common mosquitoes. It is also these mosquitoes that can then transmit the virus to us. This is therefore a typical case of 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 after receiving kidney transplants due to infection with the virus from the donor's graft. These alternative modes of transmission require us to review our blood and organ donation strategies during periods when this virus is circulating.
Key fact: West Nile virus, mainly confined to southern Europe, is now spreading further north. The Île-de-France region has been affected for the very first time, with around 20 cases identified, illustrating the extent of the threat.
This virus, which is harmless to most infected individuals, can nevertheless target the brain in certain people, particularly the elderly, causing encephalitis or meningitis (inflammation of the brain or meninges), which can be fatal. It is therefore the arbovirus that causes the most deaths in Europe, with more than 60 deaths identified in Europe in 2025, including, sadly, the first two in France this year...
Climate change as a backdrop
The current increase in cases is likely a prelude to what awaits us in the coming years... This rise, which began slowly in the early 2020s, is inevitable for many specialists.
There are many explanations for this, such as the increase in air travel facilitating the importation of cases in an increasingly interconnected world. But one of the most important factors is undoubtedly global warming. Insects such as mosquitoes, which cannot regulate their temperature, are extremely sensitive to climate variations.
In some regions, ongoing climate change is extending their period of activity. Mosquitoes are appearing earlier and earlier in the year and disappearing later and later. Furthermore, up to a certain temperature, mosquitoes' lifespan increases with heat and their accelerated metabolism promotes the multiplication of viruses in their bodies. An explosive cocktail...
To address this new situation, it is imperative to strengthen surveillance networks for these emerging viruses. In addition, 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 that they will become part of everyday life 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.