Chikungunya, dengue, West Nile: in 2025, France will face unprecedented viral circulation
Once counted on the fingers of one hand, then by the dozens, the number of indigenous cases of mosquito-borne viruses in mainland France is now in the hundreds. More than 800 cases were recorded this year, an unprecedented figure!
Yannick Simonin, University of Montpellier
The explosion in the number of cases of infection by mosquito-borne viruses in France makes 2025 a totally exceptional year. This sad record is mainly due to the very active circulation of the chikungunya virus, but our country is also confronted with the more limited but nonetheless significant circulation of the dengue and West Nile viruses.
So, for the first time, we are faced with a triple circulation of these viruses known as arboviruses (from the English arthropod-borne virus, "virus transmitted by arthropods", in this case mosquitoes). This dual problem - a significant increase in the number of cases of infection and a diversity of circulating viruses - creates a particularly complex situation.
How would you describe the 2025 season? Exceptional? Unprecedented? Unexpected? While the first two adjectives are unmistakable, the last one proves inaccurate, as specialists in the field have been warning for several years of the foreseeable, indeed inevitable, risk of these viruses, once confined to tropical regions, taking root in our latitudes.
France is not the only country concerned: Italy, Spain and other European countries are also seeing an upsurge in cases, reflecting a global dynamic, even if our country remains one of the most affected in Europe.
The chikungunya virus takes the lion's share
In France, the chikungunya virus alone has been implicated in almost 800 cases in 80 different outbreaks, i.e. groups of cases linked to the same place 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 main vector of viral diseases in Europe. While the south of France, notably 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 the number of chikungunya cases observed in France is the high level of virus circulation this year in Réunion and Mayotte, which are experiencing their biggest chikungunya epidemic in over twenty years. Regular air traffic between these overseas territories and mainland France, as well as with other areas of high viral circulation, has encouraged the introduction of the virus on the mainland.
The proof is in the pudding: this year, nearly 80% of imported cases (where the disease has been brought back from a trip) observed in mainland France come from La Réunion.
It is these imported cases that are responsible for the indigenous cases, as the tiger mosquito can bite these infected individuals and transmit the disease to other people who have not travelled. Numerous imported cases, an increasingly widespread distribution of the tiger mosquito in France: the explanations for this year's increase are all there...
What are the consequences? Symptoms of chikungunya are often flu-like, with fever, headaches and muscle aches. Most sufferers recover completely within a few days. However, in some people, a chronic form of the disease can set in, with crippling joint pain lasting up to several years after infection!
Although chikungunya is in the majority, it is not the only virus circulating in mainland France this year.
Numerous cases of dengue and West Nile virus infections
Two other viruses were in the news this year. The first is dengue fever (usually the most widely circulating arbovirus in the world), also transmitted by the tiger mosquito. Some thirty native cases of dengue fever have been identified in France. Just a few years ago, this figure would have been considered exceptional!
In addition, there have been almost 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 probably greatly underestimated, as dengue fever is mainly asymptomatic and, in those who develop symptoms, these can easily be confused with those of other illnesses, as they manifest themselves in fever, headaches and muscle aches.
Fortunately, in the vast majority of cases, dengue fever causes a mild infection. However, in around 1% of those infected, the disease can take a much more serious, hemorrhagic form, which can prove fatal.
The latest member of this unprecedented trio to send the counters into overdrive this year is the West Nile virus, with almost 60 cases reported, mainly in the south of France. Unlike the previous two viruses, this one is spread by the common mosquito(Culex pipiens), a species of indigenous mosquito that has been present in Europe for thousands of years and is widely distributed throughout the country.
What's more, we're not talking about imported or indigenous cases here, as all identified cases are local infections. Why is this? Because humans cannot transmit the virus to other humans via mosquitoes.
Infected birds, often migrating from distant countries, 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 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 by blood donation or organ transplantation. In France this year, two people were infected after a kidney transplant, as a result of the virus infecting the donor's graft. These alternative transmissions mean that we need to review our blood and organ donation strategies during periods when this virus is circulating...
The West Nile virus, mainly confined to southern Europe, is now spreading further north. Île-de-France has been affected for the very first time, with some twenty cases identified, illustrating the extension of the threat.
This virus, harmless for the majority of infected people, can nonetheless, in some people, particularly the elderly, target our brains and cause encephalitis or meningitis (inflammation of the brain or meninges), which can prove fatal. It is therefore the arbovirus that causes the most deaths in Europe, with over 60 deaths identified in Europe in 2025, including unfortunately the first two in France this year...
Climate change in the background
The current rise in the number of cases is probably a prelude to what is to come in the next few years... This rise, which began timidly in the early 2020s, is seen by many specialists as inevitable.
There are many explanations for this, such as the increase in air transport, which favours the import of cases in an increasingly interconnected world. But one of the most important factors is undoubtedly global warming. Insects such as mosquitoes are unable to regulate their temperature, making them extremely sensitive to climatic variations.
In some regions, ongoing climate change is increasing their period of activity. Mosquitoes are seen earlier and earlier in the year, and disappear later and later. What's more, up to a certain temperature, mosquitoes' lifespan increases with the heat, and their metabolism, accelerated, favors the multiplication of viruses in their bodies. An explosive cocktail...
To deal with this new situation, it is imperative to strengthen surveillance networks for these emerging viruses. In addition, the elimination of stagnant water (which favors mosquito breeding) remains, along with individual means of protection against bites (mosquito nets, repellents), the best current strategy for combating these new threats. Threats that are difficult to anticipate, but which we know will become daily occurrences 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. Read theoriginal article.