Chikungunya, dengue fever, West Nile virus: in 2025, France will face unprecedented viral circulation

Montpellier (France), Nov 6 (The Conversation) The explosion in the number of mosquito-borne virus infections in France makes 2025 a truly exceptional year. This grim record is primarily due to the highly active circulation of the chikungunya virus, but our country is also facing a more limited, but nonetheless significant, circulation of dengue and West Nile viruses.

We are therefore facing, for the first time, a triple circulation of these viruses called arboviruses (from the English arthropod-borne virus, "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 to describe the 2025 season? Exceptional? Unprecedented? Unexpected? While the first two adjectives are undoubtedly appropriate, the last proves inaccurate, as specialists in the field have been warning for several years about the foreseeable, even inevitable, risk of the permanent establishment of these viruses, once confined to tropical regions, in our latitudes.

France is not the only country affected: Italy, Spain and other European countries are also recording a resurgence of cases, demonstrating a global trend, 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 is implicated in nearly 800 cases spread across 80 different clusters , that is to say, 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 of viral diseases in Europe. While southern France, particularly the Provence-Alpes-Côte d'Azur (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 explanation for the rise 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 largest chikungunya epidemic in over 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 continent.

Proof of this is that this year, nearly 80 per cent of imported cases (where the disease was reported from travel) observed in mainland France come from Réunion.

It is these imported cases that 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 travelled. Numerous imported cases, an increasingly widespread distribution of the tiger mosquito across France: the explanations 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, headaches, and muscle aches. While most patients recover completely within a few days, in some people, a chronic form of the disease can develop, causing debilitating joint pain that can last for several years after infection.

Although 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 made headlines this year. The first is dengue fever (usually the most widespread arbovirus in the world), also transmitted by the tiger mosquito. Around 30 locally acquired cases of dengue have been identified in mainland France. Just a few years ago, this number would have been considered exceptional! In addition to these, there are nearly 1,000 imported cases, mainly from Guadeloupe, French Polynesia, and Martinique, but also from many other regions of the world.

The actual number of cases is likely a significant underestimate, as dengue is primarily asymptomatic, and in those who do develop symptoms, these can easily be mistaken for those of other illnesses, as they include fever, headaches, and muscle aches.

Fortunately, in the vast majority of cases, dengue fever causes a mild infection. However, in approximately 1 per cent of infected individuals, the disease can take a much more severe, hemorrhagic form that can be fatal.

The latest member of this unprecedented trio that has sent the numbers soaring this year is West Nile virus, with nearly 60 cases recorded, mainly in the south of France.

Unlike the two previous viruses, this one is spread by the common mosquito (Culex pipiens), a native mosquito species present in Europe for millennia and widely distributed throughout the country.

Furthermore, we are not talking about imported or locally acquired cases here; all identified cases are local infections. Why? Because humans cannot transmit the virus to other humans through mosquitoes.

It is infected birds, often migrating from distant countries, that transmit the West Nile virus to other "local" birds via common mosquitoes. These mosquitoes can then transmit the virus to humans. 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 donation or organ transplantation.

This year in France, two people were infected after a kidney transplant due to infection with the donor's graft. These alternative transmission routes necessitate a review of our blood and organ donation strategies during periods when this virus is circulating.

A significant development: the 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 expansion of the threat.

This virus, harmless for most infected individuals, can nevertheless, in some people, particularly the elderly, target the brain and cause 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 by 2025, including, unfortunately, the first two in France this year.

Climate change as a backdrop ================== The current increase in cases is probably the prelude to what awaits us in the coming years. This rise, which began timidly at the start of the 2020s, is inevitable for many specialists.

The explanations are numerous, such as the increase in air travel facilitating the importation of cases in an increasingly interconnected world. But one of the most significant factors is undoubtedly climate change. Indeed, insects like mosquitoes, unable to regulate their body temperature, are extremely sensitive to climatic variations.

In some regions, ongoing climate change is increasing their active period. Mosquitoes are appearing earlier and earlier in the year, and their disappearance is occurring later and later.

Furthermore, up to a certain temperature, mosquitoes' lifespan increases with heat, and their accelerated metabolism promotes the multiplication of viruses within their bodies. A dangerous combination indeed.

To address this new situation, it is imperative to strengthen surveillance networks for these emerging viruses. Furthermore, eliminating stagnant water (which promotes mosquito breeding) remains, along with personal protective measures 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 occurrence in the coming years. (The Conversation) NPK NPK