Mosquito-Borne Viruses on the Rise: France Faces an Unprecedented Health Threat

Mosquito-Borne Viruses in France: What to Know About West Nile, Dengue, and Chikungunya

France is facing an alarming surge in mosquito-borne illnesses. Two severe cases of West Nile fever have just been confirmed in Île-de-France, while dengue and chikungunya transmissions are climbing at record speed. Experts say climate change and the spread of the tiger mosquito are the main drivers behind this escalating health risk.

“This is both unusual and concerning,” says Rachel Bellone, a researcher with the Pasteur Institute’s Arboviruses and Insect Vectors Unit. “We’re seeing earlier and more frequent outbreaks of mosquito-related viruses than ever before, and several factors are combining to fuel this trend.”

Why the West Nile cases in Seine-Saint-Denis are significant

Bellone explains that the discovery of two local cases of West Nile fever in Seine-Saint-Denis is “quite unprecedented.” While the virus has been detected in France before, it was previously confined mostly to the southwest. “This is the first time we’ve seen such severe forms appear in Île-de-France, and so early in the season,” she notes.

Typically, most people infected with West Nile show no symptoms, meaning the diagnosed cases may represent only a small portion of the actual infections. What makes this cluster unusual is that Île-de-France lies outside the main migratory routes of birds, which are natural carriers of the virus—raising new questions for researchers.

Dengue and chikungunya spreading faster than ever

By mid-August, France had already recorded over 150 cases of locally transmitted dengue and chikungunya—nearly double the total reported for the whole summer of 2024. The first case of chikungunya appeared as early as May this year, when historically such detections don’t occur until June.

“Normally, we see transmission peak between August and September, coinciding with travelers returning from abroad,” Bellone explains. “But this year, we nearly matched last summer’s total before even reaching that peak.”

France has also seen sporadic Zika virus transmissions in recent years. Since 2022, local outbreaks of dengue or chikungunya have occurred annually, showing a clear upward trend. “Household clusters are becoming more frequent and larger in scale. That’s worrying for the years ahead,” she warns.

What’s fueling the surge?

The rise in cases stems from a mix of factors. Chief among them is the spread of the tiger mosquito. Since its arrival in southern France in 2004, the insect has expanded across nearly the entire country. Combined with increased international travel, the risk of viruses being introduced and then transmitted locally has grown sharply.

Climate conditions are also playing a major role. Warmer, wetter environments are creating ideal breeding grounds for mosquitoes. In addition, an ongoing chikungunya epidemic in Réunion Island raises the chances of further introductions to mainland France.

How individuals can reduce their risk

Preventing bites remains the first line of defense. Bellone emphasizes the importance of removing breeding sites: “Anything that can collect stagnant water—plant saucers, buckets, even decorative features—can become mosquito nurseries. Eliminating them goes a long way toward reducing transmission risk.”