Chikungunya, dengue, West Nile... Cases continue to increase in France

As of August 19, "27 episodes of chikungunya totaling 154 cases (...) have been identified in mainland France," the health agency summarized in a weekly report. The previous week, the report included 23 outbreaks for 115 cases.

These are indigenous cases, resulting from contamination within the territory, and not imported following contamination outside.

Although several of these episodes are now over, the summer of 2025 is of unprecedented magnitude in terms of indigenous cases of chikungunya, the virus of which is transmitted from one human to another via tiger mosquito bites, in mainland France.

These cases far exceed their records from previous years—never more than thirty in a single summer—a situation fueled by a major chikungunya epidemic earlier this year in Réunion, the largest in twenty years. This facilitated the arrival of imported cases, which then led to local infections.

Another disease transmitted by the tiger mosquito, dengue fever, does not currently have as many indigenous cases as it did in 2024 - with a record 66 in mainland France. But a weekly increase was also recorded, with seven identified outbreaks - one closed - for 13 cases, compared to six outbreaks and eleven cases the previous week.

This situation is part of a context where the tiger mosquito, accustomed to hot climates and still absent from the mainland a few decades ago, is now present throughout almost the entire territory, against a backdrop of global warming.

Finally, another disease has been added to this exceptional picture in recent weeks: "West Nile" fever, which is not transmitted by the tiger mosquito but the Culex variety, more common in mainland France.

The number of indigenous cases in mainland France this summer is not yet at a record high - 13 at the latest count, compared to several dozen in some years - but they are notable for their wider presence: for the first time, several have been identified in the Paris region and not just in the South.

All three diseases have similar symptoms, resembling the flu with fever and muscle aches.