Eight French régions had locally transmitted chikungunya or dengue in 2025. Three of them had never had it before.

Santé publique France published its 2025 national arbovirus retrospective in early May. The headline numbers are large but not astonishing for anyone who has been watching the curves: 809 locally transmitted cases of chikungunya in mainland France, distributed across 79 separate transmission episodes; 30 locally transmitted cases of dengue, distributed across 11 episodes. The bilan covers symptom-onset dates between 27 May and 13 November 2025.

The number that should detain a reader is not 809. It is three.

For the first time, autochthonous chikungunya or dengue transmission occurred in three French régions that had never recorded a locally acquired case before: Nouvelle-Aquitaine on the Atlantic coast, Bourgogne-Franche-Comté in the centre-east, and — most quietly significant — Grand Est, the broad block of Alsace, Lorraine and Champagne that sits against the German and Belgian borders. The other five régions with 2025 transmission — Provence-Alpes-Côte d'Azur, Occitanie, Auvergne-Rhône-Alpes, Corsica, Île-de-France — had all been on the surveillance map already. Eight régions in total. Three of them new.

What this means in operational terms is that the French chikungunya frontier crossed two long-watched lines in a single season. It crossed west to the Atlantic, where Aedes albopictus had been creeping for several years but had not yet produced a documented local transmission chain. And it crossed north into the cold belt, where the conventional wisdom held that the thermal window was still too narrow for a human-to-mosquito-to-human cycle to close.

That second crossing is the one worth dwelling on. The Centre for Ecology and Hydrology's modelling work, published in 2025 and discussed widely in European public-health circles, put the temperature threshold for sustained chikungunya transmission at around 13°C — meaningfully lower than earlier estimates. Grand Est sits in a thermal band that, on the older models, was considered safe through most of the year. The 2025 episode in Grand Est means the threshold has been crossed for long enough, in a place where the vector is present in sufficient density, for at least one viraemic traveller to seed a chain that the mosquitoes could complete.

None of this is hypothetical anymore. Aedes albopictus was established in 83 of France's 96 metropolitan départements by 1 January 2026, per Santé publique France's distribution map. The Asian tiger mosquito has been a fact of southern French life for two decades; the news is that it now bites people on home soil in places where, ten years ago, nobody bothered to look for it. Brittany's Observatoire de l'environnement en Bretagne confirmed an established population on the western coast in early 2026. The 13 remaining mainland départements without confirmed establishment are mostly in the north, the centre, and the highest Alpine departments — and even these are now subject to enhanced trap monitoring under the 2026 surveillance system.

The driver behind the 2025 chikungunya peak is not in dispute. The Réunion epidemic, which produced roughly 450,000 cumulative cases between late 2024 and June 2025, drove a sustained flow of viraemic returning travellers into the French mainland through the summer. Eighty per cent of metropolitan French chikungunya in 2025 was within four serial generations of a Réunion case. The remaining 20 per cent was traceable to imported cases from elsewhere in the Indian Ocean and the Caribbean. The PACA region absorbed roughly 60 per cent of the country's 2025 transmission load — its mature tiger-mosquito populations, dense urban tourism flows, and high seasonal temperatures combining to produce the largest single-region episode of the year. But PACA's dominance is the part of the picture that surprises nobody. The 2025 story is that the rest of the map began to look more like PACA.

The 2026 response from Santé publique France is the institutional acknowledgement of that shift. Enhanced national surveillance — the surveillance renforcée des arboviroses — was reactivated on 1 May 2026 and runs through 30 November. Every suspected case of chikungunya, dengue, Zika or West Nile virus in mainland France must now be notified within 24 hours, regardless of whether the patient has travelled. The Agence régionale de santé Île-de-France, taking the most forward-leaning posture of any French regional authority, has deployed 435 mosquito-egg traps across all eight Île-de-France départements, including at the three Paris airports, with the surveillance window running effectively year-round. ARS PACA continues its own established surveillance network. The 2026 system is, in practice, a continuous-monitoring posture calibrated to a world where the next autochthonous episode could begin in March and the first transmission could be confirmed in May.

The numbers worth holding in mind through the European summer are therefore not the 2025 totals — those are history. They are the running 2026 figures: the count of episodes that fail to close cleanly within three serial intervals, the number of régions with confirmed transmission, and whether any of the three first-time régions of 2025 record a second consecutive year. The 2024-to-2025 jump from a handful of southern régions to eight régions including Grand Est is the kind of step-change that, if it repeats in 2026, becomes a trend rather than a Réunion-epidemic artefact. If 2026 holds at eight régions or fewer, the 2025 figure can be read as a one-summer surge driven by an exceptional Indian Ocean wave. If it climbs to nine or ten, the surge has become the baseline.

What an ordinary resident of one of the newly affected régions can do about this is unchanged from anywhere else: drain standing water from saucers, gutters, paddling pools, disused buckets and the trays beneath outdoor plant pots every week through the warm months; use treated repellents on exposed skin during the daylight hours when Aedes albopictus feeds; report dead mosquitoes that match the tiger-mosquito profile through the SignalementMoustique platform; and check the national surveillance dashboard before assuming local conditions are unchanged. The personal-protection layer most worth investing in is the physical one — the long-sleeved shirt, the screened window, the bed net under which one sleeps when travelling in an active outbreak country — rather than the aerosol can. None of this is novel public-health advice. The novelty is that, as of 2025, the advice now applies in Strasbourg as well as in Saint-Tropez.

This is the steady, unromantic shape of climate change as it shows up in French primary care: not a sudden plague, but a slow rearrangement of which postcodes are at risk. The 2025 bilan is the moment that rearrangement became visible at the régional level. The 2026 surveillance system is the response. What the country does with the next eighteen months of data will determine whether 809 cases is the number we remember as the peak of an Indian Ocean episode, or the number we remember as the beginning of a long northward shift.

What we know

• 809 locally transmitted chikungunya cases in mainland France in 2025, across 79 transmission episodes (790 episode-linked + 19 isolated). Symptom-onset dates between 27 May and 13 November. [Santé publique France, Bilan 2025]
• 30 locally transmitted dengue cases across 11 transmission episodes. [Santé publique France, Bilan 2025]
• Eight régions affected by autochthonous transmission in 2025: Provence-Alpes-Côte d'Azur, Occitanie, Auvergne-Rhône-Alpes, Corsica, Île-de-France — plus Nouvelle-Aquitaine, Grand Est, Bourgogne-Franche-Comté for the first time. [SpF, Bilan 2025; Caducée.net coverage of the 6 May 2026 bulletin]
• PACA accounted for roughly 60% of mainland transmission, with 35 of the year's transmission episodes concentrated in Alpes-Maritimes, Var and Bouches-du-Rhône. [SpF press release, 7 May 2026]
• Aedes albopictus established in 83 of 96 metropolitan départements as of 1 January 2026. [Santé publique France]
• Enhanced national surveillance reactivated 1 May 2026, running through 30 November; mandatory 24-hour notification of any suspected case. [SpF; Caducée.net]
• All 2026 chikungunya clusters in France and Italy are closed as of the most recent ECDC monthly update (30 March 2026; data to 28 February 2026). [ECDC]
• Principal driver of the 2025 peak: the Réunion chikungunya epidemic (~450,000 cumulative cases, late 2024 – June 2025) feeding the introduction pool throughout summer 2025. [SpF; ECDC]

Sources

• Santé publique France — Chikungunya, dengue et Zika en France hexagonale. Bilan 2025. — https://www.santepubliquefrance.fr/maladies-a-transmission-vectorielle/chikungunya/bulletin-national/chikungunya-dengue-et-zika-en-france-hexagonale-bilan-2025
• Santé publique France — English press release, Stepped efforts to combat the tiger mosquito: activation of the surveillance and control system, 7 May 2026 — https://www.santepubliquefrance.fr/en/press/2026-stepped-efforts-combat-tiger-mosquito-activation-surveillance-and-control-system
• Caducée.net — Chikungunya 2026 et dengue en France : diagnostic, signalement et moustique tigre, May 2026 coverage of the SpF 6 May 2026 bulletin — https://www.caducee.net/actualite-medicale/16890/chikungunya-2026-et-dengue-en-france-diagnostic-signalement-et-moustique-tigre.html
• Futura Sciences — Chikungunya, dengue, Zika : pourquoi les autorités redoutent particulièrement 2026 — https://www.futura-sciences.com/sante/actualites/virus-chikungunya-dengue-zika-autorites-redoutent-particulierement-2026-134351/
• ECDC — Chikungunya Worldwide Monthly Update, 30 March 2026 (data to 28 February 2026) — https://www.ecdc.europa.eu/en/chikungunya-monthly