3b - Unprecedented number of imported cases and autochthonous transmission in Northern France: was 2023 a tipping point for dengue in mainland France ?

Background : Aedes albopictus, a mosquito vector of dengue, was first established in mainland France in 2004. Since then its presence has considerably expanded, resulting in local dengue transmission. We set up an enhanced surveillance and response system to limit the risk of local dengue transmission. We describe the results of this surveillance for 2023.

Methods : Epidemiological surveillance for dengue relies on mandatory notification. It is enhanced from May to November, when Aedes albopictus is active: to prevent or limit local transmission, awareness-raising campaigns targeting health professionals, daily monitoring of major laboratory platforms for active case finding and investigations to ascertain place of infection and of viremia are implemented. Vector control measures are implemented around both imported and autochthonous cases. Locally acquired cases trigger door-to-door cases-finding surveys, extensive vector control measures and community outreach.

Results : In 2023, 2524 imported cases of dengue were identified, the highest number since surveillance began. Most imported dengue cases had travelled to the French Antilles where a dengue epidemic occurred. Nine autochthonous transmission events of dengue were detected, amounting to 45 cases. They were limited in size (11 cases for the largest) and in duration (9 weeks for the longest). For the first time, autochthonous dengue transmission was detected in the northern half of France, in the Paris region.

Conclusions In 2023, we observed an unprecedented number of imported dengue cases and a high number of local transmission events. This data shows the ability of enhanced dengue surveillance to rapidly contain local transmission, highlighting the value of its continued implementation. We now face the following challenge: How can enhanced surveillance and vector control be maintained given the rising numbers of cases and limited resources?