Autochthonous dengue in mainland France, 2022: geographical expansion and increase in incidence

Considering the progression of Ae. albopictus colonisation, autochthonous outbreaks of viruses transmitted by this vector are now expected every year in the South of France.

However, in 2022, the situation regarding the dengue virus in metropolitan France was exceptional both in terms of number of transmission events and number of cases. From the time the surveillance system was set up in 2006 up to the start of the 2022 season, 19 events of autochthonous transmissions had been identified in metropolitan France with a total of 48 cases. This number of cases was exceeded during the 2022 season alone, which counted a total of 65 autochthonous cases from nine transmission events. Furthermore, an increase in the intensity of transmission events was observed for 2022, which included the greatest dengue outbreak ever documented in Europe. This outbreak, responsible for 34 cases from a single transmission chain, concerned two municipalities in the French department of Alpes-Maritimes. Another striking aspect of the 2022 season was the apparition of autochthonous transmission events in departments that were previously untouched by dengue. This was the case in western Occitania: outbreaks of autochthonous cases were identified in the Pyrénées-Orientales, the Hautes-Pyrénées, Tarn-et-Garonne and Haute-Garonne. Previously, only Gard and Hérault – situated in the east of the region and colonised by the tiger mosquito for longer – were affected. A first autochthonous transmission event was also described in Corse-du-Sud. 

This year appears particularly exceptional because France is the only European country to have declared autochthonous dengue cases.

There are multiple determinants of arbovirus transmission. They are mainly influenced by interactions between vector populations, virus strains and the global environment.

An autochtonous transmission cycle starts with an introduction of the virus to the metropolitan territory via persons infected during a trip to a zone where the virus circulates. International trips, in the current context of globalisation, are therefore a factor to consider in the apparition of autochtonous transmission events. However, the risk is not directly correlated to the number of imported cases: from May to November 2022, 255 imported dengue cases were identified, around three times less than in 2019 (657 cases) and 2020 (834 cases)*. An important determinant is the compatibility of the vector/pathogen pair: some viral strains are more adapted to the metropolitan vector and the introduction of a single imported case can be at the origin of a local transmission chain.

Finally, environmental conditions have a major impact on the geographical distribution of vectors, their density and activity, as well as on the multiplication and transmission of pathogens. Temperature, rainfall and land use will thus influence the efficiency of the vector system, i.e., the capacity to ensure efficient circulation of the virus. The spring and summer of 2022 were particularly warm, which may have affected the vectors’ activity and the efficiency of dengue virus transmission. 

Further research remains necessary to better characterise the determinants (climate, socio-economic, environmental) of local transmission events and their extension. 

*164 dengue cases were imported to metropolitan France in 2021.

The surveillance system set up in metropolitan France is very sensible and unique at European level; it allows the reactive deployment of mosquito control measures around cases (see box). However, it requires considerable human and material resources, especially when the importation pressure is strong or when the system is confronted by numerous autochthonous transmission events, like in 2022. 

This year certainly represents the beginning of what lies ahead in a context of the vector's continuous expansion, which in turn increases risk on the territory. On 1 January 2022, 67 metropolitan departments were considered colonised by the Aedes albopictus vector (34 in 2016). Due to the limited resources allocated to the surveillance and management of these health threats, we must reflect on how to adapt the system to anticipate the multiplication of autochthonous transmission episodes. This will probably involve prioritising surveys, whether entomological or epidemiological. 
Health professionals, like travellers, play an essential role in the prevention of arboviruses in metropolitan France. Public awareness and the involvement of local authorities remain key weapons in the fight against vector populations in order to reduce their density and the associated health risks.

To ensure a viable surveillance scheme, the implication of its main actors must be maintained by consolidating the biology laboratory network and by raising awareness, both among patients about seeking medical consultation in case of symptoms and among health professionals about diagnosing these arboviruses.