There are more and more regions at risk of spreading arboviruses such as dengue virus (DENV), chikungunya virus (CHIKV) or even Zika virus (ZIKV) due to vector expansion, Aedes albopictuscommonly known as the tiger mosquito.
In France, a system of surveillance of these arboviruses has existed since 2006 (see box) which makes it possible to identify cases imported from areas where these viruses circulate in order to limit indigenous transmissions on French territory. In metropolitan France, since the first identifications of autochthonous cases in 2010, transmissions on the national territory tend to increase every year.
In 2022, the situation regarding dengue fever in metropolitan France appears to be exceptional, both in terms of the number of transmission centers and the number of indigenous cases. Thus, at the end of October 2022, surveillance had identified 65 indigenous cases of dengue fever corresponding to nine outbreaks of transmission, compared to 48 cases identified in the last 10 years. A situation that is not reported in any other European country, France being the only country to have reported indigenous cases this year.
The article just appeared on Eurosurveillance1 bears witness to this exceptional diffusion of dengue in France.
3 questions to Amandine Cochet, Occitanie Regional Unit, Department of Regions, Public Health France and Clémentine Calba, PACA Regional Unit, Department of Regions, Public Health France.
How is the situation observed in 2022 exceptional, both compared to previous years and compared to other European Union countries?
Given the history of colonization by Oh. albopictus, the occurrence of episodes of indigenous transmission of viruses transmitted by this vector is now a situation expected every year in the south of France. However, in 2022, metropolitan France experienced an exceptional situation regarding the transmission of the dengue virus, both in terms of the number of transmission events and the number of cases. Since the implementation of the surveillance system in 2006 and before the 2022 season, 19 episodes of indigenous transmission have been identified in metropolitan France, for a total of 48 cases. In the 2022 season alone, this number of cases was surpassed, with a total of 65 indigenous cases spread across 9 transmission hotspots. In addition, an increase in the intensity of transmission episodes was observed in 2022, notably the largest ever documented dengue outbreak in Europe. This focus of 34 cases belonging to a single transmission chain concerned two municipalities in the Alpes-Maritimes department. Another significant event of this season is the occurrence of episodes of indigenous transmission in the departments so far spared. This is the case this season in the western region of Occitania: outbreaks of indigenous cases have been identified in the Pyrénées-Orientales, Hautes-Pyrénées, Tarn-et-Garonne and Haute-Garonne, whereas in previous years only Gard and L’Hérault , further east of the region and colonized the longest by the tiger mosquito, had experienced such cases. A first episode of autochthonous transmission has also been described in southern Corsica.
This year looks all the more exceptional as France is the only European country to have reported native-born cases of dengue fever.
Are there any hypotheses, particularly in relation to environmental conditions, which could explain this high number of indigenous and unprecedented cases in France this year? Do we have the ability to characterize the climatic, socio-economic and environmental determinants that favor transmission?
The determinants of arbovirus transmission are multiple, they are mainly influenced by interactions between vector populations, virus strains, hosts and the global environment.
First, the establishment of an autochthonous transmission cycle requires the introduction of the virus into the metropolitan territory through people who have become infected while traveling in an area where the virus circulates. International travel in the current context of globalization is therefore a factor to be taken into account in the occurrence of episodes of indigenous transmission. However, the risk is not directly related to the number of imported cases: 255 imported cases of dengue fever were identified from May to November 2022, about three times fewer 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 continental vector and the introduction of a single imported case can be the origin of a local transmission chain.
Finally, environmental conditions have an important impact on the geographical distribution of vectors, their density and activity, as well as on the multiplication and transmission of pathogens. Temperatures, rainfall and land use will therefore influence the efficiency of the vector system, i.e. the ability to ensure efficient circulation of the virus. The spring and summer of 2022 were particularly hot, which may have favored vector activity and the efficiency of dengue virus transmission.
Further research is still needed to better characterize the determinants (climatic, socio-economic, environmental) of local transmission episodes and their extent.
*164 cases of dengue fever imported into mainland France in 2021.
How can this exceptional situation impact the ability to identify cases through the surveillance system set up in France? What actions should be taken to ensure its sustainability?
The surveillance system set up in mainland France is very sensitive and unique at European level; allows for the reactive implementation of mosquito control measures around cases (see box). However, it mobilizes considerable human and material resources, especially when import pressure is strong or when it has to cope with numerous episodes of indigenous transmission, as was the case in 2022.
This year undoubtedly represents the beginning of what awaits us in the future, in a context of continuous extension of the carrier’s distribution area and therefore of increased risk in the area. At 1um January 2022, 67 metropolitan departments were considered colonized by the vector Aedes albopictus (34 departments in 2016). Since the resources allocated to the surveillance and management of these health events are limited, it is necessary to reflect on adapting the system, in order to anticipate the multiplication of episodes of autochthonous transmission. This will undoubtedly be based on a prioritization of investigations, be they entomological or epidemiological.
Healthcare professionals, like travellers, play an essential role in the prevention of arboviruses in mainland France. Social mobilization and the involvement of local authorities remain the main levers in the fight against vector populations in order to reduce their density and reduce associated health risks.
To ensure the feasibility of the surveillance system it is necessary to maintain the involvement of the main players by consolidating the network of biological analysis laboratories, raising awareness among patients on the need to consult a healthcare professional in case of symptoms and awareness of healthcare professionals when diagnosing these arboviruses .
 Cochet Amandine, Calba Clémentine, Jourdain Frédéric, Grard Gilda, Durand Guillaume André, Guinard Anne, investigation team, Noël Harold, Paty Marie-Claire, Franke Florian. Autochthonous dengue in mainland France, 2022: geographic extent and increase in incidence. Surveillance of the euro 2022;27(44):pii=2200818. https://doi.org/10.2807/1560-7917.ES.2022.27.44.2200818
Other references cited
Indigenous transmission of dengue virus in mainland EU/EEA, 2010-present, ECDC, 2022.
Presence maps of the tiger mosquito (Aedes albopictus) in metropolitan France