Chikungunya virus (CHIKV) is an arbovirus that threatens Europe through irreversible spread of vector mosquito Aedes albopictus. In mainland France, three CHIKV local transmission episodes were detected in the last years by sustained surveillance. This surveillance is integrated in a national preparedness and control plan. In August 2017, a real-time reverse transcription-polymerase chain reaction (RT-PCR) confirmed autochthonous case of chikungunya was notified in the Var department, (southeastern France) in the town of Le Cannet-des-Maures (LCDM). Epidemiological and entomological investigations were immediately implemented to determine the source of infection and the extent of the transmission of the CHIKV in order to guide control measures. While these investigations were ongoing, in September, a second episode occurred in Taradeau, a town located ten kilometres from LCDM. Due to the spatial and temporal proximities of these episodes, investigations were also implemented with the additional objective to explore a potential link between them.

We conducted door-to-door case-finding in all households within a 200 m-radius around the autochthonous cases’ dwellings. The aim of these investigations was to identify and test any person with symptoms compatible with CHIKV infection, among households’ members as well as visiting relatives or friends, in order to determine the source of infection and the possible link between clusters. We contacted general practitioners (GP), medical laboratories and emergency services in both towns and their surroundings, to request them to quickly report and test any patient with symptoms compatible with CHIKV infection since June 1st. In the same time, we reviewed the surveillance database looking for a possible primary case among the imported chikungunya cases notified at local and national level. For any person with symptoms compatible with CHIKV infection, blood samples were collected and tested for CHIKV infection (RT-PCR and/or serology). Molecular analyses were performed by the National Reference Laboratory on index cases for each transmission episode. Entomological prospections and vector-control measures were implemented in the area of the investigation.

Eleven cases (seven confirmed by RT-PCR and four with serological evidence of CHIKV infection) were identified in LCDM. Six cases (two confirmed by RT-PCR and four with serological evidence of CHIKV infection) were identified in Taradeau. The surveillance database reviewing has failed to identify imported chikungunya case living or travelling in the Var department. However, thanks to door-to-door investigations, we identified the primary case and a case who was the epidemiological link between the two clusters. All cases had occurred within 60 days after the onset of symptoms of the primary case (July 11th). Genomic characterisation of the virus identified for both clusters the same strain belonging to the “East Central South African” (ECSA) lineage and carrying the adaptive mutation A226V facilitating transmission by Ae. Albopictus. Despite unfavourable weather conditions, entomological prospections in both towns, detected the presence of adult mosquitoes and breeding sites attributable to a high number of rainwater storage tank in the neighbourhoods.

This outbreak is the third occurrence in France of local transmission of CHIKV. The two largest episodes involved an ECSA genotype CHIKV harbouring the A226V mutation facilitating transmission by Ae. albopictus. Thanks to previous information campaigns directed at physicians and biologists, the outbreak was detected early enough to timely implement vector-control measures and to limit its spread. This new outbreak confirms that CHIKV represents an increasing threat in Europe. Regular awareness campaigns targeting physicians and biologists, focusing on arboviruses diagnosis and reporting, have to be conducted to improve the effectiveness of this surveillance.