These are the first indigenous cases of the summer season. Two people living together in Gardanne, near Marseille, in the Bouches-du-Rhône region, have been identified as carriers of the dengue virus, even though they had not travelled to a contaminated area before their symptoms appeared. This means that they caught the disease on French territory, via a tiger mosquito – Aedes albopictus by its scientific name. Given that the number of tiger mosquitoes in mainland France has risen steadily since their arrival in Menton in 2004, the Agence Régionale de Santé (ARS, Regional Healthy Agency) in Provence-Alpes-Côte d'Azur has taken this first signal seriously.
"To prevent the spread of the disease, mosquito control was carried out [on Friday morning] in a 200-meter radius in the area near the residence of the cases (public roads, private gardens)," ARS said in a press release on Friday, August 4. This is the standard procedure in cases of indigenous transmission, along with a door-to-door survey in the neighborhood concerned to identify any other people with symptoms suggestive of the disease. "A second pass will be made next week to ensure that all risk has been eliminated," explained Grégory L'Ambert, head of the Aedes albopictus preventive control unit at the Entente Interdépartementale pour la Démoustication du Littoral Méditerranéen (EID-Med), a long-standing player in mosquito control around the Mediterranean.
Mosquito control involves spraying deltamethrin, a pesticide from the pyrethroid family, in the areas around indigenous cases or those imported by contaminated travellers. Why spray within a 200-meter radius? The tiger mosquito is known to move only a limited distance around its hatching site, a distance that entomologists estimate to be around 150 meters .
The Provence-Alpes-Côte d'Azur region, which has the highest number of communes colonized by Aedes albopictus, is under special surveillance. In 2022, there were 52 indigenous cases documented in the region out of the 66 cases identified in France, during an epidemic deemed "exceptional" by Santé Publique France (French Public Health). Since the start of the 2023 season, only 29 imported cases have so far been reported in the area, and 20 treatments carried out. "But the worst is yet to come, and things are picking up with people returning from vacation," said L'Ambert.
As for the Gardanne couple, the index case from which they were likely contaminated has been identified; he had just returned from Martinique. The latter "was treated correctly, but these two cases occurred just outside the treated area, which is rather rare," noted L'Ambert.
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