Is she the new threat of the summer? Dengue fever has become a source of “concern” for Public Health France (SPF). “This virus is transmitted by mosquitoes of the genus Aedes: Aedes aegypti And Aedes albopictus (the latter is also called tiger mosquito)”, recalls the health organization in its updated report last December. According to SPF, there are 67 metropolitan departments where dengue fever is present and active in 2022, including Paris, Yvelines or the Rhône again. Unsurprisingly, it is the southern half of the country that is the most affected. For the moment. There were 48 cases in France between 2010 and 2021. And now, 65 in three months. “This explosion of cases is not insignificant and it is the first time that we have seen such a scale in metropolitan France”, assures Thierry Baldet, specialist in insect vectors of diseases, at CIRAD, the Center for International Cooperation in Agricultural Research for Development based in Reunion.
In total, nine foci of dengue transmission have been listed and 65 indigenous cases have been identified as of October 16, 2022. “All the foci are currently closed”, specifies Public Health France. Note that indigenous cases include people who have been infected without having traveled to areas where the virus is present, such as Africa and South America. They were bitten by a “metropolitan” mosquito, which itself became infected through contact with an infected traveler returning from the areas in question. “The tiger mosquito has continued to expand in mainland France. Where it is installed, its abundance and its season of activity are increasingly important. Its only limit is altitude”, continues the researcher. According to him, we must expect, in the years to come, “epidemics” of dengue fever “more and more frequent”.
“Dengue is one of the main viral diseases transmitted by mosquitoes in the world. Between 2012 and 2020, the number of estimated cases has increased almost 8 times”, adds Anne Sénéquier, co-director of the Global Health Observatory. at Iris and author of The geopolitics of health (Editions Eyrolles, publication on April 20). The Aedes mosquito arrived in France in 2004.” In France, among the regions most affected last summer, there is Provence Alpes-Côte d’Azur. The two departments most affected were the Var and above all, the Alpes-Maritimes with 26 cases identified in three municipalities located less than 10 kilometers away from each other.In this month of February, tiger mosquitoes are already present in this area, according to France info.
High temperatures and humidity
What about this summer of 2023? “Given the acceleration of climate change in recent years in France, there is no reason for the tiger mosquito not to continue its progression on French territory”, maintains Anne Sénéquier. The cause: global warming, which creates favorable conditions for extending the period of activity of the tiger mosquito. Temperature, air humidity and precipitation play a role in the geographic distribution of mosquitoes. “Heat episodes only promote the proliferation of the mosquito, we have much shorter development cycles between eggs, larvae and adults. And if ever there is a virus, we have a system of multiplication of the virus at the within the mosquito, which is much more efficient. This contributes to better transmission”, emphasizes Thierry Baldet.
This summer, the alternation of scorching heat and intense rainy episodes led to the proliferation of mosquitoes. “The combination of heat, stagnant water in urban areas allows greater viral transmission”, continues Anne Sénéquier. Concretely, eggs need water to succeed in their development process, but they also have a great capacity for resistance. With global warming, winter has not made these mosquitoes disappear. Some water sources remained warm enough to allow the larvae to resist. They survive until the conditions for their hatching are met. In addition to the favorable weather, there is also a period conducive to travel after the borders were closed due to the Covid-19 epidemic, reports Thierry Baldet: “The French left en masse in the summer for tropical areas, some endemic, for dengue fever, bringing back for some the virus at a time when the mosquito vector is proliferating in France.”
So how can we collectively fight against these mosquitoes carrying the virus? Vector control is often preferred and consists of eliminating breeding sites and adult mosquitoes, to avoid any risk of spreading the virus. When a case of dengue fever is reported, the ARS (Regional Health Agency) mobilizes mosquito control operators to treat up to 150 meters around the bitten and infected person. These missions are accompanied by actions to raise awareness among the public and health professionals. They are piloted in collaboration with the ARS, Public Health France and mosquito control agencies. “We need more health education on this subject on the side of the population. For example, we must explain to them why there should be no stagnant water in their garden”, indicates Anne Sénéquier. According the Paca Regional Health Agency, “80% of breeding sites are found in our gardens or terraces.”
“We use insecticide sparingly”
There is also the possibility of using insecticides around declared cases. The National Health Security Agency (ANSES) was asked to draw up a surveillance plan for these chemical-resistant mosquitoes in France and in the overseas territories according to The Parisian. In Mayotte, for example, deltamethrin, the main active substance used in vector control in France, has apparently become ineffective on part of the population of adult female mosquitoes. Over time, some mosquitoes become resistant to insecticides. “Insecticidal mosquito control is used sparingly when a case is reported, to eliminate mosquitoes carrying the disease and breeding sites. They are used with caution because the impact on biodiversity is not insignificant. Research is underway for the implementation of new techniques”, adds Anne Sénéquier.
As the summer season approaches, ANSES also urges elected officials to be cautious about mosquito traps sometimes sold as miracle solutions by manufacturers. In a notice published last SeptemberANSES thus calls for waiting for scientific protocols to assess the effectiveness of these devices to be carried out before spending sometimes large sums to equip themselves with them.