Favored by global warming, the tiger mosquito continues to thrive in France, including in the middle of autumn. And after a year 2022 marked by a record number of cases of dengue fever in mainland France, experts expect a comparable 2023 report and encourage preparation for a growing threat. Last year, theaedes albopictus (the scientific name of the insect), recognizable by its black and white stripes, had in fact colonized 71 departments, a number that has been constantly increasing since its installation in France in 2004.
And this mosquito, which bites more in the morning and evening, was the cause of 65 “native cases” of dengue fever, concentrated in the south of France. These cases refer to patients who have not traveled to areas where the virus is widely circulating, such as the Antilles, but were bitten by a mosquito itself infected through contact with an infected traveler.
This year, the trend looks “quite similar”, with around forty indigenous cases recorded for the moment, but the season is not yet over, indicates Marie-Claire Paty, coordinator of vector-borne disease surveillance at Public health France. France had nine “outbreaks” last year, and eight, for now, in 2023.
Vigilance for the 2024 Olympics
For the first time, an indigenous case was recorded in Île-de-France in October, in Limeil-Brévannes (Val-de-Marne), about fifteen kilometers southeast of Paris. These cases were previously observed in southern regions, with a climate a priori more favorable to the tiger mosquito. “This is the most northern case in France, and even in Europe, ever recorded,” specifies Dr Marie-Claire Paty. The northward expansion of the tiger mosquito is favored by global warming: the warmer it is, the more the mosquito’s development cycle shortens. The speed of multiplication of the virus inside the insect also increases, under the effect of temperature.
The expansion of the tiger mosquito, however, remains mainly driven by travel and human behavior. The health authorities will therefore have to be extra vigilant during the 2024 Olympic Games, which will encourage the mixing of populations in the middle of the summer season. “This is one of the anticipated risks, we are preparing for it,” assures Dr. Paty.
Dengue is a viral disease which results in high fever with, in rare cases, progression to a more serious form causing bleeding. Deaths are rare. Overseas, the Antilles have been in an epidemic phase since mid-August, and health authorities are monitoring risk profiles for serious forms. This epidemic has contributed to an already record number of cases of dengue imported into France, more than 1,300 to date.
The arrival of West Nile virus
This year, another virus surprised health authorities: West Nile, carried by birds and transmitted to humans by mosquitoes of the “Culex” genus, a species distinct from the tiger mosquito. Around forty cases have been identified in New Aquitaine although, until now, only the Mediterranean region was concerned. How can we explain the colonization of this new territory? “We think that climate change could modify avian migration corridors,” says Yannick Simonin, professor and researcher in virology at the University of Montpellier.
Although the majority of cases are asymptomatic, 20% of infected people present with a flu-like syndrome. Neurological complications occur in less than 1% of cases, particularly in immunocompromised or elderly people, and can even lead to death. Overall, “we remain at fairly low levels, but it is clear that the upward trend in the circulation of viruses transmitted by insects in recent years is accelerating in France; all experts expect a regular increase in the number of cases,” warns Yannick Simonin. “This means preparing by adapting our surveillance networks.”