Dengue: the upward trend in the number of “native” cases seems to be confirmed in mainland France

At the end of October, the tiger mosquito continues to thrive in France. After a year 2022 marked by a record number of cases of dengue fever in mainland France, experts expect a comparable 2023 outcome and encourage preparation for a growing threat. Overseas, the Antilles have been in an epidemic phase since mid-August, and health authorities are monitoring profiles at risk of serious forms, particularly patients with sickle cell anemia. This epidemic has contributed to an already record number of cases of dengue imported into France with more than 1,300 to date.

Dengue is a vector-borne infectious disease, which results in high fever with, in rare cases, progression to a more serious form causing bleeding. However, deaths are very rare (around 0.01% of all cases). The pathology is caused by a virus (the infectious agent) carried by insects (the vectors) such as mosquitoes of the Aedes genus.

As of January 1, 2023, the Aedes albopictus (scientific name of the insect), recognizable by its black and white stripes, had colonized 71 departments, a number in constant increase since its installation in France in 2004, and had been the origin of 65 “indigenous cases” of dengue 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 and a season which is not yet over, indicates Marie-Claire Paty, coordinator of vector-borne disease surveillance at Public health France.

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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,” Dr Marie-Claire Paty told AFP.

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. But it is travel and human behavior that brings it into the territory. 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.

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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.

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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 for several 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.” “It’s a threat that will not go away, it is set to increase,” confirms Marie-Claire Paty.