An indigenous case of "West Nile virus" has been detected in France: what is this disease?

For the first time this year, a case of West Nile virus has been reported in mainland France.

Mosquitoes are definitely giving us a hard time this year! While numerous cases of chikungunya, dengue fever, and even Zika have been detected in metropolitan France for several months, a case of West Nile virus has even recently been reported. The case, indigenous (i.e. contracted on the territory), was detected in the Var department in Hyères, according to a report by Public Health France published on July 30, 2025. In 2024, 38 indigenous cases were reported in humans in France. One of them died.

But what is this little-known disease? The West Nile virus is transmitted by mosquitoes. Not tiger mosquitoes like chikungunya or dengue, but those of the Culex genus, the most common. These mosquitoes are "established throughout mainland France" and are active "between May and November," according to Public Health France.

West Nile virus normally affects mostly birds, but horses and humans can also be infected by mosquito bites. However, "the amount of virus in their blood (viremia) is insufficient to infect a mosquito during a bite and thus allow the transmission of the disease," reassures Public Health France .

In those affected, "in the majority of cases, West Nile virus infection is asymptomatic," according to the Pasteur Institute . In symptomatic cases, the disease manifests itself by "the sudden onset of a high fever after a 3 to 6-day incubation period. This fever is accompanied by headaches and backaches, muscle pain, a cough, swollen lymph nodes in the neck, and often a rash, nausea, abdominal pain, diarrhea, and respiratory symptoms," the Pasteur Institute lists.

In some rare cases, complications—particularly neurological ones—can occur. The disease can even be fatal, "mainly in older adults" and immunocompromised individuals, according to the Pasteur Institute. There is no treatment for West Nile virus, only symptomatic treatments. There is also no specific prevention for humans, other than protection against mosquitoes.