Coming from birds and sometimes deadly: what is the West Nile virus, a case of which has been identified in France?

After dengue fever, it is the most widespread flavivirus in the world. Like other RNA viruses—Zika, yellow fever, and dengue fever—it infects mammals, including humans, through the bite of an infected mosquito. In mid-July, the first case of West Nile virus appeared in the Provence-Alpes-Côte d'Azur region.

West Nile virus, active between May and November, is mainly carried by birds: domestic birds such as ducks and pigeons or wild birds, particularly migratory species , which carry the virus from Africa to the temperate zones of Europe and Asia where they settle in the summer.

Common mosquitoes of the genus "Culex" - to be distinguished from the tiger mosquito - are a particularly disease-carrying species. They become infected by biting these birds carrying the virus, before transmitting it to humans, considered by the Pasteur Institute to be "accidental hosts."

On its online page , the French foundation indicates that, in the vast majority of cases (80%), "infection by the West Nile virus is asymptomatic."

However, in rare cases, West Nile virus can cause a high fever three to six days after the bite, but is accompanied by "headache, backache, muscle pain, cough, swollen glands in the neck, and often a rash, nausea, abdominal pain, diarrhea, and respiratory symptoms."

To summarize, 20% of people bitten by a mosquito infected with the West Nile virus will suffer from a flu-like syndrome. The Pasteur Institute indicates that neurological complications can be caused by infection in less than 1% of cases, particularly in immunocompromised or elderly people.

In these cases, while the patient can usually recover on their own, sometimes with after-effects, "the viral infection can prove fatal, especially in older adults."

The West Nile virus was first identified on the African continent, in Uganda, in 1937. In Europe and France, the first human cases were recorded in the 1960s.

"Today, it is endemic in the Mediterranean region, in Central Europe and in North America where it is responsible for fatal human cases," explains the Pasteur Institute.

"The first indigenous case of West Nile in 2025 has been identified in mainland France. The case is located in the Provence-Alpes-Côte d'Azur region and presented the first symptoms on July 15," Public Health France indicates in its latest bulletin .

To date, cases have been identified in three other European countries in July: Greece, Italy, and Romania. In 2024, 39 people contracted West Nile virus in France, mainly in the southeast of the country, and 1,300 in Europe.