Mosquitoes: Potentially deadly West Nile virus detected for the first time in Île-de-France

For the first time, indigenous cases of "West Nile" fever, transmitted by mosquitoes, have been observed in the Île-de-France region, health authorities announced on Wednesday, while outbreaks of chikungunya are multiplying in mainland France at a particularly high level .

"Two indigenous cases of West Nile virus infection have been detected in people living in Seine-Saint-Denis," the Île-de-France regional health agency said in a statement. "These are the first reports of local vector-borne transmission of West Nile virus in the Île-de-France region."

This virus is transmitted through mosquito bites . However, unlike chikungunya or dengue fever, it is not the tiger mosquito that is the cause, but the Culex genus, which is much more widespread in mainland France.

Another difference from these two other diseases is that West Nile fever is not transmitted from human to human by mosquitoes, but from an infected bird.

The infection is usually asymptomatic, but in about one-fifth of cases it results in a flu-like illness. In less than 1% of cases, serious complications can occur, sometimes leading to death.

Indigenous cases - that is, resulting from contamination on site - have already been reported in mainland France in previous years (around forty in 2024) but never so far north.

This summer, apart from the two cases in the Paris region, five other indigenous cases were recorded in the South, in Provence-Alpes-Côte-d'Azur, the national agency Santé publique France specified in another report, published Wednesday.

Public Health France also continues to monitor the development of indigenous cases of chikungunya in mainland France on a weekly basis. These cases are at a particularly high level this summer, following an epidemic in Réunion Island that facilitated the importation of the virus.

Now, "23 episodes of chikungunya totaling 115 cases (with) 1 to 23 cases per episode" have been recorded in mainland France, according to Public Health France, specifying that six of these episodes are closed. The previous week, the total stood at 16 outbreaks for 63 cases, an already record level. As for dengue fever , six outbreaks were recorded for a total of eleven cases.

The transmission of dengue and chikungunya in mainland France is a consequence of the establishment of the tiger mosquito. This mosquito was still absent from mainland France a few decades ago, but has now become widespread , against a backdrop of global warming.