West Nile virus: first indigenous cases observed 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, August 13, while outbreaks of chikungunya are multiplying in mainland France at a particularly high level. "Two indigenous cases of infection by the West Nile virus have been detected in people living in Seine-Saint-Denis," summarizes the Île-de-France regional health agency in a press release . "These are the first identifications of local vector transmission of the West Nile virus in the Île-de-France region."

This virus is transmitted by 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 one human to another by mosquitoes, but by 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, cases resulting from local contamination – 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 Île-de-France region, five other indigenous cases were recorded in the south, in Provence-Alpes-Côte d'Azur, the national agency Santé publique France (SPF) specified in another report, published Wednesday.

SPF also continues to monitor the development of indigenous cases of chikungunya in mainland France on a weekly basis. This summer, they are at a particularly high level, following an epidemic in Réunion that facilitated the importation of the virus. Now, "23 episodes of chikungunya totaling 115 cases [with] one to twenty-three cases per episode" have been recorded in mainland France, according to SPF, specifying that six of these episodes are closed. The previous week, the total stood at 16 outbreaks for 63 cases, an already unprecedented level. As for dengue fever, 6 outbreaks were recorded for a total of 11 cases.

The transmission of dengue and chikungunya in mainland France is a consequence of the establishment of the tiger mosquito. While the mosquito was absent from mainland France a few decades ago, it has now become widespread amid global warming.