France has recorded 39 locally acquired cases of West Nile virus infection as of August 30, nine more than the previous week, according to a weekly bulletin from the public health agency Santé publique France. The agency published the update on Wednesday.
The cases were located in the Provence-Alpes-Côte d'Azur, Occitanie, Île-de-France and Auvergne-Rhône-Alpes regions. For two of the cases, health officials said they were unable to identify a precise location of contamination.
The first locally acquired case of the year was detected in mid-July in southern France, marking the first human circulation of West Nile virus on French soil in 2026.
The virus is usually mild or causes no symptoms at all, but it can occasionally be fatal. In the Netherlands, one patient infected over the summer died in late August. The Dutch National Institute for Public Health and the Environment said on Thursday that the country had recorded nine infections as of September 2.
How the virus spreads
West Nile fever is caused by a virus in the Flaviviridae family. It is most commonly transmitted by Culex mosquitoes, the most widespread mosquito species, which become infected exclusively through contact with infected wild birds.
A 2023 surveillance study by the Institut Pasteur in France found that the Asian tiger mosquito, better known as a carrier of chikungunya, dengue and Zika, has also acquired the ability to transmit West Nile virus.
Horses and, occasionally, humans are considered "accidental hosts" of the virus. They can become infected when bitten by an infected mosquito but cannot pass the virus on to others afterwards.
More rarely, the disease can spread through blood transfusion or the transplantation of organs, tissue or cells, France's health ministry says on its website. It adds that cases of transmission from mother to child during pregnancy, childbirth and breastfeeding have also been observed.
Symptoms in horses and humans
In horses, the disease causes fever that can develop into a nervous form marked by listlessness, tremors and neurological problems. Recovery generally happens on its own within four weeks, though severe cases involving paralysis and death can occur.
In humans, most infections produce no symptoms at all. About 20% of infected people develop a flu-like illness with fever, muscle pain and headaches, sometimes accompanied by a skin rash. In rare cases, affecting around 1% of those infected, the virus can cause serious neurological complications, particularly in people who are immunocompromised or elderly, which can lead to death.
There is no vaccine for humans. The World Health Organization says the only way to reduce the number of human infections is to raise public awareness of risk factors. That means protecting against mosquito bites, working to curb mosquito breeding around homes, and avoiding direct contact with sick animals.
Spread across France and Europe
The virus was first identified in Uganda in 1937, in the West Nile region from which it takes its name. It has since spread across the African continent as well as to Asia, Australia, the Americas, the Middle East and Europe, particularly around the Mediterranean.
According to Santé publique France, the virus has circulated more widely in Russia and Europe since 2010, with cases reported for the first time in Germany and the Netherlands in 2019 and 2020 respectively.
West Nile virus first appeared in France in the 1960s and for a long time was observed only around the Mediterranean coast. It returned to mainland France in August 2015, first in horses in the Gard, Hérault and Bouches-du-Rhône departments, then in humans in October, after a long gap with no reported cases dating back to 2003.
Infections have since been recorded along the Atlantic coast, confirming the virus's spread. In 2024, a locally acquired case was reported for the first time in the Gironde department.
Santé publique France said 62 infections were reported in total last year across France, spanning six regions, three of them affected for the first time: Provence-Alpes-Côte d'Azur, Nouvelle-Aquitaine, Occitanie, Île-de-France, Auvergne-Rhône-Alpes and Normandy.
