West Nile virus infection is a viral disease transmitted by mosquitoes (of the genus Culex) that become infected exclusively after biting infected birds. Unlike other arboviruses (such as dengue, chikungunya, or Zika, which are transmitted by the tiger mosquito), humans and horses are considered “accidental hosts”: they cannot, in turn, transmit the virus through a mosquito bite.

The Culex mosquito is the most common mosquito in metropolitan France. It bites mainly in the evening and at night, unlike the tiger mosquito, which bites primarily during the day.

In most cases, West Nile virus infection in humans is asymptomatic. In some cases, the disease presents as flu-like symptoms (fever, headache, muscle aches), sometimes accompanied by a rash. More rarely, it can cause neurological complications that may lead to long-term effects and can even be fatal.

In horses, the disease manifests either as a febrile form—often asymptomatic—or as a neurological form (depression, tremors, paresis, ataxia). Spontaneous recovery generally occurs within 20 to 30 days, but severe cases involving paralysis and death can occur.

The circulation of the West Nile virus is monitored in France through an integrated surveillance system that combines human and animal components, focusing on horses and birds. In humans, surveillance involves mandatory reporting by healthcare professionals and diagnostic confirmation by the National Reference Center for Arboviruses. In animals, it includes early detection and diagnosis by the ANSES National Reference Laboratory.

In the departments along the Mediterranean coast and in Vaucluse, surveillance of mortality among bird species susceptible to West Nile virus is active from June through November. During this period, any mortality among raptors (buzzards, falcons, owls, etc.) and corvids (crows, magpies, rooks): excluding roadkill, ducks, and pigeons.

Healthcare professionals are being educated by the ARS and Santé publique France on how to identify, screen for, and report cases (through informational letters and webinars).

Human cases reported to the ARS are investigated in collaboration with Santé publique France Paca-Corse to identify possible sites of transmission.

The Mediterranean Interdepartmental Mosquito Control Agreement (EID) is conducting investigations in these areas to identify and eliminate potential breeding sites for Culex mosquitoes.

Finally, measures to ensure the safety of blood and organ donations are temporarily being implemented by the French Blood Establishment (EFS) and the Biomedicine Agency (ABM) in the affected departments.

Protect Yourself from Bites

• Wear loose-fitting, long-sleeved clothing, especially in the evening, since Culex mosquitoes are primarily active at night;
• Use mosquito nets on openings (doors and windows);
• If necessary, use electric mosquito repellent diffusers inside your home;
• Use repellents recommended by your pharmacist on exposed skin;
• Eliminate standing water where mosquitoes breed: regularly empty containers (watering cans, buckets, etc.), tightly cover rainwater storage containers, and drain unused swimming pools and ponds.

Consult your primary care physician immediately if you experience symptoms suggestive of the disease

Sudden onset of fever, which may be accompanied by headaches, muscle aches, a rash, or even behavioral changes or incoherent speech.

Equine Owners

Contact your veterinary public health officer immediately if you notice that an equine is exhibiting neurological signs accompanied by hyperthermia.

The main preventive measures to minimize contact with vectors:

• pest control treatment for equines;
• monitoring the entire herd during the risk period, with daily temperature checks for early detection of animals with hyperthermia;
• bring horses indoors as much as possible at dawn and dusk, when mosquito activity is high;
• insecticide treatment of facilities, installation of mosquito traps in stalls, and mosquito netting above the stalls;
• Disinfecting transportation vehicles;
• Minimize standing water where mosquitoes breed (tarps, ruts, trampled areas, etc.): drain the water, restrict access, etc.;

In addition, vaccination of horses is possible and provides good protection against the disease in the majority of vaccinated animals.