Update - January 2026
What are the causes?
West Nile fever is caused by West Nile virus, an arbovirus belonging to the Orthoflavivirus genus of the Flaviviridae family that is closely related to the viruses responsible for dengue and Zika. It is an RNA virus measuring approximately 40 to 50nm in diameter. The main reservoir of West Nile virus is migratory birds.
How does the disease spread?
West Nile virus is spread by bites from mosquitoes of the genus Culex. After biting infected birds, the female mosquitoes multiply and then transmit the virus to humans during blood meals. Any factors conducive to the proliferation of mosquito vectors – heavy rainfall, irrigation, standing water, above-average temperatures, etc. – may increase the incidence of West Nile virus infection in the regions where the virus circulates. Unlike the tiger mosquito (Aedes albopictus), Culex pipiens mosquitoes colonize larger breeding grounds that are generally highly polluted.
It is worth bearing in mind that mammals – dogs, cats, horses and humans – are accidental "dead end" hosts of the virus: a mosquito that bites an infected mammal is not capable of transmitting the virus to another mammal as the host does not develop high enough levels of virus in the bloodstream to infect mosquitoes.
Human-to-human transmission is rare but may occasionally occur through blood transfusions or organ transplants.
What are the symptoms?
In 80% of cases, infection with West Nile virus does not result in any symptoms. In the remaining 20% of people, after three to six days of incubation they develop West Nile fever, with symptoms such as the sudden onset of high fever, together with headache, a cough, back and muscle pain, and swollen lymph glands in the neck. Other symptoms may include a skin rash, nausea, abdominal pain, diarrhea and various respiratory symptoms.
Neurological complications (meningitis and encephalitis) occur in around 1% of cases. More rarely still, the heart, liver and pancreas may be affected. In most cases the symptoms disappear on their own, but infection can be severe or even fatal, mainly in elderly or immunocompromised individuals.
How is West Nile virus diagnosed?
Diagnosis is based on specific testing for the virus: either RT-PCR to detect viral genetic material, or virus isolation in cell culture, but the latter technique is complex and requires a high-security laboratory.
Another diagnostic method involves using the ELISA technique to detect specific West Nile virus antibodies.
What treatments are available?
No specific vaccine or treatment for West Nile virus is currently available for humans. Treatment mainly involves alleviating symptoms through rest, fluids, and medication to reduce fever and pain. Severe cases require hospitalization, sometimes in intensive care to treat neurological complications.
How can West Nile virus be prevented?
Prevention of West Nile virus infection is based on individual and collective protective measures against mosquitoes:
- Reducing and eliminating stagnant water, where mosquito larvae can develop (for example in containers, buckets, clogged gutters and crawl spaces)
- Wearing long clothes
- Using mosquito nets and/or repellent
Who is affected?
West Nile virus is thought to be the second most widespread flavivirus after dengue. It has been found in several countries in Africa, Europe, the Middle East, North America and West Asia.
In Europe, human cases have been reported since the 1960s, but infections have become increasingly frequent over the past 15 years. West Nile virus is currently endemic in several European countries, and its distribution area is growing. It is particularly active in southern Europe.
In France, the first human and equine cases were diagnosed in the early 1960s. The virus re-emerged in horses in Camargue in 2000, and seven human cases were reported in 2003 in Var.
In 2024, 38 locally acquired cases were identified in the south of France, mainly in Var and Hérault.
In 2025, locally acquired cases were detected in Greater Paris for the first time.
For more information, see:
The Santé publique France information page (in French)
