West Nile, a virus transmitted by mosquitoes from birds to mammals

How is the virus transmitted? 

West Nile virus is mainly transmitted by mosquitoes of the genus Culex. They contract the virus after biting an infected bird, and can then transmit it to mammals, mainly humans and horses. These are "dead-end hosts", because mosquitoes cannot become carriers after biting an infected mammal. 

Which species of birds can carry the virus?

As many as 250 bird species can carry West Nile virus. The groups most affected are passerines, which include sparrows and corvids, and birds of prey. When the virus infects a bird population for the first time, it can cause high mortality. This was the case, for example, when the virus first arrived in the United States in 1999. In Europe, on the other hand, West Nile virus infection in birds is rare and mainly asymptomatic. It poses no risk to poultry farms.

What are the symptoms of the disease? 

Horses and humans are the only mammals that can develop symptoms. However, 80% of infections are asymptomatic. 

In 20% of cases, horses and humans develop a febrile form characterised by fatigue, high fever, headaches, and muscle and joint pain. It passes after a few days. 

Less than 1% of cases may go on to develop a neurological form. This causes encephalitis and meningoencephalitis, and can be fatal. Neurological sequelae may persist for several years, or even for life. 

Where does the virus come from and how has its geographical distribution changed?

West Nile virus was discovered in Uganda in 1937, in the district that gave the virus its name. Since then, its range has expanded. According to WHO, West Nile virus is distributed worldwide and is now present in Africa, Europe, the Middle East, North America and Western Asia. The first French cases of infected horses and humans were reported in the 1960s. These were sporadic cases occurring around the Mediterranean. Since the late 1990s, they have become more common in Mediterranean countries. 

In 2022, cases of horses infected with West Nile virus were diagnosed for the first time on the French Atlantic coast, in Gironde. In 2023, cases in humans and horses again were reported in the Nouvelle-Aquitaine region, more specifically in the Gironde and Charente-Maritime départements. In 2025, cases in humans, horses and/or mosquitos carrying the virus were detected in the Ile-de-France, Centre-Val de Loire and Auvergne-Rhône-Alpes regions, as well as in Haute-Garonne. 

What is causing the spread of the virus?

There are many reasons for the spread of the virus, including international trade, global warming, urbanisation and biodiversity loss. These factors can modify the behaviour of mosquitoes and birds carrying the virus. This is particularly true for migratory birds, which may change their migration patterns.

How can it be prevented in humans?

There is no vaccine or treatment for humans. Only the symptoms can be treated. People should take the usual preventive measures against mosquito bites to protect themselves from the virus: 

• Use repellent,
• Wear long, loose, light-coloured clothing,
• Avoid going out at dusk, when Culex mosquitoes are most active. 

Elderly or immunocompromised people are at greatest risk of developing severe forms of the disease, making it particularly important for them to follow these recommendations. 

It is also advisable to eliminate mosquito egg-laying sites, by emptying saucers and pots filled with water, covering rainwater collection tanks with mosquito netting and cleaning out gutters. 

How can it be prevented in horses? 

A vaccine against West Nile virus is available for horses. Equidae should be vaccinated in regions where cases have been reported. 

How is the virus monitored?

Monitoring for West Nile virus in animals is coordinated by the Directorate General for Food, and in humans by the Directorate General for Health and Santé Publique France. 

ANSES is involved in the confirmation of avian and equine cases. Infections in wildlife are reported by the French wildlife disease surveillance network (SAGIR), coordinated by the French Biodiversity Agency (OFB). Monitoring for cases in horses takes place in close collaboration with the Epidemiological surveillance network for equine diseases (RESPE), independent veterinarians and Departmental Directorates for the Protection of Populations (DDPPs).

Identification of human cases is managed by the National Reference Centre for arboviruses, i.e. pathogens transmitted by blood-sucking arthropods such as mosquitoes and ticks. This work is supported by the French National Institute for Health and Medical Research (Inserm) and the French Armed Forces Biomedical Research Institute (IRBA).

What is ANSES's role?

The Virology Unit of ANSES's Laboratory for Animal Health is the national and European reference laboratory for West Nile virus. It also conducts research to find out more about the epidemiological situation and the factors influencing the virulence of the virus. 

• The national reference mandate

The National Reference Laboratory (NRL) for West Nile virus coordinates a network of seven approved testing laboratories. If an animal is suspected of being infected with West Nile virus, the laboratories in this network carry out the initial serological analyses, which examine whether the animal has actually been infected by the virus. The NRL then carries out confirmatory analyses.  Molecular diagnosis, which involves searching for the viral genome in blood, cerebrospinal fluid or organs targeted by the infection (such as the brain, liver or spleen), is only carried out by the NRL. When a case is confirmed, the NRL alerts the authorities, in particular the DGAL and the relevant DDPP. 

The NRL's role is also to determine the reliability of the analytical methods used by the network's laboratories. It regularly checks their detection capabilities by conducting interlaboratory proficiency tests using reference material it supplies. 

• The European reference mandate

ANSES has been the European Union Reference Laboratory (EURL) for equine diseases since 2008. In addition to West Nile virus, this group of viruses includes Japanese equine encephalitis virus, Eastern equine encephalitis virus, Western equine encephalitis virus and Venezuelan equine encephalitis virus. West Nile virus is the most common in Europe. 

The role of the EURL is to harmonise detection practices between the NRLs of the countries in the European Union and conduct interlaboratory tests with these laboratories. It can carry out confirmatory analyses on request. 

• Research to improve understanding of the virus

ANSES carries out epidemiological investigations to obtain a comprehensive view of horse infections by the virus, beyond symptomatic cases. These studies are conducted in France and abroad. 

The Agency also conducts research to gain a better understanding of the factors determining the virulence of the virus. For example, it studies the molecular and genetic determinants that explain why certain strains are more virulent than others, depending on the species infected. 

Another topic of study is the effect of co-infection by Usutu virus, a virus similar to West Nile, carried by the same species of birds and transmitted by the same mosquitoes. This co-infection, which has been reported in migratory birds and in a zoo in Germany but not in France, could influence virus circulation. Another research objective is to find out whether the infections were simultaneous or successive. 

Comparing the two viruses could also provide clues as to how to treat West Nile virus. This is because birds develop symptoms when infected with Usutu virus, but there are very few cases in horses or humans. The symptomatic species therefore seem to be the reverse of those for West Nile virus. Understanding why could help us combat it more effectively.