what you need to know about this mosquito-borne disease

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Present in France since the 1960s, the West Nile virus, transmitted by mosquitoes, is spreading more and more quickly. In Hérault, 14 cases were detected in horses, two in humans. Here is what we know about this disease which affects birds, horses and humans.

How is West Nile virus transmitted?

West Nile virus is a virus transmitted by mosquitoes of the Culex genus, found everywhere on the planet, in tropical, temperate climates and even at altitude. These mosquitoes bite birds to feed on their blood, and this is how the virus is transmitted from bird to bird. However, mosquitoes are opportunistic and are not limited to birds. They can also bite other animal species, such as horses or humans, allowing the virus to spread to other hosts.

Haoues Alout, researcher at INRAE, explains: “Culex can grow in very different environments, whether urban areas around polluted waters or marshes in the Camargue. »

This adaptability makes their presence dangerous in many regions. They are found throughout France, particularly in Aquitaine, where cases in horses have been reported.

Since the 1960s, West Nile virus has been established in France, where it was first detected in the Camargue. Initially, cases of infection occurred sporadically. But from the 2000s, their frequency and number increased, and in 2018, a wave of unprecedented scale hit Europe (more than 1,500 cases). This change in pace, which has become annual, worries researchers.

What are the symptoms of West Nile virus infection?

Symptoms vary between species and individuals. Humans and horses are the main victims of the virus; the most visible signs are serious neurological disorders, but they are uncommon. In horses, we often observe ataxia, that is to say a loss of coordination of movements, and tremors. These symptoms can lead to the death of the animal.

Humans, on the other hand, rarely develop symptoms. About 80% of human infections are asymptomatic. However, in cases where symptoms do appear, they are usually mild, manifesting as moderate fever, headache or muscle pain. But in certain cases, particularly in the elderly or immunocompromised, the infection can lead to severe neurological complications. “The most seriously affected patients suffer from meningitis, encephalitis, coordination disorders and sometimes partial paralysis,” explains Haoues Alout. The virus is in the same family as dengue and yellow fever, with a similar ability to target the central nervous system. In patients with severe symptoms, hospitalization in intensive care is often necessary.

Is there a treatment or a vaccine?

To date, there is no specific treatment to treat people infected with West Nile virus. Medical management focuses on treating symptoms, particularly in severe forms where intensive care is often required to manage neurological disorders. Regarding prevention, a vaccine has recently become available to protect horses only. For humans, there is no vaccine available to date.

Faced with the absence of curative treatment and vaccine, the best defense against the virus remains the prevention of mosquito bites. Using repellents, wearing protective clothing and installing mosquito nets on windows are effective measures to reduce the risk of being bitten. In certain risk areas, municipalities and regional health agencies (ARS) intervene by spraying insecticides to reduce mosquito populations.

Another aspect to watch out for is the risk of transmission through blood transfusion. If a human case is detected in a department, strict measures are put in place to monitor blood donations. People who have been infected with the virus cannot donate blood for at least 28 days after infection, to minimize the risk of accidental transmission.

Why is West Nile virus spreading more and more?

Several factors explain the resurgence of West Nile virus epidemics in Europe. The first of these is probably global warming. “With higher temperatures and milder winters, mosquitoes proliferate more easily, which favors the transmission of the virus,” says Haoues Alout. This is because Culex mosquitoes breed more quickly in warm, humid environments, and climate change is expanding the areas where these mosquitoes can survive and multiply.

In addition, the virus itself evolves. Scientists found that some more pathogenic variants of the virus spread more quickly and further in northern Europe, causing symptoms more often in humans. Mosquitoes themselves appear to be becoming more efficient at transmitting these variants, which could explain the intensification of epidemics. As West Nile virus continues to spread, vigilance is required. With more than 700 human cases recorded in Europe in 2023, the risk remains very present, extending to new geographical areas, and mosquitoes could continue to gain ground in the years to come.

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