Three key approaches in the fight against West Nile virus

West Nile virus is spreading. During the summer of 2026, more than 1,000 infections have already been reported across more than 100 locations in Europe. Several human cases have also been identified in the Netherlands, including in a blood donor. Although most people - around 80 per cent - do not develop symptoms, the virus can cause health problems in some of those infected. These range from flu-like symptoms and headaches to neurological complications, encephalitis and even death. People with weakened immune systems, including older people, are particularly vulnerable.

Horses and birds are also susceptible to the virus. Birds also play a crucial role in its spread, says Gorben Pijlman, Professor of Arbovirology and Medical Biotechnology at the Laboratory of Virology. “When a mosquito bites an infected bird, the mosquito itself becomes infected because birds can develop very high concentrations of the virus in their blood. Songbirds often carry the virus without becoming ill, whereas corvids and birds of prey can develop severe neurological symptoms and may die in large numbers.”

Rising temperature is one of the factors contributing to the spread of the virus across Europe, says Sander Koenraadt, Professor of Ecology and Control of Disease Vectors at the Laboratory of Entomology. “Higher temperatures lead to larger mosquito populations. The more mosquitoes there are, the greater the chance that some of them are carrying the virus. But a mosquito carrying the virus still has to be able to transmit it to a person or animal. That happens through the salivary glands. At higher temperatures, the virus spreads through the mosquito’s body more quickly, reaching the salivary glands sooner and allowing it to be transmitted through the mosquito’s saliva.”

The first human case of West Nile virus in the Netherlands was recorded in 2020. The next registered human infection was not until 2025. According to Koenraadt, however, that does not mean the virus was absent in the intervening years. “I’m almost certain that several people were infected during those years. We also detected the virus in mosquitoes and birds that we examined. The people who were infected probably had no symptoms or only very mild symptoms, so they were never tested for the virus.”

WUR has been researching West Nile virus for more than 15 years, long before the virus first appeared in the Netherlands. Koenraadt: “At the time, we could see the virus spreading through southern Europe. Initially, we thought it could only be transmitted by mosquitoes in the warmer parts of southern Europe, but that turned out not to be the case. In 2015, research by virologist Jelke Fros showed that the common house mosquito found in the Netherlands can also transmit the virus. Once we saw how important temperature was to its spread, we knew it was only a matter of time before the virus appeared here too. So I’m not surprised that we are now seeing it increasingly often.”

To assess the risks posed by West Nile virus and how it is transmitted, researchers need to know where potential vectors occur. Citizen science plays an important role in this work. Through the Muggenradar project, members of the public can report how much mosquito nuisance they experience. These reports provide researchers with valuable information about the presence and activity of mosquitoes throughout the year and help improve our understanding of the risk of transmission of mosquito-borne viruses.

Koenraadt and his colleagues are currently investigating, among other things, whether and how the virus can survive the winter in mosquitoes. “We know that some mosquitoes enter a kind of winter dormancy, called diapause. What we want to know is whether warmer winters increase the chances of mosquito populations surviving, and whether mosquitoes can act as a reservoir for the virus, providing a bridge for transmission into the following year. That would not mean a major outbreak immediately in spring, but it could increase the likelihood of further spread. We already know that a similar virus – Usutu virus – can overwinter in mosquitoes.”

Much of WUR’s research into West Nile virus takes place in the high-containment Biosafety Level 3 Laboratory (BSL3 Lab). Here, researchers from Entomology and Virology work together to build fundamental knowledge about virus transmission, interactions between viruses and the risks of infection for people and animals. Pijlman, Koenraadt and Fros were among the founders of the BSL3 Lab in 2013. Pijlman: “The close collaboration between Virology and Entomology in a single location makes us unique. Together, we study West Nile virus at many different levels, from molecular processes to the role mosquitoes play within an ecosystem. In addition to West Nile virus, we study other vector-borne viruses in the lab, including Zika, chikungunya and tick-borne encephalitis.”

One of the activities carried out in the lab is dissecting mosquitoes, says Pijlman. “This allows us to see what happens to a virus once it has entered a mosquito. When a mosquito takes a blood meal, the midgut is infected first. The virus then has to make its way to the salivary glands. How does that process work, and what barriers does the virus have to overcome? We also look at ways of weakening the virus by making highly targeted modifications to its viral RNA. This reduces its ability to replicate. A weakened virus like this could be useful in the development of vaccines. At present, West Nile virus vaccines are only available for horses.”

According to Pijlman, developing vaccines is an important way of combating the virus. “Studies in mice at Wageningen Bioveterinary Research in Lelystad have shown that vaccination with a weakened virus produces a strong immune response and protects against subsequent infection with West Nile virus. Because these are live attenuated viruses, we also need to establish whether they can still be transmitted to mosquitoes. In the BSL3 Lab, we therefore feed mosquitoes chicken blood containing a specific amount of vaccine material. Two weeks later, we check whether the virus is present in the mosquitoes’ saliva. If the mosquitoes are no longer able to transmit the virus, that would support the safe use of these vaccines in the future.”

It may still take some time before such a vaccine becomes available, Pijlman stresses. “The main obstacle is not so much technological as financial. Before pharmaceutical companies develop a vaccine and bring it to market, they want to know whether there will be sufficient demand. Otherwise, it is not commercially viable. The number of West Nile virus infections is increasing, but in relative terms it is still low compared with other viruses. Seasonal flu causes many more deaths. Even so, I believe we should invest in a vaccine. It could, for example, be made available to vulnerable groups or people who spend a lot of time outdoors.”

Another avenue being explored by researchers in Wageningen is modifying the microbiome of the common house mosquito – the community of microorganisms living in its body. The aim is to boost the mosquito’s immunity to West Nile virus, explains researcher Jelke Fros. “There are various techniques we can use to do this. Our main focus is on viruses that are harmless to humans but infect mosquitoes and activate their immune system. This makes it more difficult, or even impossible, for West Nile virus to establish itself in the mosquito. The major challenge, however, is how to build immunity across a large mosquito population. Our research has not yet reached that stage.”

In the shorter term, the most effective way to protect against West Nile virus is to protect ourselves better against mosquitoes, says Koenraadt. “Indoors, that means using insect screens, mosquito nets and insect repellent, but there are measures we can take outdoors as well. The main thing is to prevent mosquitoes from laying eggs in small amounts of stagnant water. So empty water from plant saucers and flowerpots, and change the water in bird baths regularly. At the same time, however, we also need to create space in built-up areas to store water during wet periods. It is therefore an extremely complex issue that requires us to look at the whole system. And that is exactly what makes it so interesting.”