West Nile fever is caused by the West Nile virus. The west nile virus virus is found in birds and transmitted via common house mosquitoes (Culex mosquitoes). It can be passed on birds and sometimes also to humans and other mammals, including horses.
Most people (80%) who have a West Nile virus infection do not have any symptoms. About 19% of infected people develop mild flu-like symptoms, such as:
- fever
- headache
- muscle pain
- stomach pain
- diarrhoea
- skin rash (sometimes)
- swollen lymph nodes (sometimes)
In very rare cases (about 1% of infections), people can become seriously ill. If that happens, they may develop encephalitis or meningitis. The risk of death from these severe infections is 4 to 14%. In people aged 70 years and older, the risk can be even higher: 15 to 29%.
Who can become very ill from West Nile fever?
People over 50 and people with reduced immunity due to illness or medicine use have a higher risk of becoming very ill from West Nile fever.
How does West Nile fever spread?
Humans can contract the West Nile virus from the bite of an infected mosquito. The virus is carried by the common house mosquito (Culex pipiens), which is the most common type of mosquito in the Netherlands. Mosquitoes become infected when they feed on infected birds. Then these mosquitoes can spread the virus via their bite to other birds and sometimes also to humans, horses and other mammals. It does not work the other way around: mosquitoes cannot pick up the virus from people and other mammals.
The time between a bite from an infected mosquito and the first symptoms is 2 to 14 days. This interval is usually 2 to 6 days. Once a person has had West Nile fever, they cannot get the disease a second time.
How can I prevent West Nile fever?
The most important measure is to protect yourself against mosquitoes. The common Culex mosquito is especially active at dusk. At those times, you can follow these recommendations to prevent mosquito bites as much as possible:
- Wear covering clothing (long sleeves, long trousers)
- Keep mosquitoes outside (screens on windows and doors)
- Use mosquito repellents on uncovered skin
- Sleep under a mosquito net (bed net)
There is no vaccine available against West Nile fever.
Is there any treatment for West Nile fever?
There is no medication to treat West Nile fever. You can take medication to alleviate the symptoms, such as paracetamol for fever. A person with severe West Nile fever needs to be admitted to hospital.
How common is West Nile fever?
The West Nile virus has spread over large parts of the world in the past few decades. The disease has been present for some time in parts of Africa, the Middle East, Western Asia, North America, the southeast and central parts of Europe, and the Mediterranean region. In recent years, the disease has expanded further north within Europe, including central Germany. On the website of the ECDC (the European Centre for Disease Prevention and Control), you can see where in Europe the virus is currently present.
In 2020, an infection acquired in the Netherlands was identified for the first time. The patient most likely contracted the virus in the Utrecht region through a mosquito bite. In August 2026, West Nile virus was again confirmed in a person who had acquired the infection in the Netherlands.
Can people pass West Nile virus on to one another?
People cannot normally pass West Nile virus on to each other. There are a small number of rare exceptions, involving transmission through blood transfusion or organ donation. This is therefore closely monitored in high-risk areas where the virus is widespread. In certain parts of the Netherlands, the blood bank Sanquin is now also screening donor blood for West Nile virus.
Is West Nile virus dangerous during pregnancy?
West Nile virus does not appear to pose a greater risk to pregnant women than to other people. Pregnant women are not more likely to become seriously ill.
Can pregnant women pass West Nile virus on to their unborn child?
There have been a few cases worldwide of newborns with West Nile virus, but this is extremely rare. Transmission of the virus to babies through breast milk is also extremely rare. Women can continue to breastfeed or give expressed milk.
When should people contact their GP?
Contact your GP or out-of-hours GP service if you:
- develop a fever and flu-like symptoms, possibly with a rash; and
- feel clearly more unwell than you would with ordinary flu.
Call your GP or an emergency number immediately if you have:
- a sudden, severe headache;
- a stiff neck;
- confusion, drowsiness or difficulty speaking;
- signs of paralysis, for example difficulty moving an arm or leg.
These may be signs of encephalitis or meningitis, for which prompt medical care is important.
How common is West Nile virus in the Netherlands?
In 2020, an infection acquired in the Netherlands was identified for the first time. The patient probably got the virus from a mosquito bite in the greater Utrecht area.
No human infections were identified between 2022 and 2025, although infected birds or mosquitoes were found occasionally. In 2025, infections in horses were identified in the Netherlands for the first time.
In August 2026, West Nile virus was again identified in a person who had contracted it in the Netherlands. Infections have also been found in birds, mosquitoes and horses.
Find out more about the situation in the Netherlands on the Updates on West Nile fever page.
How does the Netherlands monitor the spread of West Nile virus?
In October 2020, West Nile fever was identified for the first time in someone who had acquired the virus in the Netherlands. Since then, RIVM and other organisations have carried out ongoing research into West Nile virus in the Netherlands. These organisations monitor humans, mosquitoes, birds and horses in the Netherlands. Proactive monitoring allows us to see where West Nile virus is present and how this changes over time. This is important so that measures can be taken in good time. Such measures may include more intensive testing and screening blood for the virus. It is also important to be able to actively inform people about precautionary measures.
Read more about research into West Nile virus in the Netherlands (only in Dutch, translation is expected).
