West Nile fever is caused by the West Nile virus. This virus is found in birds and transmitted via mosquitoes. It can also be passed on through mosquito bites 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. These mosquitoes then spread the virus to other birds, and sometimes also to humans 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 best way to protect yourself against the virus is to avoid mosquito bites. This is especially important at dusk, when the mosquitoes are most active. For example, you can wear protective clothing, apply mosquito repellent that contains DEET, and/or sleep under a mosquito net.
There is no vaccine 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 first infection of a human in the Netherlands was identified in October 2020. The patient probably got the virus from a mosquito bite in the greater Utrecht area. Birds and mosquitoes that carried the West Nile virus were found in that area in August and September 2020. In total, another 6 infections were identified in and around Utrecht in 2020 and 1 in the greater Arnhem area. No new infections have been identified in humans in the Netherlands since that time.
Over the past few decades, West Nile virus has spread across large parts of the world. For some time now, the disease has been common in parts of Africa, the Middle East, West Asia, North America, southeastern and central Europe, and the Mediterranean. In recent years, it has been spreading northward within Europe, including central Germany. The European Centre for Disease Prevention and Control (ECDC) posts regular updates on West Nile virus infections in Europe.
Record cases of West Nile fever in humans were reported in Europe in 2018. Within that one year, there were 2083 confirmed and suspected patients. This was more than the total number in the seven years before that. This outbreak was probably caused by very warm weather and high numbers of mosquitoes and birds.
West Nile virus transmission mechanism: between mosquitoes and birds, then from mosquitoes to humans and horses. (Source: Reusken C, Braks M, et al. De mug. Over steekmuggen en de verspreiding van ziekten. Uitgeverij Lias; 2022)
