West Nile fever is caused by the West Nile virus, which can be found in birds and is transmitted by mosquitoes. The virus can also be transmitted to humans and certain other mammals, such as horses.

First infection cases in the Netherlands

October 2020 marked the first time a patient in the Netherlands was found to have been infected with the West Nile virus. This patient had probably contracted the virus in the Utrecht region, due to a mosquito bite. Birds and mosquitoes infected with the West Nile virus were found in this region in August and September 2020. It was the first time an infection with this virus was found in a person who had contracted the virus in the Netherlands. In early November, a further five people were found to be infected in the Utrecht region, as was one person in the Arnhem region.

West Nile fever symptoms

Once you have contracted the virus, it typically takes three to 15 days for you to develop symptoms. Most people (80%) do not develop any symptoms at all. Approximately 20% of infected people will develop mild flu-like symptoms, such as fever, headache and muscle ache. Only a very small percentage of them (1%) will experience severe illness, in the form of diseases such as encephalitis or meningitis. Some 4-14% of patients who contract a serious disease like this will die, although the mortality rate is higher in people aged 70 or older:  15-29%.

How does one get infected?

People are infected by mosquitoes that carry the disease. Typically, these will be common house mosquitoes, the most common mosquito species in the Netherlands. Mosquitoes are infected when they feed on infected birds. These mosquitoes then spread the virus to other birds and sometimes to humans and other mammals. Under normal conditions, the virus is not transmitted from humans to other humans. However, humans can be infected through blood or organ transplants.

How to prevent getting infected

The main measure to take in order to prevent being infected with the virus is to protect yourself against being bitten by mosquitoes. It is particularly important to do so at dusk, which is when mosquitoes are most active. For instance, you can wear clothes that cover most of your body and/or sleep under a mosquito net.

Where is the West Nile virus endemic?

In recent decades, the West Nile virus has spread to large parts of the world. For some time now, the disease has been found in the United States, southeastern Europe and the Mediterranean. In recent years, the disease has spread towards Central Europe, including central Germany. The first West Nile virus infection in the Netherlands was found in 2020.

2018 European outbreak

In 2018, a record number of West Nile fever infections were reported in Europe. In a single year, 2,083 confirmed and suspected infection cases were reported, which is more than the total number of reported cases in the seven preceding years combined (a total of 1,832 reported cases). This outbreak was likely due to a combination of very high temperatures and the presence of many mosquitoes and birds.

West Nile virus transmission mechanism: between mosquitoes and birds, then from mosquitoes to humans and horses.