A mosquito bite. Usually, it is nothing more than an itchy bump that disappears after a few days. But sometimes, such a tiny creature can transmit a virus we would rather not encounter.
Recently, the West Nile virus was detected in a blood donor who had contracted the infection in the Netherlands. This is noteworthy because West Nile virus still only occurs sporadically in the Netherlands.
But how does a virus like this end up in a Dutch mosquito? And should you be worried if you get bitten this summer?
From bird to mosquito… and sometimes to human
West Nile virus has a unique transmission cycle. The virus mainly circulates between birds and mosquitoes. A mosquito can pick up the virus when it bites an infected bird. If that mosquito then bites another animal or a human, it can transmit the virus.
And there is an important detail here: not every mosquito can spread the virus. It involves specific mosquito species that can acquire the virus and subsequently transmit it. Humans and horses are essentially accidental hosts in this cycle. They do not play an important role in further spreading the virus.
You can think of it like this: 🐦 infected bird → 🦟 mosquito → 👤 human
An infected bird cannot transmit the virus directly to a human. The mosquito is needed to bridge that gap.
Is West Nile virus already present in the Netherlands? Yes… West Nile virus was first detected in the Netherlands in 2020. At the time, infected birds and mosquitoes were found, and people in the Netherlands also became infected. Research by the Dutch National Institute for Public Health and the Environment (RIVM) has also shown that the virus continued to circulate in the Netherlands between 2021 and 2023, including among several birds and a horse. This does not mean that the virus is present everywhere in the Netherlands. It occurs sporadically, and its presence can vary depending on the region and the season. That is precisely why the spread of the virus is closely monitored. The recent infection in a blood donor is therefore noteworthy: it shows that the virus can still circulate in the Netherlands and infect people who have not travelled abroad.
What happens if you are infected? And now for the good news: most people do not notice anything at all when they are infected.
According to the RIVM, around 80% of infected people develop no symptoms. About 20% develop mild, flu-like symptoms such as fever, headache and muscle aches. Sometimes these can be accompanied by symptoms such as a rash, abdominal pain or diarrhoea. In a small proportion of cases, the infection can be more severe. The virus can then affect the nervous system and, for example, cause encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes surrounding the brain and spinal cord). Fortunately, this happens only rarely.
But why is a blood donor important? Feeling healthy does not automatically mean that there is no virus present in your blood. And that is exactly what makes an infection in a blood donor relevant. West Nile virus can be transmitted through blood products. For this reason, blood donations are tested for the virus in certain situations, for example when a donor has recently been in an area where West Nile virus occurs. In healthy people, the body can usually clear the virus on its own. But blood transfusions are also given to people who are particularly vulnerable. That is why it is important to prevent an infected blood product from reaching a patient.
Should we be afraid of every mosquito now? No, definitely not. A mosquito bite does not mean that you are infected with West Nile virus. The virus still occurs only sporadically in the Netherlands. Nevertheless, it is good to remain aware of the risk. Especially because mosquitoes thrive in warm conditions that allow them to reproduce. West Nile virus has been present for some time in parts of Southern and Southeastern Europe and has also spread further north in recent years. And although you obviously cannot avoid every mosquito—they sometimes seem rather determined to find you—you can reduce your chances of being bitten: use insect repellent when necessary, wear long sleeves and long trousers when mosquitoes are particularly active, use window and door screens and avoid standing water around your home, for example in buckets, plant pots and saucers.
A virus to keep an eye on
The detection of West Nile virus in a Dutch blood donor is therefore mainly an important signal to continue monitoring the spread of the virus closely. The virus is not new to the Netherlands, but this recent infection once again shows that West Nile virus can make the jump from mosquito to human here as well. A mosquito may seem like just a tiny part of nature. Yet this little creature can play an important role in the spread of viruses. So, the next time you hear a mosquito buzzing next to your ear: maybe close that window after all.