West Nile virus is also gaining a foothold in the Netherlands

In a few months, at least seven people have already become infected.

Originally, the West Nile virus was only found in Africa. But in recent decades it has spread quite quickly around the world and has also advanced towards Central Europe. It seemed therefore a matter of time before the virus would reach the Netherlands. And now the time has come. After an infection was first detected in our country in October, six more infections followed this month. And with that, the virus now seems to have gained a foothold in the Netherlands.

About the West Nile virus
The virus occurs among birds, but is spread by house mosquitoes, the most common mosquito species in the Netherlands. The mosquitoes pick up the virus when they bite infected birds. Once infected, such a mosquito can spread the virus to other birds, but also to humans and other mammals, such as horses (which can become very sick from the virus). Once infected, animals or humans cannot transmit the virus to others. Most people (about 80 percent) do not notice an infection, but 10 to 20 percent develop mild complaints such as fever or flu-like symptoms. In addition, there is a very small chance – about 1 percent – that infected people will develop a serious neurological disease, such as encephalitis. The virus poses a particular risk to elderly people and people with reduced resistance. In the US (where the virus was first found in 1999, after which it spread rapidly across the continent), the virus claims between 100 and 200 people every year.

The West Nile virus was first diagnosed in a Dutch man in October. It was believed that he contracted the virus from being bitten by a mosquito in Utrecht. Recent research by scientists at Wageningen University confirms this. The scientists found mosquitoes carrying the West Nile virus in the same environment where the man would have been bitten. And in the summer, a warbler was caught in that same area that had the West Nile virus. It can now be established with certainty that the virus can be found in the Netherlands and is already transmitted by mosquitoes to both humans and animals.

In line with expectations
That is not very surprising, says entomologist Sander Koenraadt, associated with the Wageningen university (WUR). “It was in line with expectations when we look at the countries around us. There too, more and more reports were made of positive birds, mosquitoes, horses or people. ” Although it has now been proven that the West Nile virus also occurs in the Netherlands, it remains unclear to what extent it has spread – unnoticed – in our country. “For example, it is still difficult to say how large the actual number of infected mosquitoes among the entire population is now,” says Koenraadt. “Often it only concerns tenths of a percentage.”

Climate change
The fact that the West Nile virus is spreading more and more across the world and now also occurs in more northerly areas – such as the Netherlands – can probably also be traced back to climate change, says Koenraadt, who has been researching the West Nile virus and others in his laboratory for almost ten years. viruses that are transmitted by mosquitoes, such as the zika and chikungunya virus. “Climate change certainly affects the transmission of viruses by mosquitoes. Because mosquitoes are cold-blooded animals, their development is highly dependent on temperature. In general, the warmer it is, the faster it will go. And if that goes faster, you can also get larger populations of mosquitoes in a shorter time, which means a greater risk of disease transmission. In addition, the development of the virus in the mosquito is also strongly dependent on temperature: the warmer it is, the faster the virus can multiply from the mosquito’s stomach and eventually find its way to the salivary glands. ”

Origin
The virus is therefore in the Netherlands. But it remains unclear exactly how it ended up here. “That remains speculative. But on the basis of the genetic code of the virus we can of course say something about its origin. If you make a kind of family tree of previously sequenced viruses and the virus found in the Netherlands, you can trace this. ” Researchers at the Erasmus Medical Center have made such a family tree and have to conclude that the virus found in the Netherlands is very similar to the virus we know from Germany (where it first appeared in 2018) and Austria. “The exact route is unclear, but it is probably natural (migratory) birds that spread it over longer distances. The flight range of a mosquito is many times smaller (a few hundred meters to a maximum of a few kilometers), although there is also a chance that an infected mosquito, for example, hitches a ride in a (freight) car from Southern Europe. However, that chance is much smaller. ”

Looking for a virus
Scientists are rushing to get a better grip on the spread of the virus. The fact that most people – fortunately – notice little of infection does not make things any easier. “Much of the transmission remains under the radar,” confirms Koenraadt. And that’s why scientists need to actively look for it. To this end, birds and mosquitoes in the Netherlands are regularly examined to determine whether they carry the virus. It was during such a study that in August a warbler in the Utrecht region came across that tested negative for the virus in the spring, but was now carrying the virus. And some of the mosquitoes caught in this area in July and August were found to also carry the virus. When it turned out in October that a man in the same area had also become infected with the virus, the circle was complete. Where we may now only see the tips of the iceberg, Koenraadt expects that we will hear and notice much more of the virus in the future.

A good reason for scientists to closely monitor the virus (and its carriers) and to study how it behaves here in the Netherlands. For example, it is currently also being investigated which role hibernating mosquitoes play. “We want to find out whether and how the West Nile virus hibernates in mosquitoes, and how it can reappear in the next season. To this end, we are now taking samples from wintering locations of mosquitoes. We want to know which species these are, how well they can survive the winter (the winters are of course also getting milder), and whether they may be carrying the virus. Hopefully we will get more clarity about this soon. These are exciting times for an entomologist. ”

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