West Nile Virus – “We set up mosquito traps to identify viruses”


The West Nile virus could soon reach Switzerland. One expert suggests an early warning system by monitoring mosquitoes. The BAG sees no need for action.

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Mosquitoes multiply in hot and humid summers.


The West Nile virus is transmitted by various mosquitoes.  It came to Europe with the help of the mosquitoes.

The West Nile virus is transmitted by various mosquitoes. It came to Europe with the help of the mosquitoes.


The virus has also been known in Serbia since 2012, in the picture Belgrade.

The virus has also been known in Serbia since 2012, in the picture Belgrade.

Wikimedia Commons / Orjen / CC BY-SA 3.0

  • The West Nile virus, which can also be dangerous for humans, spreads through birds and mosquitoes in Europe.

  • There have not yet been any cases in Switzerland – that could change, according to an expert.

  • Infection is fatal in only 0.1 percent of those infected.

  • Nevertheless, the expert suggests an early warning system – also to protect blood donations.

Serbia is preparing for a mosquito plague and more infections with the West Nile virus. According to the website of the Federal Office of Public Health (FOPH), it cannot be ruled out that the virus will also occur in Switzerland.

An infection is usually harmless, around a quarter of those infected suffer from flu symptoms. But it can also lead to dangerous inflammation of the meninges and meninges or even death (see below). Expert Pie Müller explains the danger posed by the virus for Switzerland.

Have infected mosquitoes been found in Switzerland?
Pie Müller*:
Not until now and as far as I know no infected birds. However, the mosquitoes are not systematically examined, as is the case in Italy, for example. After all, mosquito traps have been set up in Ticino and Graubünden for a year or two. We use a process that we have further developed in order to be able to identify viruses efficiently. In the Basel region, we also tested mosquitoes for the West Nile virus last year, but none of them were positive.

So could the virus already be here without us noticing? That is theoretically possible. Infected mosquitoes have already been discovered in various neighboring countries, so it is only a matter of time before it is also in Switzerland.

In individual cases, the pathogen can attack the nervous system and cause meninges and meningitis. However, an infection usually has no consequences. Around 20 percent develop flu-like symptoms such as aching limbs, nausea, dizziness, headache and swollen lymph nodes. Around 80 percent of those infected survive the infection without symptoms. There are no vaccinations.

The West Nile virus is mainly transmitted between wild birds by mosquitos. Mosquitoes infected by birds can also transmit the virus to mammals such as horses and humans. Often, clusters of cases of dead birds and sick horses serve as a trigger for extending the search for cases to humans. It is now also known that the virus can be transmitted via organ transplants, blood transfusions and during pregnancy. In Switzerland, medical services such as blood donation services have already responded.

Why is the virus spreading in Europe right now? This is probably due to climatic reasons: In recent years we have had above-average summers in which there was still enough moisture. This encourages the mosquitoes to spread and the virus to develop.

How does the virus affect people? It is mainly transmitted via the house mosquito, but could also be spread by the Asian tiger mosquito. Both can also be found in Switzerland. The virus actually circulates among birds. If a mosquito bites an infected bird and then a human, the virus can be transmitted.

Is it transferable from person to person? No. The virus does not survive very well in humans. It can therefore be transmitted from birds to humans, but that is usually the end of the line.

How do you assess the danger posed by this virus? In most cases, the infection is asymptomatic, i.e. without disease. Only in a very small percentage of diseases do serious complications such as meningitis or meningitis occur, which in extreme cases can lead to death. But there would have to be a lot of infections for that to be statistically significant. However, blood transfusions pose a problem.

“Anyone who has become infected but noticed no symptoms and then donates blood can unintentionally pass the virus on.”

Pie Müller

In what way? If a person becomes infected with the virus but has no symptoms and then donates blood, they can unintentionally pass the virus on. On the other hand, a systematic review of the mosquito population could help and serve as a kind of early warning system.

Serbia is currently fighting the mosquitoes with insecticides because the authorities fear a plague and many infected people. If the going gets tough and we notice a strong spread of infected mosquitoes, it is also conceivable that we would have to contain the mosquito populations. However, as mentioned, an early warning system with mosquito traps would make more sense.

Like Sars-Cov-2, WNV is a zoonosis. Do we have to expect more viruses that are dangerous for humans in the future?This is quite possible and on the one hand has to do with globalization: it is much easier for a virus to spread over long distances today. In addition, humans have penetrated deeper and deeper into the forests. Viruses often circulate between animals and then jump to populated areas due to their proximity. It is therefore quite conceivable that in the future more viruses will be transmitted from animals via mosquitoes to humans.

* Pie Müller is Head of Vector Biology at the Swiss Tropical and Puclib Health Institute.

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