A new challenge for Europe: West Nile Virus

The West Nile Virus (WNV) is transmitted mainly by mosquitoes of the genus Culex. The bite of an infected mosquito might lead to West Nile fever in humans. For some years now, the WNV has been migrating northwards. 

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Culex species are the major vector of West Nile Virus.

Culex species are the major vector of West Nile Virus. Image: © Kaan Mika


C. pipiens

WNV on the rise

WNV was discovered in Uganda in 1937 and it has been continuously spreading around the globe. France, Italy, Germany, and Austria have all been affected by WNV in recent years. Moreover, Switzerland houses some mosquito species such as Culex pipiens, Culex torrentium and Aedes japonicus that can spread WNV. Given the proximity of the already affected countries to Switzerland, the spread of the virus was inevitable. Indeed, in 2022, WNV-infected mosquitoes were detected in Switzerland for the very first time. Luckily, no casualties from WNV have been reported until now (as of 2023), however, the following years are expected to be more problematic if necessary measures are not taken. 

How does WNV spread?

Birds are the main reservoirs for the WNV, most remaining unaffected or surviving infection. Nevertheless, mosquitoes can act as vectors by transmitting WNV from birds to humans and horses. In these cases, humans and horses are considered dead-end hosts since the virus does not replicate as efficiently within their bodies. Consequently, the levels of the virus in their bloodstream remain relatively low, insufficient to infect mosquitoes that subsequently feed on them. This ultimately restricts the virus's further transmission.

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WNV circulates among birds, and infected mosquitoes transmit the disease to humans. Image: © Kaan Mika

Is WNV dangerous? 

Even though most infected people are asymptomatic, around 20% of the infections will lead to complications such as fever, headache, diarrhea, etc. In rare cases (1:150 ratio), the virus can cause inflammation of the nervous system, which in very rare cases can be fatal, especially in people over 50.

Find more information about the West Nile Virus in this fact sheet from the Centers for Disease Control and Prevention USA (CDC) 

How do scientists detect the virus in Switzerland?

Due to WNV outbreaks in northern Italy, the virus has been on the radar of authorities and scientists in Switzerland for a few years, especially in the Canton of Ticino. In most cases, the presence of a certain virus is confirmed when hospitals or veterinaries notify public health authorities of the related diseases. Often, this happens only after a considerable number of people have been infected by a mosquito, since most cases of a WNV infection are asymptomatic. Earlier detection of infected mosquitoes could provide the necessary time to take safety measures, thus is vital to prevent disease transmission in new locations. 

How to catch a mosquito?

A common practice for early detection of viruses involves trapping thousands of mosquitoes and screening them for circulating viruses. To achieve this, scientists place mosquito traps at strategic locations and visit them regularly to collect mosquitoes and change batteries for continuous capturing. Particularly effective in catching Culex mosquitoes are gravid box traps, which consist of a tray and a box containing a fan and a collection chamber. To attract gravid (pregnant) mosquitoes, hay infusion  is poured into the tray (mosquitoes are attracted to the smell of decating hay in stagnant water). When a female mosquito arrives to lay her eggs on the water, she is immediately sucked with the help of a fan into the collection chamber from which she cannot escape.

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Although they look like ordinary toolboxes from the outside, gravid boxes are a very effective means of trapping mosquitoes in the wild . Image: Fairfax County, CC licence

After capturing the mosquitoes, scientists identify the species under a light microscope, isolate genetic material by molecular biology techniques and search for the presence of specific viruses by PCR*. However, this method has two major drawbacks: (i) it is expensive and laborious, (ii) it is hard to detect the presence of a virus when the transmission rate is very low. To overcome these problems, scientists place honey-baited FTA cards into the gravid traps.

What are honey-baited FTA cards?

FTA (Fast Technology for Analysis of nucleic acids) cards are filter papers that can preserve genetic material for a long time. When honey is applied to the paper, some of the trapped mosquitoes will feed on it and release their saliva on the FTA card. Proteins and cells in the saliva will denature quickly, but nucleic acids (DNA and RNA) will be preserved. Later on, the nucleic acids can be extracted and used to assess virus and bacteria species that circulate in these mosquito populations.

Mosquito FTA card 001

Gravid box and FTA cardplaced inside the trap. Once mosquitoes have fed on honey-baited FTA cards, genetic material of the circulating viruses can be captured. Image: Adapted from Wipf NC et al. 2019, CC licence

Current situation

In 2022, mosquitoes carrying WNV were found in 8 out of 12 locations in the Canton of Ticino that were monitored by researchers at SUPSI (University of Applied Sciences and Arts of Southern Switzerland)! Due to global warming, mosquitoes are finding increasingly favorable conditions for their development in the northern parts of Europe. As disease vectors will keep spreading, close monitoring of their migration patterns is required to take the necessary measures to prevent disease outbreaks. The presence of the disease in neighboring countries has already complicated the blood donation process in Switzerland and in the following years it is expected to have a mandatory WNV screen for blood donations as an additional safety measure to diminish WNV spread. 

Moreover, WNV also poses health risks to some bird species such as bearded vultures, a bird of prey that has been reintroduced to Switzerland. Therefore, vaccines against WNV for birds are being developed and trials have already started across zoos in Europe.


PCR: Abbreviation for polymerase chain reaction. This method is used to amplify and detect a precisely defined short DNA segment in a sample.


Fynmore N et al. 2022. Honey-baited FTA cards in box gravid traps for the assessment of Usutu virus circulation in mosquito populations in Germany. Acta Tropica 235:106649. https://doi.org/10.1016/j.actatropica.2022.106649

Wipf NC et al. 2019. Evaluation of honey-baited FTA cards in combination with different mosquito traps in an area of low arbovirus prevalence. Parasites Vectors 12:554. https://doi.org/10.1186/s13071-019-3798-8 

Bergman F et al. 2023. Vaccination of Zoo Birds against West Nile Virus—A Field Study. Vaccines 2023, 11(3):652. https://doi.org/10.3390/vaccines11030652

Vulture Conservation Foundation 2023. Tackling the threat of West Nile Virus within the Bearded Vulture Captive Breeding Network. https://4vultures.org/blog/tackling-the-threat-of-west-nile-virus-within-the-bearded-vulture-captive-breeding-network/ Accessed 10.11.2023

SUPSI. Ecologia dei vettori (ECOVET) https://sites.supsi.ch/im/ricerca/Ecologia-dei-vettori.html Accessed 10.11.2023

The author

20221110 2108 copy

Dr. Kaan Mika

ETH Zurich - The Biocommunication Group

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