Sandra Junglen used to have to travel far south for her research. The biologist works at the Institute of Virology at the Charité Hospital in Berlin and is an expert on arboviruses – which include all viruses transmitted by mosquitoes. And these viruses are no longer just a problem in the global south.
Mosquito-borne viruses cause diseases such as dengue fever, yellow fever, or Zika virus, and are a major problem in many countries in Asia, South America, and Africa. According to the World Health Organization (WHO), a record 2023 million cases of dengue fever were registered in 6,5 alone.
"Mosquitoes are the deadliest animals in the world," says Junglen. About a million people die each year from mosquito-borne diseases.
Today, biologists no longer have to leave Berlin to research the viruses they transmit. Because one arbovirus has already successfully become established and is spreading in eastern Germany: the West Nile virus (WNV).
What is West Nile virus?
In most cases, people infected with West Nile virus do not experience any symptoms. About one-fifth of those infected develop flu-like symptoms – fever, chills, headache and muscle aches – within 14 days of infection. This is West Nile fever. Symptoms usually disappear after three to six days.
In one in 100 infected people, the virus attacks the central nervous system and can cause meningitis or encephalitis, leaving permanent consequences. Five to ten percent of these so-called neuroinvasive diseases end in death.
Main vector of the virus: the common mosquito
Unlike some other diseases transmitted by the invasive tiger mosquito, West Nile virus is mainly transmitted by the common mosquito (Culex pipiens), which is also native to northern regions. The virus itself was transported from the tropics to the north by migratory birds.
The VZN virus was first detected in birds and horses in Germany in 2018. By 2024, individual cases of animal infection had been reported in almost all German federal states. Since 2019, the virus has also been detected in humans several times, mainly in eastern Germany.
How does a mosquito get the virus?
In order for a mosquito to transmit the virus, it must first become infected itself. Only female mosquitoes bite, as they need blood to produce eggs.
When it comes to West Nile virus, mosquitoes feed on the blood of wild birds: the mosquito bites an infected bird and transmits the virus to another bird during the next bite.
Some birds die from the infection, while others remain asymptomatic carriers. "We still don't know which birds in Europe are particularly conducive to the spread of the virus," says Junglen.
Mosquitoes can also transmit the virus to mammals, primarily horses and humans. However, the chain of transmission stops there, as the virus does not reproduce efficiently in their bodies. Therefore, humans and horses are not a source of infection for mosquitoes and do not transmit the virus further.
Why do mosquito-borne diseases spread?
“Rising temperatures have a strong impact on mosquitoes and their potential to transmit disease,” says Sandra Junglen. In other words, climate change is playing into the hands of the bloodsuckers.
"That's why mosquitoes stay active longer: they hatch earlier, bite more often, reproduce faster and lay more eggs," the scientist explains. All mosquito species also need water to lay eggs and develop larvae. Heat and a little rain are enough for the expansion of the mosquito population.
Viruses also thrive in heat: "At higher temperatures, the virus multiplies faster in the mosquito's body and reaches its salivary glands more easily," explains Junglen. From there, it only takes one bite for the next infection.
Countries like France, Spain, and Italy have long had a problem with dengue fever: the tiger mosquito (Aedes albopictus), native to the Asia-Pacific region, has successfully become domesticated there.
The dengue virus itself, which is transmitted by this type of mosquito, has become indigenous in these countries. This means that it is no longer just brought in by travelers from abroad, but is also transmitted and circulated locally. "It's only a matter of time before we have this situation in Germany," says Junglen.
More mosquitoes don't always mean more viruses
It's not just rising temperatures in the Northern Hemisphere that are benefiting mosquitoes – both native and invasive species. The decline in biodiversity, which is crucial for the stability of healthy ecosystems, also plays an important role.
Sandra Junglen sees this in Berlin too. She and her team are investigating mosquito populations and the presence of West Nile virus in various locations around the city – from urban concrete surfaces to cemeteries to parks.
They found that the number of mosquitoes and viruses is highest in cemeteries. There are plants, water and animals that can serve as hosts for mosquitoes. In the natural park, there are even more mosquitoes – but almost no viruses. This is related to the diversity of species in the location.
"High biodiversity reduces the presence of pathogens. Every pathogen needs a host, and the more diverse the ecosystem, the lower the density of individual hosts – and therefore the lower the amount of viruses," explains Junglen.
Protection against West Nile virus: prevent mosquito bites
Since there is no vaccine or specific treatment for West Nile virus infection, the only protection is to prevent bites: pants and long-sleeved clothing make it harder for mosquitoes to bite. Mosquito nets on windows also help.
Those who have a garden should be careful not to provide breeding grounds for mosquitoes: mosquitoes love when water collects in pots or barrels. The same applies in the garden: the greater the diversity of species, the better – bats, frogs and spiders like to feed on mosquitoes.
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