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Mosquito season is in full swing in Europe, and with it, mosquito-borne virus season.

Nearly 470 cases of West Nile virus, chikungunya, and dengue have been reported so far this year, according to the European Centre for Disease Prevention and Control (ECDC), which tracks cases in the European Union and a few other countries in the region.

These viral diseases, which usually occur when an infected mosquito bites someone, are becoming more common in Europe due to increased travel and warmer summer temperatures driven by climate change.

Here’s what you need to know about these illnesses and where cases have been detected so far this summer, as of mid-August.

West Nile virus

Eight European countries have reported West Nile virus infections this year: Italy (274), Greece (35), Serbia (nine), Romania (six), France (four), Hungary (two), Spain (one) and Bulgaria (one).

Those counts include both probable and confirmed cases in people. 

Most cases so far this year have been among men aged 65 and older, and most people have been hospitalised. At least 10 people in Italy have died from West Nile virus in 2025.

About 1 in 150 people who are infected with the virus will develop severe illness, such as encephalitis, which is inflammation of the brain, or meningitis, which is inflammation of the protective membranes around the brain and spinal cord.

These complications can be fatal, according to the World Health Organization (WHO).

Chikungunya

France has reported 111 chikungunya cases this summer, amid an uptick in cases in recent weeks. There have been 22 clusters of chikungunya infections this summer, and 16 are still active.

Italy, meanwhile, has recorded seven cases of chikungunya.

Chikungunya is not endemic in the EU mainland, but warmer weather during the summer months make it easier for mosquitoes to spread the virus. Most cases are typically reported in July or August, the ECDC said.

There are two chikungunya vaccines approved in the EU, but there are no specific treatments.

Chikungunya can cause fever, nausea, headache, fatigue, rash, muscle aches, joint swelling, and joint pain, which can be debilitating and long-lasting. Severe symptoms and death are rare consequences, though babies and older adults are at higher risk.

Dengue

France has seen 11 dengue cases this year, while Italy has seen four and Portugal has reported two cases.

EU health authorities believe the Portuguese infections, which were reported in Madeira in January, were likely contracted last year.

The ECDC said one type of mosquito that spreads dengue has been “established in a large part of Europe” and that another is fairly common around Madeira, Cyprus, and the Black Sea.

Globally, there are 100 million to 400 million dengue infections per year, according to the WHO.

Most people who are infected with dengue have no or mild symptoms, but the virus can cause fever, headache, body aches, nausea, and a rash. In extreme cases, dengue can be fatal.

There is no specific medication to treat dengue, and the best way to avoid infection is to take steps to prevent mosquito bites.