West Nile, chikungunya, dengue: Breaking down summer’s mosquito-borne diseases

West Nile virus in Italy, chikungunya in China, dengue in France. With summer in full bloom, mosquitoes are doing what mosquitoes do: keeping you up at night and spreading vector-borne viruses.

European countries, particularly in the south, have already reported their share of outbreaks this summer, while China is seeing an alarming spike in chikungunya cases. Although these diseases are not new, their presence in Europe is shifting as climate change makes environmental conditions more habitable for certain types of mosquito.

Here’s what you need to know.

West Nile virus

The West Nile virus is endemic to Europe — meaning it is found across the continent all year round. The virus is spread by Culex pipiens, or the northern house mosquito (native to Europe), which spreads it from infected birds to humans and some animals. It is asymptomatic in 80 percent of humans — but can lead to fever and, in severe cases, cause neurological diseases and become deadly.

So far in 2025, five countries in Europe have reported human cases of the West Nile virus: Bulgaria, France, Greece, Italy and Romania. Italy in particular has seen an uptick over the past few weeks, with 145 confirmed cases as of Aug. 5, including 12 deaths.

It is normal to see a “steep increase” in the number of cases around this time of year, said Tamas Bakonyi, principal expert on vector-borne and zoonotic diseases at the European Centre for Disease Prevention and Control (ECDC).

“We may expect that this year will be also a relatively intense West Nile season, but we don’t know it yet, because we are at the very beginning of the season,” Bakonyi said.

Though present in Europe since the 1960s, in recent years there has been an increase in the number of cases reported. This has coincided with a geographical spread northward — from the virus being predominantly present in southern Europe, it’s now emerging in Germany, Poland, the United Kingdom and even some Baltic countries.

“There will be increasing numbers in more northern countries, and the geographic spread may extend to Scandinavia or more to the northwestern part of Europe, where there have been so far only sporadic cases,” said Bakonyi.

There is currently no human vaccine for West Nile virus. There is, however, a vaccine available for horses and some are in development for birds.

Dengue and chikungunya

Dengue and chikungunya are not endemic to Europe, as they currently only manifest in the summer. These viruses are spread by infected Aedes albopictus, or Asian tiger mosquitoes, an invasive species in Europe, and host animals are not involved in the transmission cycle.

Dengue can cause flu-like symptoms, such as fever, headache, nausea, and joint and muscle pain. In severe cases, it can cause bleeding from the gums, abdominal pain and even death. The chances of developing severe dengue, and it turning hemorrhagic, are higher if someone has been infected more than once. It is particularly dangerous for older people, pregnant women, and children. However, mortality is low in Europe — there were no reported dengue deaths in 2024.

Chikungunya’s symptoms are similar to dengue’s, and include high fever, headache, nausea, rash, and muscle and joint pain. The word chikungunya comes from the Kimakonde language of southern Tanzania and means “that which bends up,” describing the way patients often double over from severe joint pain. Most people recover, although 30 to 40 percent of those affected develop chronic arthritis. It is particularly dangerous for older people, pregnant women, and children.

Most of the dengue and chikungunya cases reported in Europe are imported cases, meaning in people traveling back from areas with ongoing outbreaks, which can then be spread locally.

In 2025, major outbreaks have been ongoing in several Indian Ocean islands, including the French islands of La Réunion and Mayotte, as well as Mauritius. The virus is also spreading to Madagascar, Somalia and Kenya.

The World Health Organization issued an urgent call for action to prevent these outbreaks from turning into an epidemic.

“WHO is seeing the same pattern emerge since the beginning of 2025, where the same islands that were affected 20 years ago had very large outbreaks, such as Réunion, Mayotte, and Mauritius,” said Diana Rojas Alvarez, WHO’s team lead on arboviruses, during a recent press briefing. “We are calling for urgent action to prevent history from repeating itself.”

In addition to imported cases, the number of locally transmitted cases in Europe is also increasing as Aedes mosquitoes have spread throughout the continent over the past two decades, driven here by warming temperatures and changing weather.

A study published in The Lancet Planetary Health earlier this year examined how climate and other factors have influenced the spread of dengue and chikungunya in Europe over the past 35 years, finding that outbreaks have become more frequent and severe since 2010, in line with rising temperatures.

“Our findings highlight that the EU is transitioning from sporadic outbreaks of Aedes-borne diseases towards an endemic state,” the study said.

As of July 30, two countries in Europe had reported cases of dengue and chikungunya in 2025: France and Italy. In France, there have been 49 local cases of chikungunya in 14 clusters this year.

The high number of chikungunya virus disease clusters in France is “unusual compared to previous years,” said Bakonyi, but can be explained by the ongoing outbreaks in Réunion and Mayotte, which see many visitors from mainland France.

For diseases like dengue and chikungunya, which are increasingly present due to the spread of tiger mosquitoes in Europe, experts expect case numbers to continue to rise and have warned they may become endemic. Under the worst-case climate scenarios, the Lancet study estimated that the spread of dengue and chikungunya could increase to five times the current rate by 2060.

“We do expect that there will be an increase in the number of local outbreaks in the future if the current trends remain,” Bakonyi said, citing international travel and warm summers.

Vaccines against dengue and chikungunya have been authorized for human use in Europe. There are two dengue vaccines — including a version recommended for people who have already had a dengue infection, since that puts them at higher risk. There are also two chikungunya vaccines available, though there are safety concerns around one of them.