In September 2024, a handful of patients showed up at the emergency room in the coastal city of Fano, Italy, with mysterious symptoms: a rash on their hands and feet, fever, nausea, and diarrhea. Doctors sent for various tests until one came back with a positive result for dengue, a viral infection spread by mosquitoes, usually in tropical regions of the world. They tested the other patients, and, sure enough, they had dengue too. The strange thing? None of the patients had recently traveled to a country where they might expect to pick up a dengue infection. The only thing they had in common was visiting a particular neighbourhood in Fano.

As climate change increases temperatures across Europe, invasive mosquitoes are spreading and introducing disease agents that Europeans would usually think about only when traveling to other continents. The Fano outbreak wasn’t the first time dengue had been detected as spreading locally in mainland Europe, but it was significantly larger than previous incidents, with 199 cases detected between mid-August and mid-October of 2024. Over the past few years, European countries have reported outbreaks of other mosquito-borne viruses such as chikungunya, a painful disease that causes joint swelling, and West Nile virus. In a heating world, so-called tropical diseases are no longer restricted to tropical regions—and researchers expect the trend to continue.

The main current culprit is the mosquito Aedes albopictus, commonly known as the Asian tiger mosquito (or just the tiger mosquito, given it’s now found across all continents). Unlike many mosquito species, it bites in the daytime as well as at night. The tiger mosquito already has established populations across Europe, particularly in the south, and we can expect it to move north, says Riccardo Moretti, a senior researcher at the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), who has been working on tiger mosquitos for more than 20 years.

The tiger mosquito thrives between 20 and 30 degrees Celsius (68 to 86 Fahrenheit), but it can still live at around 10 Celsius (50 Fahrenheit). “This means that at the moment there are already conditions that could allow for the spread of the Asian tiger mosquito, even in England,” Moretti says. At the moment, these conditions are limited to the warmest part of the year in northern European countries. But farther south, such as in Greece, the mosquito season is elongating. Moretti says researchers have found adults buzzing around until December and reappearing already in March; they overwinter as eggs. “There is a concrete risk that in the next few years, we could have a population capable of surviving throughout the year,” Moretti says.

At the moment, cases of mosquito-borne diseases in Europe tend to occur toward the end of summer. This is when tiger mosquitoes are most prevalent, and it’s also when more people return from travel. Someone unwittingly brings back an infection that isn’t found in Europe—dengue, say—and gets bitten by a tiger mosquito. The mosquito goes on to bite other people, and the disease spreads locally. But as the weather cools, the mosquitoes get less active and cases subside. If the mosquito season lasts longer, however, more infections can occur. Ultimately, says Moretti, there’s a risk that tropical diseases like dengue could become endemic in parts of Europe, meaning the disease is always there. “It is completely different dealing with occasional cases of virus transmission, and instead dealing with a permanent status of possibility of transmission,” he says.

A difference of a couple of degrees can make a big difference. Earlier this year, a study found that the conditions required for tiger mosquitoes to transmit chikungunya were cooler than previously thought. The mosquitoes can spread the virus at just 14 degrees Celsius (57 degrees Fahrenheit) rather than 16 (61 degrees Fahrenheit), meaning outbreaks could occur in most European countries and for longer periods. “That difference of two degrees matters a lot when we are talking about global warming,” says Sandeep Tegar, an epidemiological modeler at the UK Centre for Ecology and Hydrology, who led the study.