In September 2024, a number of patients showed up at the emergency room in the coastal town of Fano, Italy, with mysterious symptoms: rashes on the hands and feet, fever, nausea, and diarrhea. Doctors sent out various tests until one came back positive for dengue, a viral disease spread by mosquitoes, usually in tropical regions of the world. They tested the other patients, and sure enough, they had dengue again. Something weird? None of the patients who had recently visited the country could expect to get dengue. The only thing they had in common was visiting a certain place in Fano.
As climate change increases temperatures across Europe, invasive mosquitoes are spreading and introducing disease agents that Europeans used to think about only when traveling to other continents. The Fano outbreak was not the first time that dengue was found to be spreading locally in the European country, but it was much larger than previous events, with 199 cases detected between mid-August and mid-October of 2024. In the past few years, European countries have reported outbreaks of other mosquito-borne viruses such as chikungulling disease, the West Niswe virus that causes West Niswe pain. In a tropical world, so-called tropical diseases are no longer confined to the tropics—and researchers expect this trend to continue.
The main reason now is mosquitoes Aedes albopictuscommonly known as the Asian tiger mosquito (or just the tiger mosquito, if it is now found on all continents). Unlike most types of mosquitoes, it bites day and night. The tiger mosquito has already established populations throughout Europe, especially in the south, and we can expect it to go north, said Riccardo Moretti, senior researcher at the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), who has worked on tiger mosquitoes for more than 20 years.
The tiger mosquito lives between 20 and 30 degrees Celsius (68 to 86 Fahrenheit), but can survive as low as 10 Celsius (50 Fahrenheit). “This means that at the moment there are conditions that can allow the spread of the Asian tiger mosquito, even in England,” said Moretti. Currently, these conditions are limited to the warm part of the year in northern European countries. But in the far south, like in Greece, the mosquito season is increasing. Moretti says researchers have found adults buzzing through December and re-emerging in March; in winter like eggs. “There is a real risk that in the next few years, we could have people who can live all year,” Moretti said.
Currently, mosquito-borne disease outbreaks in Europe tend to occur in late summer. This is where tiger mosquitoes are most common, and where most people return from trips. Someone unknowingly brings back an infection not found in Europe—dengue—and is bitten by a tiger mosquito. The mosquito continues to bite other people, and the disease spreads in the area. But as the weather gets colder, the mosquitoes become less powerful and the cases decrease. If the mosquito season lasts long, however, many diseases can occur. Ultimately, Moretti said, there is a risk that tropical diseases like dengue could spread to parts of Europe, meaning the disease is always there. “It’s completely different to deal with occasional cases of transmission, and instead to deal with the constant possibility of transmission,” he said.
A difference of a few degrees can make a big difference. Earlier this year, research found that the conditions needed for tiger mosquitoes to transmit chikungunya were cooler than previously thought. Mosquitoes can spread the virus at 14 degrees Celsius (57 degrees Fahrenheit) rather than 16 (61 degrees Fahrenheit), which means outbreaks can occur in many European countries and for a long time. “That difference of two degrees is very important when we talk about global warming,” said Sandeep Tegar, an epidemiologist at the UK Center for Ecology and Hydrology, who led the study.
