Tropical diseases such as dengue and chikungunya are on the rise in Europe

In September 2024, A group of patients showed up at the emergency room in the coastal city of Fano, Italy, with vague symptoms: a rash on their hands and feet, fever, nausea, and diarrhea. doctors Sent for various tests Until someone came back with a positive result dengue fever, A viral infection spread by mosquitoes, usually in tropical regions of the world. They tested other patients, and sure enough, they had dengue as well. The strange thing? None of the patients had recently traveled to a country where they might be expected to become infected with dengue. The only thing they have in common is a visit to a certain neighborhood in Fano.

As climate change Temperatures rise across EuropeInvasive Mosquitoes They spread and introduce pathogens that Europeans typically only think about when traveling to other continents. The Fano outbreak was not the first time a local outbreak of dengue had been detected in mainland Europe, but it was much 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 swollen joints, and West Nile virus. In a warming world, so-called tropical diseases are no longer limited to the tropics, and researchers expect this trend to continue.

The current main culprit is mosquitoes Aedes albopictusknown as the Asian tiger mosquito (or just the tiger mosquito, since it is now found on all continents). Unlike many mosquitoes, it bites during the day as well as at night. The tiger mosquito has already established populations around the world Europe“Especially in the south, we can expect them 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 mosquitoes for more than 20 years.

Tiger mosquitoes live between 20 and 30 degrees Celsius (68 to 86 Fahrenheit), but they can still survive around 10 degrees Celsius (50 Fahrenheit). “This means that at the moment there are already conditions that could allow the spread of the Asian tiger mosquito, even in England,” says Moretti. Currently, these conditions are limited to the warmer part of the year in northern European countries. But in the south, such as in Greece, the mosquito season is longer. Researchers have found that adults hang around until December and then actually reappear in March, Moretti says. They spend the winter as eggs. “There is a tangible risk that in the next few years, we may have a population that is able to survive all year round,” Moretti says.

Currently, cases of mosquito-borne diseases in Europe tend to occur towards the end of summer. This occurs when tiger mosquitoes are most prevalent, and also when more people are returning from travel. Someone unwittingly brings back an infection not found in Europe, such as dengue fever, and is bitten by a tiger mosquito. The mosquitoes continue to bite others, and the disease spreads locally. But as the weather cools, mosquitoes become less active and cases decline. However, if mosquito season lasts longer, more infections could occur. Ultimately, Moretti says, there is a risk that tropical diseases like dengue will become endemic in parts of Europe, meaning the disease is always there. “It’s very different to deal with episodic cases of transmission and instead deal with the permanent situation of potential transmission,” he says.

A difference of a few degrees can make a big difference. Earlier this year, a study found that the conditions necessary for tiger mosquitoes to transmit chikungunya are: Cooler than previously thought. Mosquitoes can spread the virus at only 14 degrees Celsius (57 degrees Fahrenheit) instead of 16 degrees (61 degrees Fahrenheit), meaning outbreaks could occur in most European countries and for longer periods. “This two-degree difference is very important when we talk about global warming,” says Sandeep Tigar, an epidemiological modeler at the UK Center for Ecology and Hydrology, who led the study.