Climate Change: It Bites During the Day and Is Moving North - The Asian Tiger Mosquito Is Conquering Europe

In September 2024, patients began arriving at the emergency room in the Adriatic coastal city of Fano, Italy, with symptoms no one could quite explain. They had rashes on their hands and feet, high fevers, and severe stomach problems. Doctors kept sending sample after sample to the laboratory until one finally came back positive for dengue, a mosquito-borne viral disease normally associated with the tropics. The remaining patients were tested as well, and they too carried the virus. What made the outbreak so puzzling was that none of them had traveled abroad. The only thing they had in common was that they had all spent time in the same neighborhood of Fano.

It was not the first time dengue had spread locally on mainland Europe, but it was by far the largest outbreak. Between mid-August and mid-October 2024, 199 cases were recorded. During the same period, European countries also reported other mosquito-borne viruses, including chikungunya, which causes severe joint swelling, and West Nile virus. In a warming world, so-called tropical diseases are no longer confined to the tropics. The primary culprit has a name that tells the story of its own migration. Aedes albopictus, the Asian tiger mosquito, has already established itself on every continent. Unlike many of its relatives, it bites both during the day and at night. In Europe, it has become firmly established across much of the south, and according to Riccardo Moretti, who has studied the species for more than twenty years at Italy's ENEA research agency, it will continue moving north.

The mosquito thrives best in temperatures between 68 and 86 degrees Fahrenheit, but it can still survive when temperatures drop to around 50 degrees. According to Moretti, conditions already exist that would allow the species to spread even into England, at least during the warmest part of the year. Farther south, however, its season continues to grow longer. Researchers in Greece were still observing adult mosquitoes in December 2025, and by March 2026 they had already returned. The insects survive the winter as eggs. There is now a very real possibility that within only a few years some populations could remain active year-round.

Until now, Europe's dengue cases have typically appeared toward the end of summer. That is when mosquito populations reach their peak, and it is also when people return from international travel. One traveler unknowingly brings home an infection that is still uncommon in Europe, gets bitten by a mosquito, that mosquito bites someone else, and the virus begins circulating locally. Once temperatures drop, mosquito activity declines and the outbreaks gradually fade away. If the season continues growing longer, those outbreaks will become more frequent. Eventually, dengue could become established in parts of Europe. There is an enormous difference between dealing with occasional local transmission and living with the permanent possibility of recurring outbreaks.

Just how significant a difference two degrees can make is illustrated by a study published this year. Researchers found that the Asian tiger mosquito can transmit the chikungunya virus at 57 degrees Fahrenheit instead of only at 61 degrees, making outbreaks possible across much larger parts of Europe and for much longer periods each year. That two-degree difference carries enormous consequences when discussing global warming, says Sandeep Tegar, who led the study at the UK Centre for Ecology and Hydrology. But warmer temperatures do not automatically benefit the mosquito. Once temperatures rise above 95 degrees, the insects begin struggling to survive. Southern France and Spain, both affected by extreme heat waves and wildfires, could actually experience fewer mosquitoes this year. "It hasn't been a good mosquito season," says ecologist Frederic Bartumeus of Spain's CEAB research center. "It's simply been too hot and too dry." Rainfall also plays a critical role because mosquitoes need standing water to breed. Too little rain means too few breeding sites. Too much rain washes those breeding sites away, explains Felipe Colón González of the Wellcome Trust in the United Kingdom. Humidity also matters, as does the effect climate has on the viruses themselves and on human behavior. All of those factors make accurate predictions extremely difficult and continuous monitoring absolutely essential.

Researchers can reduce mosquito populations if they act early enough. Moretti's team released more than one million male mosquitoes in Italy that had been modified so females would lay infertile eggs after mating. The number of fertilized eggs dropped significantly. The program, however, proved expensive because the mosquitoes had to be bred in a dedicated facility. They came from Google's Debug program in Miami and had to be shipped to Italy. Completely eliminating the mosquito is impossible, Moretti says, because the species is simply too effective at colonizing new areas. The real objective is to reduce the risk of disease transmission as much as possible.

And this is where the real test begins. It is not about predicting the next outbreak. It is about building a system that can respond the moment the risk appears, says Colón González. That is where Europe still holds an advantage because reliable public health systems and disease surveillance networks have so far kept case numbers lower than in other parts of the world with similar climates. But that advantage is borrowed, and it will only last as long as people continue investing in it.

The process of adaptation has already begun. In Spain, many people have installed screens on their windows and routinely empty standing water from flower pots and containers around their homes. In the evenings, when people spend more time outdoors, many wear long pants and carry mosquito repellent with them. There is not much more anyone can do. Soon, however, the Asian tiger mosquito may no longer be alone. The yellow fever mosquito, Aedes aegypti, has already established itself in Cyprus and Madeira. In addition to yellow fever, it can also transmit dengue, chikungunya, and Zika. It tolerates cold weather less effectively than the Asian tiger mosquito, but global shipping routes continue carrying it farther north. Its most recent destination was a gas station in Luxembourg.

In the end, every degree of warming erases a boundary that existed only in our minds. For generations, the word tropical described a place on a map. Nature has never cared about our maps. It knows no distance, only temperature, and wherever temperatures rise, what we once believed was far away steadily moves closer.