In September, Portuguese authorities released 30,000 specially treated mosquitoes in Lisbon. These were not ordinary insects: the males had been sterilised through irradiation, so they could mate with females but produce no offspring. The goal is to shrink populations of the Asian tiger mosquito, an invasive species capable of transmitting dengue, chikungunya, and Zika.
This is the second time Portugal has deployed the technique, as European countries confront a growing reality: diseases once confined to the tropics are gaining a foothold on the continent. International travel brings viruses into Europe, while warmer temperatures and the northward spread of invasive mosquitoes create conditions for local transmission.
“A lot of people are coming from their vacation back to Europe from Thailand or Brazil or places where we have these tropical diseases and they bring those viruses with them,” said Jan Semenza, an environmental epidemiologist, in an interview with Euronews. “They get bitten by a local mosquito and that local mosquito bites someone else and infects someone else and then you have a local outbreak.”
The numbers are already drawing attention. By the end of October 2026, Italy had reported 76 locally acquired dengue cases and France eight—roughly double the figures from 2025. As cases rise, experts worry that sporadic outbreaks could evolve into sustained transmission and eventually become endemic.
Moving north
The Asian tiger mosquito, originally from South-East Asia, has spread across much of southern Europe since arriving in Italy in the 1990s. It can hitch rides in vehicles, helping it colonise new areas, while infected travellers can introduce viruses into places where the mosquito is already established.
“The Asian tiger mosquito is moving something like 60 kilometres a year further north,” Semenza said. Outbreaks are moving even faster, at 120 kilometres a year. “They're coming later, but they're coming faster and we can see that that puts every year 50 million people at risk from these types of tropical diseases,” he added.
This northward shift also creates a diagnostic challenge. Doctors may not immediately recognise tropical infections in patients who have not travelled abroad. “Often you get an infection even if you haven't been abroad,” Semenza said. “And then how can [doctors] figure out that these types of weird symptoms that they have never seen before could be a potential tropical disease?”
That is why the EU's Health Emergency Preparedness and Response Authority (HERA) is investing €10 million in the development of new diagnostics. The funding aims to equip health systems across the bloc with tools to detect and respond to these emerging threats.
Portugal's sterile mosquito programme is part of a broader European effort to contain the spread. Similar initiatives are being explored in other Mediterranean countries, including Spain and Greece, where the tiger mosquito has also established populations. The technique, known as the sterile insect technique, has been used successfully in other parts of the world to combat agricultural pests, but its application to disease vectors in Europe is still relatively new.
While the immediate focus is on controlling mosquito populations, experts stress that a comprehensive strategy is needed. This includes public awareness campaigns, improved surveillance, and cross-border cooperation. The EU's climate policies also play a role, as rising temperatures are a key driver of the mosquito's northward expansion.
As Europe's climate warms, the window for tropical diseases to establish themselves is widening. The question is whether the continent can act quickly enough to stop them in their tracks.
