Portugal is releasing sterile mosquitoes to curb the spread of an invasive species. But rising dengue cases and warmer temperatures are forcing European authorities to rethink how they prepare for tropical diseases.
In September, Portugal released 30,000 mosquitoes in Lisbon. But these were no ordinary insects. The specially treated male mosquitoes had been sterilised through irradiation, meaning they could mate with females but produce no offspring.
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The aim is to reduce populations of the Asian tiger mosquito, an invasive species capable of transmitting dengue, chikungunya and Zika.
It is the second time Portugal has used the technique, as European countries face growing concerns that diseases traditionally associated with tropical regions could become established on the continent.
International travel is bringing viruses into Europe, while warmer conditions and the spread of invasive mosquito species are creating opportunities 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," Jan Semenza, an environmental epidemiologist, told 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 attracting attention. By the end of October 2026, Italy had reported 76 locally acquired dengue cases and France eight. That’s roughly twice as many as in 2025.
As cases increase, experts fear sporadic outbreaks could eventually become sustained transmission and become endemic.
Moving up 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 travel 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 said.
That 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’s why the EU's Health Emergency Preparedness and Response Authority (HERA) is pouring €10 million into the development of new diagnostics.
“We are also missing the right medicines to treat these new diseases that are coming to Europe," said Laurent Muschel, director and deputy head of HERA, adding that the agency was funding research into antivirals and new vaccines.
