Mosquitoes advance into northern Europe: insect sterilisation in Lisbon and a rise in dengue cases

Sterile males against an invasive species

In September, Portugal released 30,000 mosquitoes in Lisbon that had been specially treated with radiation: these are sterile males that can mate with females but produce no offspring. The goal is to reduce the population of the Asian tiger mosquito, an invasive species capable of transmitting dengue, chikungunya and the Zika virus. According to Euronews, this is already the second time Portugal has used this technique.

Cases are rising, the threat is moving north

The cause for concern is a combination of two factors: international travel brings viruses into Europe, while warming temperatures and the spread of invasive mosquitoes create conditions for local transmission. Eco-epidemiologist Jan Semenza explained the mechanism to the outlet: travellers return from Thailand or Brazil carrying viruses, a local mosquito bites the infected person, and a local outbreak begins. The numbers are already noticeable: by the end of October 2026, Italy had reported 76 cases of dengue acquired within the country, and France eight — roughly twice as many as in 2025. Experts fear that sporadic outbreaks could turn into sustained transmission and become endemic.

60 kilometres per year northward

The Asian tiger mosquito originates from Southeast Asia and, since the 1990s, when it arrived in Italy, has spread across most of southern Europe. It can travel by transport, colonising new territories, while infected travellers carry viruses to places where the mosquito has already established itself. According to Semenza's estimate cited by Euronews, the mosquito is advancing northward at roughly 60 kilometres per year, and disease outbreaks even faster, about 120 kilometres per year; every year around 50 million people are at risk from this type of tropical disease.

The diagnostic problem and the EU's response

The northward shift also creates a diagnostic challenge: doctors may fail to recognise a tropical infection in a patient who has not travelled abroad — Semenza notes that symptoms doctors have not previously encountered are difficult to link to a tropical disease. For this reason, the EU's Health Emergency Preparedness and Response Authority (HERA) is allocating €10 million to develop new diagnostic methods. Laurent Muschel, Director and Deputy Head of HERA, added that for new diseases arriving in Europe there is a shortage of suitable medicines, and the agency is funding research into antiviral drugs and new vaccines.