The scene repeats every summer in thousands of Catalan homes: the light off, the silence of the night, and suddenly, the sharp, insistent buzzing that grazes our ear. The reaction is almost a Pavlovian reflex: we turn on the light, look for the little invader on the wall, and with a sharp slap of a slipper, we put an end to the annoyance. Until recently, this domestic victory returned us the tranquility of a bite-free sleep. Today, however, this gesture hides a much more disturbing reality: the mosquito is no longer just an annoying neighbor, it has become the vector for diseases that just four days ago we considered exotic.Catalonia has entered a new epidemiological era. The dengue outbreak in Vila-seca in the summer of 2024, with eight autochthonous cases, was the definitive alarm signal: the virus no longer only arrives by plane, but circulates among us through the tiger mosquito ("Aedes albopictus). This invasive species, recognized by its white stripes and its diurnal activity, has joined the common mosquito (Culex pipiens), the one of a lifetime, which is nocturnal and acts as the main transmitter of the Nile virus. As researcher Rachel Lowe, director of the Global Health Resilience group at the Barcelona Supercomputing Center (BSC-CNS), explains, "Barcelona is a global hub with a high risk of emergency or re-emergence of various pathogens." According to Lowe, rising temperatures and altered rainfall patterns create a "perfect cocktail" for the expansion of these vectors.

In recent weeks, there has been a decrease in incidents caused by mosquito bites. Although high temperatures favor these insects, the extreme heat that Catalonia has suffered in recent months does not seem to please them much. Unlike mammals, mosquitoes do not have an internal thermal regulation mechanism. This makes them very vulnerable to heat and the dehydration it causes, leading them to seek refuge in vegetation during the day to survive.

In the summer of 2004, two decades ago, one of the major media stories was the presence of a mosquito with black stripes that was larger than the native mosquito and caused more painful bites. Originating from Southeast Asia, Sant Cugat del Vallès was the first place in Catalonia where it was detected, almost twenty years after it entered Europe camouflaged in trucks transporting tires. Little by little, the insect has now become a integrate into everyday life every time the thermometer gets hot, to the point that it feels like the tiger mosquito (Aedes albopictus) has become the lord and master of the entire country. A study published this Thursday by a Catalan team in the journal Insects, The study, which includes the first map of invasive mosquitoes, indicates that this species has colonized some 780 of the 947 existing Catalan municipalities. The study is based on the analysis of data collected by both technicians and citizens who have reported the presence of the mosquito through the application. Mosquito Alert, a citizen science project that brings together around forty scientific entities, including the CSIC, the UPF, and the Blanes Center for Advanced Studies (CEAB).

"When we detect cases in humans, we see the tip of the iceberg. We may have many more circulating that we haven't identified," warns Jacobo Mendioroz, Deputy Director General of Surveillance and Emergency Response at Public Health, regarding mosquito-borne diseases. He is referring primarily to dengue, West Nile virus, and chikungunya.

Dengue fever is no longer just an imported threat arriving on airplanes. It's a homegrown reality in Catalonia. By the summer of 2024, the outbreak of eight cases in Vila-seca (Tarragonès) –the largest recorded in Spain– forcefully confirmed it: the virus is circulating here, transmitted by native mosquitoes. This episode is not an anecdote, but an alarming sign that reflects a profound transformation: global warming and mass mobility have shattered old epidemiological boundaries. A patient with fever and bone pain can no longer rule out a tropical disease without traveling. Faced with this new normal, the response cannot be fragmented. Human, animal, and environmental health are intrinsically linked. The solution has a name, One Health', an integrative approach that becomes an imperative necessity.

This summer will undoubtedly be a mosquito season. The frequent rainfall in Catalonia during spring, alongside high temperatures, will lead to a high number of insects. On the other hand, the heat will also mean cockroaches, rats and other rodents. The Catalan Association of Environmental Health Companies (Adepap) emphasises that in most cases (especially mosquitoes and rodents) these are species that transmit viruses and diseases, and therefore recommends taking preventive measures and relying on specialised professionals to combat pests and carry out disinfections.