First assessment of Aedes albopictus vector competence for chikungunya virus in the westernmost limit of its European range (Algarve, Portugal)

In the quiet corners of public health, a constant race unfolds between the movement of people and the spread of invisible threats. One such threat is the chikungunya virus, a pathogen that causes fever and severe joint pain, carried by mosquitoes that have expanded their territory far beyond the tropics. For decades, scientists have watched the Asian tiger mosquito, a striped insect known for its aggressive bite, establish itself in temperate climates across Europe. This mosquito is not just a nuisance; it is a highly efficient vehicle for viruses. When a mosquito bites a person carrying the virus, the pathogen enters the insect's body, replicates, and eventually travels to the salivary glands, ready to be injected into a new host. The speed and efficiency of this journey determine how quickly an outbreak can ignite. While the mosquito has been present in southern Portugal for several years, a critical question remained unanswered: could the local populations in this specific region actually transmit the virus to humans, or were they merely bystanders in a changing climate?

To answer this, researchers turned their attention to the Algarve, a popular tourist destination in southern Portugal where the mosquito has taken root. They collected eggs from the wild in the town of Olhão and raised the resulting mosquitoes in a controlled laboratory setting. The goal was to simulate a natural infection event. The team fed these mosquitoes a blood meal containing a specific strain of the chikungunya virus, one that had caused a major outbreak in the Indian Ocean region. The mosquitoes were then kept at a steady, warm temperature that mimics the summer conditions of the Algarve. Over the course of two weeks, the scientists checked on the insects at regular intervals to see what was happening inside them. They looked for the virus in the mosquito's abdomen, where the blood meal was digested, and in the thorax, the central body section that connects to the legs and wings. Most importantly, they tested whether the virus had reached the saliva, the only place from which it could be passed to a human.

The results revealed a mosquito population that is exceptionally well-suited to spreading the virus. Within just three days of feeding on the infected blood, nearly ninety percent of the mosquitoes had the virus successfully moving from their gut into their main body tissues. This rapid spread is significant because it means the mosquito becomes capable of transmitting the disease much faster than previously feared in some other regions. By the tenth day, the infection rate in the abdomen peaked at nearly eighty-eight percent, and the virus had spread to the rest of the body in almost all of those infected insects. The researchers then performed a forced salivation test, a method where they gently stimulated the mosquitoes to spit out a tiny drop of fluid to see if the virus was present. They found that infectious saliva could be detected as early as three days after the initial bite, and the proportion of mosquitoes capable of transmitting the virus continued to rise, reaching a maximum of over forty percent by the end of the two-week period.

This study provides the first concrete evidence that the Asian tiger mosquitoes living in the Algarve are fully competent to transmit the chikungunya virus under local conditions. The data shows that the barriers inside these mosquitoes that usually stop a virus from spreading are weak against this specific strain. The virus replicates aggressively and moves quickly through the insect's body, bypassing the usual delays that might prevent an outbreak. While the number of mosquitoes carrying the virus in their gut dropped slightly after the tenth day, the ones that remained infected maintained high levels of the virus in their bodies and saliva, ensuring that the potential for transmission remained high. These findings suggest that if a traveler carrying the virus were to visit the Algarve during the summer months, the local mosquitoes could pick it up and spread it to others within a matter of days.

The implications of these findings are clear for public health planning in southern Portugal. The region, with its high volume of international tourism and established mosquito populations, now has a verified risk factor for local outbreaks of chikungunya. The researchers emphasize that this is not a prediction of an inevitable epidemic, but rather a confirmation of biological capability. The mosquitoes have the tools to transmit the virus; they simply need the virus to be introduced. This knowledge allows health officials to move from guessing to knowing, providing the specific data needed to model risks and design targeted surveillance. By understanding exactly how fast and how efficiently these local mosquitoes can spread the disease, authorities can better prepare for the possibility of an outbreak, ensuring that if the virus does arrive, the response is swift and informed by the reality of the local insect population.