When Few Mosquitoes Are Enough: Dengue outbreaks in non-endemic areas

Dengue is no longer limited to tropical regions, as recent outbreaks in Southern Europe show. In 2024, nearly 150 local cases were reported in Fano, Italy, raising concerns that dengue could become established in temperate areas. In this work, we question the common belief that large outbreaks require high numbers of mosquitoes and a high basic reproduction number ( $R_0$ ). We show that significant outbreaks can happen even when $R_0$ is below one, triggered by the random introduction of infected travelers. Using a mathematical model that includes imported cases and mosquito populations, we explain how rare but possible transmission events can cause substantial local spread, even under conditions usually thought too weak for sustained outbreaks. Our findings point out an important gap in current risk assessments, which often rely only on fixed invasion thresholds and basic reproduction numbers. By considering the complex interactions between human travel, mosquito ecology, and the environment, our work supports the One Health approach to understanding disease emergence. We recommend combining stochastic modeling, data on human movement, and mosquito surveillance in early warning systems to better predict outbreaks' risk and improve preparedness in regions not traditionally affected by dengue.