Increasing dengue outbreaks in temperate Brazil are associated with Aedes aegypti invasion and infestation levels

Dengue fever, caused by dengue virus (DENV), is one of the most significant mosquito-borne diseases globally, with increasing risk due to the expansion of Aedes vectors into subtropical and temperate zones. Brazil accounted for 70% of dengue cases reported worldwide in 2024, and the virus has recently become endemic in the southernmost state of Rio Grande do Sul. Here, we integrate entomological, epidemiological and environmental data and generate complementary genomic data to investigate DENV dynamics in Southern Brazil from 2015 to 2023. During this period, Aedes aegypti expanded across state municipalities, with a marked increase in dengue cases from 2020 onward. Nearly half of the municipalities surpassed the incidence threshold of ≥20 cases per 100,000 residents per year by 2023, indicating endemic transmission. Genomic analysis of 665 new DENV genomes is consistent with the local persistence of viral lineages across years, driving seasonal outbreaks. Moreover, Northwest and Metropolitan regions are identified as key hubs for viral spread within the Rio Grande do Sul state. These findings highlight the north-to-south expansion of dengue into temperate regions, emphasizing the urgent need for intensification of vector control and surveillance in newly endemic areas to mitigate dengue burden in a mostly naive human population.