Vector surveillance during a major chikungunya outbreak in northwestern São Paulo state, Brazil

Brazil constitutes a hotspot for arbovirus emergence, posing persistent public health challenges. In São José do Rio Preto (SJdRP), dengue has historically predominated, whereas only 62 chikungunya virus (CHIKV) cases were reported between 2015 and 2023, suggesting cryptic circulation. However, from 2024 onwards, CHIKV notifications began to increase across São Paulo State. Because arboviral febrile illnesses frequently overlap clinically, early detection necessitates robust surveillance systems. To address this, mosquito collections were conducted over 15 months in SJdRP and screened for CHIKV RNA. Positive specimens were further analyzed through virus isolation and whole-genome sequencing. Overall, 5.6% of specimens tested positive, with prevalence rising from December 2023 and peaking in May 2024, coinciding with human cases. Phylogenetic reconstruction indicated multiple introductions, primarily from southeastern Brazil, driving viral establishment. These events likely facilitated transmission within a dense, susceptible vector population, contributing to extensive spread and culminating in a major CHIKV outbreak in northwestern São Paulo.