Entomological surveillance during a major CHIKV outbreak in northwestern São Paulo: insights from São José do Rio Preto.

Background: Brazil is considered an epicenter for emerging and re-emerging arboviruses that significantly impact public health. The mid-sized city of São José do Rio Preto (SJdRP) in northwestern São Paulo state is considered hyperendemic for arboviral diseases, with case numbers climbing each year. Only 45 cases of chikungunya (CHIKV) were reported in the city from 2015 to 2022, indicating cryptic circulation of this virus, but cases in the state increased notably in 2023. This study investigates the use of active entomological surveillance to detect new arbovirus introductions in specific areas like SJdRP. Methodology/Principal findings: We used molecular testing to investigate the presence of CHIKV in adult culicids collected monthly from various neighborhoods in SJdRP. Positive samples underwent whole-genome sequencing and phylogenetic analysis. Entomological surveillance successfully detected the early spread of CHIKV across SJdRP, revealing an infection rate of 6.67%, with the well-established vectors Conclusions/significance: Extensive circulation of CHIKV was documented within the human and vector population, marking the onset of the first major CHIKV epidemic in SJdRP and neighboring cities. Because multiple arboviruses co-circulate in several locations in Brazil, entomological surveillance, along with ongoing monitoring of patient samples, is a key to help health authorities to implement more effective measures to interrupt transmission cycles and mitigate new epidemic waves.