Background Brazil is considered an epicenter for emerging and re-emerging arboviruses that significantly impact public health. The mid-sized city of Sao Jose do Rio Preto (SJdRP) in northwestern Sao 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 Aedes aegypti and Ae. albopictus as well as Culex sp. carrying the virus. The vector positivity rate increased from December 2023 to April 2024, which correlates with rising numbers of chikungunya fever cases reported in SJdRP during the same period. The resurgence of CHIKV in this region is attributed to several introduction events, mainly from the Southeast and North of Brazil, which facilitated establishment of the virus within the highly dense vector population and led to extensive spread and, in turn, a major CHIKV epidemic in this geographical area. 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.
