Vector surveillance during a major CHIKV outbreak in southeastern Brazil

Brazil is considered an epicenter for the emergence and re-emergence of arboviruses, which pose significant challenges to public health. The mid-sized city of São José do Rio Preto (SJdRP), located in northwestern São Paulo State, is hyperendemic for arboviral diseases, with an increasing number of reported cases each year. Notably, despite this epidemiological context, only 62 chikungunya fever (CHIKF) cases were officially reported in SJdRP between January 2015 and October 2023, suggesting cryptic circulation of chikungunya virus (CHIKV). In contrast, CHIKF notifications increased markedly across São Paulo State in 2024. Given the clinical overlap among arboviral febrile illnesses, particularly in dengue-endemic areas, the rise in CHIKV cases complicates differential diagnosis and highlights the need for robust surveillance systems. In this context, entomological surveillance is a valuable tool for the early and accurate detection of arboviruses. It enables the identification of emerging or increasing viral activity, supports the prediction of potential outbreaks, and guides targeted control efforts to mitigate public health impact. Therefore, in this study, we conducted monthly collections of adult mosquitoes across multiple neighborhoods in SJdRP and applied molecular assays to detect CHIKV RNA. Positive specimens were further analyzed through virus isolation, whole-genome sequencing, and phylogenetic reconstruction. Our results demonstrate that entomological surveillance allowed for early detection and tracking of CHIKV dissemination across the city, revealing an overall CHIKV RNA detection rate of 5.5% in mosquito samples. Detection rates began rising in December 2023, peaking in May 2024, in parallel with an increase in confirmed human CHIKF cases in the same period. Phylogenetic analysis indicated that the re-emergence of CHIKV in the region was driven by multiple introduction events, primarily originating from southeastern Brazil. These introductions likely facilitated viral establishment within a dense and susceptible vector population, contributing to extensive transmission and culminating in a large-scale CHIKF outbreak. Together, our findings reinforce the critical role of entomological surveillance in areas with co-circulating arboviruses. When integrated with ongoing clinical and laboratory surveillance of human cases, vector monitoring constitutes a powerful approach for guiding timely public health interventions, disrupting transmission chains, and reducing the likelihood of future epidemic waves.巴西被视为虫媒病毒（arboviruses）出现与再出现的热点区域，该类病毒对公共卫生构成显著挑战。位于圣保罗州西北部的中等城市圣若泽杜里约普雷图（São José do Rio Preto, 简称SJdRP）是虫媒病毒性疾病的高度地方性流行区，年报告病例数逐年递增。值得注意的是，尽管存在上述流行病学背景，2015年1月至2023年10月期间，圣若泽杜里约普雷图仅官方报告了62例基孔肯雅热（chikungunya fever, CHIKF）病例，这提示基孔肯雅病毒（chikungunya virus, CHIKV）存在隐性传播循环。与之形成对比的是，2024年圣保罗州的基孔肯雅热报告病例数显著攀升。由于虫媒病毒性发热疾病存在临床表型重叠，在登革热流行区这一现象尤为突出，基孔肯雅病毒病例数的增加为鉴别诊断带来了复杂性，同时也凸显了构建完善监测体系的迫切需求。在此背景下，虫媒监测（entomological surveillance）是早期精准检测虫媒病毒的重要工具，其可用于识别新发或活跃度上升的病毒活动，辅助预测潜在暴发风险，并指导针对性防控措施以减轻公共卫生影响。因此，本研究对圣若泽杜里约普雷图多个街区的成蚊开展了月度采样，并通过分子检测技术筛查基孔肯雅病毒RNA。阳性样本进一步通过病毒分离、全基因组测序及系统发育重建开展深入分析。研究结果表明，虫媒监测实现了对基孔肯雅病毒在该市传播扩散的早期检测与追踪，蚊媒样本中基孔肯雅病毒RNA总体检出率达5.5%。检出率自2023年12月起持续上升，于2024年5月达到峰值，与同期人类基孔肯雅热确诊病例数的增长趋势完全一致。系统发育分析显示，该地区基孔肯雅病毒的重现源于多起病毒输入事件，主要病毒来源为巴西东南部地区。这些输入事件可能促使病毒在密度高且易感的媒介种群中成功定植，进而引发广泛传播并最终导致大规模基孔肯雅热暴发。综上，本研究结果证实了虫媒监测在虫媒病毒共同流行地区的关键作用。若将其与持续开展的人类病例临床及实验室监测相结合，媒介监测可成为指导及时公共卫生干预、阻断传播链并降低未来疫情波发生概率的有力手段。