Retrospective serology reveals chikungunya exposure among hospitalized dengue suspected cases in Minas Gerais, Brazil, 2024.

The 2024 dengue outbreak in Brazil was accompanied by intense co-circulation of other arboviruses, complicating clinical diagnosis. In this retrospective study, serological evidence of unrecognized Alphavirus chikungunya (CHIKV) exposure was investigated among patients hospitalized with dengue-like illness during the 2024 outbreak in Minas Gerais, Brazil. A total of 1119 serum samples from 553 patients admitted to a referral hospital for infectious diseases between March and May 2024 were analyzed. Patients were initially managed under the clinical suspicion of dengue and had previously undergone molecular testing for arboviruses. Retrospective serological testing for CHIKV-specific IgM and IgG antibodies was performed using a commercial ELISA. CHIKV IgM antibodies were detected in 16.6% of patients, while IgG antibodies were identified in 8.3%. Notably, 31% of patients who tested negative by PCR for all investigated arboviruses tested positive for CHIKV IgM, indicating a substantial burden of unrecognized CHIKV exposure. Furthermore, 7.5% of patients who were PCR-positive for DENV were also CHIKV IgM-positive. Hematological analysis of CHIKV-positive (PCR and/or IgM) patients revealed marked thrombocytopenia, accompanied by mild to moderate leukopenia. These findings demonstrate that CHIKV exposure was frequently overlooked among hospitalized patients initially suspected of dengue, underscoring the limitations of molecular testing alone in late-presenting cases and highlighting the importance of combined molecular and serological diagnostic strategies during outbreaks.