Clinical and virological characterisation of Oropouche, dengue, and chikungunya during simultaneous co-circulation in the State of Espírito Santo, Brazil: a retrospective observational study

Background Dengue (DENV), chikungunya (CHIKV), and Oropouche (OROV) viruses co-circulate in Brazil and present with overlapping acute febrile syndromes, complicating clinical differentiation. Comparative data integrating clinical phenotyping and viral load dynamics across these arboviruses are limited. Methods We conducted a retrospective observational study using linked surveillance and laboratory databases from Espírito Santo State, Brazil (2024–2025). We aimed to compare DENV, CHIKV, and OROV infections and to explore their virological correlates. Cycle threshold (Ct) values, used as a relative semi-quantitative proxy for viral RNA levels, were analysed within each virus to characterise temporal dynamics and clinical expression. Multinomial logistic regression identified clinical features associated with arboviral infection; associations between Ct values and clinical domain scores were assessed using multivariable linear regression adjusted for age, sex, and time since symptom onset. Findings Among 10,793 cases (4058 DENV, 2508 CHIKV, 4227 OROV), Oropouche affected older individuals (median age 49 years, interquartile range [IQR] 43–56) and was characterised by near-universal fever (97·8%), headache (96·2%), and intense arthralgia (86·4%), with fewer gastrointestinal and mucocutaneous manifestations. After adjustment for age, sex, and time since symptom onset, increased odds of arthritis (OR 5·22, 95% CI 4·08–6·68) and rash (OR 2·39, 95% CI 1·83–3·12) were observed for CHIKV versus OROV. Conversely, nausea (OR 2·98, 95% CI 2·52–3·53), vomiting (OR 2·54, 95% CI 2·01–3·21), and retro-orbital pain (OR 8·00, 95% CI 6·69–9·56) were positively associated with DENV versus OROV. Dengue and chikungunya showed progressive Ct increases over time, consistent with viral decay. Oropouche displayed a non-monotonic, with lower Ct values and higher systemic and musculoskeletal domain scores after adjustment for age, sex, and time since symptom onset. Interpretation The distinct clinical and temporal patterns identified provide descriptive foundations that may inform future development of diagnostic algorithms and sampling strategies, pending formal predictive validation. Funding This study was partially funded by the Fundação de Amparo à Pesquisa e Inovação do Espírito Santo (FAPES), with additional institutional support from the Brazilian Ministry of Health and the Government of the State of Espírito Santo.