Exploring the challenge of diagnosing and tracking Oropouche virus during Brazil's largest recorded dengue epidemic in 2024.

BACKGROUND: In 2024, Brazil experienced its largest dengue virus (DENV) epidemic alongside the widest spread of Oropouche virus (OROV) to date. This scenario may have strongly impacted public health surveillance efforts, as DENV and OROV infections often lead to similar clinical signs and symptoms, making clinical differential diagnosis challenging. Herein, we investigated whether the co-circulation of DENV and OROV in Brazil in 2024 may have hampered OROV detection and tracking, potentially contributing to possible underdiagnosis. METHODS: The number of confirmed cases of dengue and Oropouche in 2024, as well as the probable states of infection, were collected on June 9, 2025, from the Brazilian Ministry of Health database. For dengue, cases were analyzed according to the diagnostic approach: laboratory testing or diagnosis by clinical-epidemiological criteria. All Oropouche cases considered here were confirmed by laboratory testing. Finally, dengue and Oropouche cases were grouped by state and epidemiological week, and their temporal-spatial overlap was assessed to discuss its potential negative effect on OROV reporting. RESULTS: A total of 5,956,257 dengue cases were recorded in Brazil, of which only 2,191,326 were laboratory confirmed. All states, including the Federal District, registered dengue cases, with highest number in São Paulo and Minas Gerais states (Southeast region). Dengue peaks occurred in early 2024 for Brazil overall and for individual states. Regarding Oropouche, 13,856 cases were recorded in 22 states, the majority in Espírito Santo (Southeast region) and Amazonas (North region). Oropouche cases in Brazil (total cases) showed two peaks: a smaller one at the beginning of 2024 and a higher one at the end of the year. On the other hand, OROV peaked in different periods depending on the state analyzed. In general, Oropouche cases were described close to the dengue reporting period, mainly in Amazonas, Pará and Rondônia states (northern Brazil), where there was evident overlap between the dengue and Oropouche curves. CONCLUSION: DENV and OROV co-circulated in most of Brazil in 2024. Furthermore, most dengue diagnoses were based on clinical-epidemiological criteria. This approach, combined with the clinical and temporal-spatial overlap between dengue and Oropouche cases, may have contributed to misdiagnoses, potentially hindering OROV detection and tracking. These findings reinforce the importance of laboratory testing for accurate arbovirus identification and surveillance in Brazil.