Oropouche Virus Importation in Southern Brazil and Emerging Concern Calling for Enhanced Public Health Surveillance

Abstract

Oropouche virus (OROV), an arthropod-borne virus transmitted by Culicoides paraensis, is an endemic arbovirus that historically circulates mostly in the Amazon basin. Between 2022 and 2024, it reemerged as a more widespread public health concern in South America. We conducted a pooled-sample molecular surveillance study to understand the prevalence of Oropouche fever in Brazil's southernmost state. Over 18 months, we analyzed 4060 samples to monitor the virus emergence in the Rio Grande do Sul state. We detected the first human case of OROV in the state, and our phylogenetic reconstruction indicated a travel-related introduction from the Amazon region into Rio Grande do Sul. Despite the absence of local transmission, the invasion of Culicoides paraensis and enzootic circulation of the OROV in Rio Grande do Sul highlight the risk of Oropouche fever outbreaks in the region. We demonstrated that pooled-sample surveillance effectively monitors virus introduction during periods of low endemic circulation, serving as an essential active surveillance tool for the timely detection of virus emergence and enhancing public health preparedness. The multiple introductions of distinct OROV lineages into southern Brazil underscore the importance of genomic surveillance and public health strategies to monitor and mitigate arbovirus spread in the region.

Keywords: Brazil; Oropouche; arbovirus; surveillance; virus emergence.

Figure 1

Distributions of confirmed Oropouche fever cases reported in Brazil from 2023 to 2025 and geographic distribution of the samples included in this study. (A) Number of confirmed Oropouche fever cases reported in Brazilian states per year, including data up to February 18, 2025, as reported by the Brazilian Ministry of Health. (B) The map of Rio Grande do Sul state, showing the geographic distribution of clinical samples from individuals presenting arbovirus infection symptoms, tested for OROV infection in this study.

Figure 2

Phylogenetic analysis of Oropouche virus (OROV) in Brazil, highlighting a sample from Rio Grande do Sul. (A) Maximum Clade Credibility (MCC) time‐scaled phylogenetic tree showing six major OROV clades defined by Naveca et al. (2024). The Rio Grande do Sul sample (red) clusters within the AM‐I lineage from Amazonas. A subtree within this well‐supported clade (posterior probability = 1) includes sequences from Erechim (Rio Grande do Sul) alongside five Amazonas cities. The Most Recent Common Ancestor (MRCA) is estimated to have emerged between mid‐October and mid‐November 2023. The map in (B) shows the transmission route from the central region of Amazonas to Erechim. (C) Zoomed‐in view of the clade containing the Erechim (Rio Grande do Sul) sequence. The asterisks in the left side of nodes in (A) and (C) represent posterior probabilities higher than 0.9. In (A), these probabilities are visually represented by gradually sized points at the tree's nodes.