Oropouche virus attack rate in an urban setting outside the Amazon Basin: findings from a post-outbreak...

Oropouche virus (OROV) is transmitted by the biting midge Culicoides paraensis. Historically, it was restricted to small outbreaks in the Amazon region. Since 2023, genomic reassortment and human-environment interactions facilitated its geographic expansion, including into urban areas. We estimate the OROV attack rate and identify factors associated with infection in an urban area outside the Amazon.

A randomised cross-sectional serosurvey in the urban area of Baturité, Brazil. Households were selected using stratified probability sampling proportional to census tract population; one resident per household was randomly chosen. Participants completed a questionnaire, and serum samples were tested for OROV IgM using MAC-ELISA. Associations were estimated using prevalence ratios (PR) with 95% confidence intervals from Poisson regression with robust variance. Spatial clustering of seropositives was assessed using kernel density and nearest-neighbours.

Of 392 participants, 212 were seropositive (54·0%). Most infections were asymptomatic/oligosymptomatic (72·6%-154/212). Living near rivers (PR = 1·80; 95% CI: 1·20-2·50) and working from home (PR = 1·70; 95% CI: 1·30-2·30) were associated with higher prevalence of infection. Clustering of seropositivity was found in census tracts close to cultivated and forested areas, and in central urban neighbourhoods.

We show large-scale urban OROV transmission, with attack rates exceeding 50%. The high proportion of infections among low mobility individuals suggests peridomestic transmission and vector adaptation to urban microenvironments. Results highlight the need to incorporate OROV into routine surveillance, outbreak preparedness, and differential diagnoses of acute febrile illnesses.

Health Surveillance Secretariat/Brazilian Ministry of Health, and Japan International Cooperation Agency.