The current number of Oropouche virus infections in Brazil may be up to 200 times higher than officially confirmed cases, with an estimated 5.5 million Brazilians potentially infected, according to a new study led by the Faculty of Medicine of the University of São Paulo (FMUSP). The research, conducted in partnership with the State University of Campinas (Unicamp), the Hematology and Hemotherapy Hospital Foundation of Amazonas (Hemoam), and the University of Kentucky in the United States, was published on Tuesday (24/03) in the scientific journal Nature Medicine and suggests that the scale of the outbreak has been significantly underestimated.
This Content Is Only For Subscribers
To unlock this content, subscribe to INTERLIRA Reports.
Transmission
Oropouche fever is caused by the Orthobunyavirus oropoucheense (OROV) and is transmitted primarily by the Culicoides paraensis mosquito, commonly known as Maruim or sandfly. The virus was first identified in Brazil in 1960 and has since caused sporadic outbreaks, especially in the Amazon region, where it is considered endemic. In urban environments, where circulation is less common, the Culex quinquefasciatus mosquito can also act as a vector. Since 2023, however, the disease has reemerged in Brazil and other Latin American countries, expanding across the national territory at an unprecedented scale. In July 2024, Brazil confirmed the first recorded deaths from Oropouche fever worldwide, as well as cases of vertical transmission from infected pregnant women to their fetuses.
Underestimated Numbers
According to the researchers responsible for the study, official figures significantly underestimate the spread of the disease. Over the last three years, the Ministry of Health has confirmed approximately 26,700 infections—834 in 2023, 13,856 in 2024, and 11,988 in 2025, based on epidemiological data. However, the study estimates that around 5.5 million people in Brazil may have been infected between 1960 and 2025. Across Latin America and the Caribbean, projections exceed 9.4 million infections during the same period. In Manaus, identified as a recent epicenter of transmission, the real number of cases may be up to 200 times higher than officially reported.
Amazon
In the capital of Amazonas, between the end of 2023 and mid-2024, the proportion of individuals with antibodies against the Oropouche virus more than doubled, increasing from 11.4% to 25.7%, according to analyses conducted with blood donors. The city has experienced two major outbreaks in its history, separated by a 42-year interval, in 1980–1981 and 2023–2024. Both outbreaks peaked during the rainy season and affected more than 12% of the population, reinforcing the role of environmental and seasonal factors in transmission dynamics.
Dissemination
Regarding the spread of the virus to new regions, the study indicates that factors such as high population density and increased air travel contributed to the expansion of Oropouche fever to all Brazilian states over the past three years, as well as to new Caribbean countries and isolated cases linked to travel in Europe and North America. The research also identified a new viral strain with surface mutations that may facilitate immune evasion, reducing the effectiveness of antibodies developed from previous infections. Another study suggests that this lineage replicates more efficiently in human cells. Additional factors cited by the Pan American Health Organization (PAHO) include climate change, deforestation, and unplanned urbanization, all of which favor the proliferation and geographic expansion of disease vectors.
Symptoms and prevention
The symptoms of Oropouche fever are like those of dengue and chikungunya and typically last between two and seven days. They include sudden onset of fever, intense headache, back and lower back pain, joint pain, cough, dizziness, pain behind the eyes, skin rashes, chills, photophobia, nausea, and vomiting. The incubation period, defined as the time between the infectious mosquito bite and the onset of symptoms, generally ranges from four to eight days. Prevention measures are focused on reducing exposure to mosquito bites, especially in endemic and high-risk areas.
Analysis:
The new estimates regarding Oropouche virus infections in Brazil reveal a substantial gap between official surveillance data and the likely real scale of transmission, pointing to structural limitations in epidemiological monitoring. A central factor behind this discrepancy is the low detection capacity, particularly in remote regions of the Amazon, where access to healthcare services remains highly uneven. In many cases, individuals must travel long distances—sometimes more than 24 hours—to reach medical facilities, which significantly reduces the likelihood of diagnosis and reporting. This logistical barrier creates a scenario in which a large share of infections never enters official records, distorting the understanding of the virus’s actual spread.
In addition to geographic constraints, the clinical profile of the disease further complicates detection. A considerable proportion of infected individuals are either asymptomatic or experience only mild symptoms, which reduces the incentive to seek medical attention. From a public health perspective, these findings suggest that Oropouche fever may represent a far more significant and geographically widespread challenge than previously assumed.
Sources: O Globo.
