The recent Oropouche virus outbreak in 2023 drew attention in Brazil and other Latin American countries not only because of its scale—with more than 30,000 cases recorded nationwide—but also because of the first confirmed death in the country caused by the disease and its rapid spread to all states, extending beyond the Amazon region. In light of this situation, the World Health Organization expressed concern earlier this year and called for the accelerated development of prevention and control tools against this pathogen, which was virtually unknown until then.
Two studies published in Nature Medicine and Nature Health have shown that the impact of the Oropouche virus is much greater than official data suggests. Using mathematical calculations, historical data, and blood bank analysis, researchers estimate that since 1960, approximately 9.4 million people in Latin America and the Caribbean have been infected with Oropouche virus. In Brazil alone, there have been an estimated 5.5 million cases.
The disease causes fever and symptoms similar to dengue. It can lead to serious complications including neurological problems such as meningitis and meningoencephalitis, as well as microcephaly when transmitted from mother to fetus.
An estimated 300,000 people in Manaus were infected between 2023 and 2024—nearly 260 times more than confirmed cases—with antibody prevalence rising from 11.4% to 25.7% over one year, according to researchers' findings. William de Souza, a professor at the University of Kentucky in the United States and co-author of one study, said: "The capital of the state of Amazonas is a city with over two million inhabitants and is considered the gateway to the Amazon region. The striking underreporting occurred due to several factors, mainly because the virus circulated silently before reaching the outskirts of the urban center, with many cases being asymptomatic or mild and going undiagnosed." Souza also said, "Patients in remote regions of the Amazon often face travel times of more than 24 hours to reach a healthcare facility. That means that many cases likely went undiagnosed, allowing the virus to circulate silently until it reached the outskirts of a major urban center."
Researchers found continuous circulation at low levels often escapes standard surveillance systems; they identified two major outbreaks in Manaus—in both instances infecting over twelve percent of residents—as Módena said: "In our study we identified two major Oropouche virus outbreaks in Amazonian capital: one in 1980s and one in 2023. Each one infected more than 12% of the population." They also discovered individuals infected decades ago could still neutralize recent strains: "That suggests long-lasting cross-protection which could inform future vaccination strategies," Souza explained.
Unlike other arboviruses transmitted by Aedes aegypti mosquitoes primarily breeding near standing water sources within cities—the Oropouche virus spreads via gunpowder midge (Culicoides paraensis), making it eleven times more prevalent rurally. Souza said: "Unlike Aedes aegypti [the mosquito that transmits dengue, Zika, chikungunya], which breeds in standing water, the gunpowder midge lays its eggs in moist soil rich in organic matter. It's a bush mosquito found in humid areas. That's why cases are predominantly found in rural rather than urban areas." Strategies targeting typical mosquito habitats are therefore less effective for controlling this outbreak.
Researchers stress structural changes are needed for surveillance—including serological studies using blood banks as early warning systems—and integrating digital/genomic tools while decentralizing laboratory testing across rural areas.
