The recent outbreak of the Oropouche virus, which occurred in 2023, drew attention in Brazil and other Latin American countries not only due to its magnitude (more than 30 thousand cases registered in the national territory), but also due to the first confirmed death in the country caused by the disease and its rapid spread to all states, no longer restricted to the Amazon region.
Faced with this scenario, at the beginning of the year, the WHO (World Health Organization) also expressed concern and made an appeal to accelerate the development of prevention and control tools against this pathogen, until then almost unknown.
Two studies published today (24/03) in journals proved that the impact of the Oropouche virus is much greater than that portrayed in official data.
Through mathematical calculations, historical data and analysis of blood from blood centers, researchers estimate that, since 1960, the has already infected around 9.4 million people in Latin America and the Caribbean. In Brazil alone, there would be approximately 5.5 million cases.
The disease, which causes fever and symptoms similar to dengue fever, can develop into serious complications, including neurological problems (meningitis and meningoencephalitis) and even microcephaly in cases of mother-to-child transmission.
“We are facing a disease with a much greater magnitude than imagined, which requires more attention. We estimate that one in every thousand diagnoses of the disease evolves into serious complications, such as neurological diseases, microcephaly, miscarriages and liver complications, which raises the level of priority for public health”, says , coordinator of the Laboratory for Studies of Emerging Viruses (Leve) at the State University of Campinas (Unicamp) and co-author of the studies, which are supported by FAPESP.
The work was also funded by the National Council for Scientific and Technological Development (CNPq), the National Institutes of Health (NIH), the United States, the British philanthropic institution Wellcome Trust and the Todos pela Saúde Institute.
Manaus, epicenter of the crisis
In Manaus (AM), the largest metropolis in the Amazon region, it is estimated that 300,000 people were infected between 2023 and 2024, almost 260 times more than the confirmed cases. According to the researchers, the prevalence of antibodies against the virus jumped from 11.4% in November 2023 to 25.7% in November 2024, indicating the widespread spread of the disease.
“The capital of Amazonas is a city with more than 2 million inhabitants and considered the gateway to the Amazon region. The impressive underreporting occurred due to several factors, mainly due to the fact that the virus circulated silently before reaching the edges of the urban center, with many cases being asymptomatic or mild, and without diagnosis”, says William de Souza, professor at the University of Kentucky, in the United States, who also authored the study.
This dynamic helps to explain the spread of the virus throughout all Brazilian states and neighboring countries, in addition to reinforcing the scenario that motivated the to issue an international alert.
In the case of patients in remote regions of the Amazon, researchers highlight the dynamics and logistics of the region. “Patients in remote regions of the Amazon often face travel times of more than 24 hours to reach a healthcare facility. This means that many cases were likely undiagnosed, allowing the virus to circulate silently until it reached the edge of a large urban center,” says Souza.
Researchers have detected that the Oropouche virus is in continuous circulation, although often at levels so low as to be almost undetectable by common surveillance systems. “In our work, we identified two large waves of Oropouche in the Amazonian capital, one in the 1980s and one in 2023, which each infected more than 12% of its population,” says Módena.
From this tracking, the researchers also identified that individuals infected in the 1980s were still capable of neutralizing the recent viral strain. “This suggests lasting cross-protection, capable of guiding future vaccination strategies”, explains Souza.
A bush virus
The re-emergence of the Oropouche virus in 2023 confirmed its expansion across the country. The State of Espírito Santo had the highest accumulated rate, with 318 cases per 100 thousand inhabitants. The Southeast region concentrated 57.9% of notifications, becoming the new epicenter of the disease.
Unlike other better-known arboviruses, it is transmitted by the sandfly, porvine or maruim (Culicoides paraensis), which means that the incidence of the disease in rural areas is 11 times higher than in cities.
“Contrary to Aedes aegypti [mosquito transmissor da dengue, zika e chikungunya]which reproduces in stagnant water, the maruim lays its eggs in moist soil rich in organic matter. It is a bush mosquito, from humid areas. Therefore, the predominance of cases in rural and non-urban areas”, explains Souza.
“Historically, this disease was closely linked to areas with banana and cocoa plantations, but, when studying the ecology of the virus, we identified that the issue is not the fruit itself, but the ideal condition of moist soils with plenty of . High temperatures and rain are also favorable conditions for the spread of maruim”, says the researcher.
The authors emphasize that the rural nature of the disease impacts public policy strategies. “The fight against the disease is very different from other arboviruses transmitted by mosquitoes, which are more urban. Strategies such as fumigation [o fumacê] in squares and paved streets they are probably of little use against Oropouche. The maruim does not live in the drains of houses, but in the humidity of forest areas and in the peripheral vegetation of cities”, explains Souza.
Another important characteristic of the maruim is that it is three times smaller than a common mosquito, an ideal size for crossing mosquito nets. But the reason behind this aggressive reemergence lies not just in climate, but in new viral recombination (reassortment).
In the work, the researchers also identified the emergence of a new viral lineage, the result of a process of genetic rearrangement or reassortment that occurs when two different viruses infect the same cell. This increased the replication capacity and made it difficult for antibodies to neutralize previous infections, making the pathogen more suitable for new territorial expansions (read more at: ).
“The reemergence of Oropouche shows us that we cannot combat all arboviruses with the same recipe, as maruim does not follow the same rules as Aedes. This makes current surveillance against the Oropouche virus insufficient and drastically underestimates the real scale of the disease”, says Módena.
For him, surveillance needs to go beyond big cities. “Although long-term immunity appears to exist for those who have already been infected, the speed with which the virus has expanded throughout all Brazilian states shows that the health system needs new detection systems, focused even on surveillance far from large centers”, he states.
The researchers highlight the need for structural changes, such as the adoption of continuous serological studies, the use of blood banks as early warning and the integration of digital and genomic tools to monitor outbreaks and mutations.
They also highlight the importance of decentralizing laboratory tests and creating active and permanent surveillance, capable of combining environmental, serological and genomic data to anticipate risks and guide vaccination strategies.
