New research has indicated that a hidden virus, Oropouche, has spread far more widely than reported, exposing major gaps in disease surveillance.
Until recently, few people had heard of the Oropouche virus, but after a massive 2023 outbreak swept across Brazil, infecting tens of thousands of people, causing the country’s first confirmed death, and spreading far beyond its traditional Amazonian range, the virus quickly became an international concern.
The World Health Organisation (WHO) had recently urged the rapid development of new tools to improve surveillance, prevention, and control. Two studies published in Nature Medicine and Nature Health suggest that Oropouche virus has affected far more people than official figures show.
By combining mathematical modeling, historical data, and blood bank analysis, the researchers estimate that the virus has infected about 9.4 million people across Latin America and the Caribbean since 1960. Brazil alone may account for roughly 5.5 million infections.
The illness causes fever and symptoms that can resemble dengue. In some cases, it can trigger severe complications, including neurological conditions such as meningitis and meningoencephalitis, as well as microcephaly when the virus passes from a mother to a fetus.
“We’re facing a disease of much greater magnitude than previously imagined, which requires greater attention.
We estimate that one in every thousand diagnosed cases progresses to serious complications such as neurological disorders, microcephaly, miscarriages, and liver complications, raising the priority level for public health,” said José Luiz Proença Módena, coordinator of the Laboratory for the Study of Emerging Viruses (LEVE) at the State University of Campinas (UNICAMP) and co-author of the studies, which were supported by FAPESP.
In Manaus, the largest city in the Amazon region and epicenter of the crisis, an estimated 300,000 people were infected between 2023 and 2024, a figure nearly 260 times higher than the confirmed case count.
According to the researchers, antibodies against the virus rose from 11.4 percent of the population in November 2023 to 25.7 percent in November 2024, a sign that transmission was widespread. “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,” says professor at the University of Kentucky in the United States and co-author of the study, William de Souza.
That pattern helps explain how Oropouche virus reached all Brazilian states and spread into neighboring countries, leading the WHO to issue an international alert.
The situation is especially difficult in remote parts of the Amazon, where access to medical care can be slow and logistically complex. “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,” Souza said. The researchers found that Oropouche virus can keep circulating continuously, often at levels too low for standard surveillance systems to detect.
