outbreaks of Oropouche viruses have spread in the Amazon for decades, but historically the pathogen hasn’t bothered the rest of the world much. But that seems to be changing. In 2024, the virus showed it could travel.
Most of this year’s more than 11,000 cases have occurred in Brazil and Peru, where the virus is well known, but in 2024 it was also found in Bolivia, Colombia, Ecuador, Guyana, Panama and Cuba, which reported 603 cases as first-time transmission within the country. Infected travelers have also brought the virus to North America and Europe: It has been found twice in Canada and 94 times in the United States this year — with 90 cases reported in Florida — while 30 imported cases have been found in Spain, Italy and Germany.
For those who study Oropouche and other arboviruses—a family of viruses transmitted by arthropods such as mosquitoes and ticks—the situation is troubling. Despite having information about its transmission cycle, there is insufficient information to accurately predict Oropouche’s future behavior. “We have several pieces of the puzzle, but there is no absolute certainty as to what role each one plays,” says Juan Carlos Navarro, director of research at SEK International University, where he leads the Emerging Diseases and Epidemiology Group.
The first symptoms of the disease appear suddenly between 3 and 12 days after the bite and usually last between four and six days. Symptoms include headache, muscle and joint pain, chills, nausea, vomiting and sensitivity to light. Skin rashes and bleeding from the gums or nose may appear, in severe cases meningitis or encephalitis – inflammation of the brain and its membranes – may develop. Oropouche infection is generally uncomplicated, if unpleasant, although Brazil has recorded two virus-related deaths for the first time this year.
Where cases have occurred, researchers are increasingly looking for something that may explain why the virus is emerging and spreading: deforestation. Alteration of natural land to grow crops, drill for oil or extract resources “seems to be the main driver of outbreaks,” Navarro says. “It brings together three bonds: the virus, the vector, and the people.”
A natural cycle with gaps
In 1955, a young coal miner fell ill after spending two weeks working and sleeping in a forest near the Oropouche River in Trinidad and Tobago. He had a fever for three days. This was the first documented case of Oropouche virus disease. Since then, dozens of outbreaks have been reported, mostly in the Amazon basin.
Navarro has dedicated 30 years to the study of arboviruses such as dengue, equine encephalitis, Mayaro and, since 2016, Oropouche. It has two transmission cycles. In the jungle, the reservoirs of the Oropouche virus—animals that keep the virus circulating even when not sick themselves—are believed to be nonhuman primates such as Neotropical marmosets and capuchin monkeys, sloths, rodents, and birds. The virus has either been isolated from these creatures or antibodies have been found in their systems. In fact, this disease is also known as “sloth fever”. It’s not known what role sloths and nonhuman primates play in the transmission cycle, Navarro says. “They are probably amplifying hosts” – meaning they probably allow the virus to multiply rapidly in high concentrations in their bodies.
When an epidemic occurs between humans, a second cycle of transmission occurs. In this, humans are the amplifying hosts, and the virus is transmitted between them by blood-feeding insects. The main vector that transmits the pathogen between humans is the midge Culicoides paraensis, which is the size of a pinhead and is found from Argentina to the United States. Some studies suggest that Culex and Aedes mosquitoes can also transmit Oropouche. In fact, the first isolation of the virus in Trinidad and Tobago was from Coquillettidia venezuelensis, a different kind of mosquito.