outbreaks of Oropouche virus have flared up in the Amazon for decades, but historically the pathogen has little troubled the rest of the world. But this seems to be changing. In 2024, the virus showed that it can travel.
Most of this year’s 11,000-plus cases occurred in Brazil and Peru, where the virus is an old acquaintance, but it has also been found in 2024 in Bolivia, Colombia, Ecuador, Guyana, Panama, and Cuba—the latter reporting 603 cases as well as in-country transmission for the first time. Infected travelers also transported the virus to North America and Europe: This year it was found twice in Canada and 94 times in the United States—with 90 cases reported in Florida—while 30 imported cases were found across Spain, Italy, and Germany.
For those who study Oropouche and other arboviruses—the family of viruses transmitted by arthropods such as mosquitoes and ticks—the situation is worrying. Despite having clues about its transmission cycle, there’s insufficient information to accurately predict Oropouche’s future behavior. “We have some pieces of the puzzle, but there is no total certainty as to what role each one plays,” says Juan Carlos Navarro, director of research at SEK International University, where he heads the emerging diseases and epidemiology group.
The first symptoms of the disease appear suddenly between three and 12 days after being bitten, and usually last between four and six days. Symptoms include headaches, muscle and joint pain, chills, nausea, vomiting, and sensitivity to light. Skin rashes and bleeding from the gums or nose may occur, and in severe cases, meningitis or encephalitis—inflammation of the brain and its membranes—may develop. An Oropouche infection is generally uncomplicated, if unpleasant, though for the first time this year Brazil recorded two deaths linked to the virus.
Where cases have occurred, researchers are increasingly detecting something that may explain why the virus is emerging and spreading: deforestation. Changing natural land to grow crops, drill for oil, or mine for resources “seems to be the main driver of outbreaks,” says Navarro. “It brings together three links: the virus, the vector, and humans.”
A Natural Cycle With Gaps
In 1955, a young charcoal burner fell ill after spending two weeks working and sleeping in the forest near the Oropouche River in Trinidad and Tobago. He had a fever for three days. That was the first documented case of Oropouche virus disease. Since then, dozens of outbreaks have been reported, most occurring in the Amazon basin.
Navarro has dedicated 30 years to studying arboviruses such as dengue, equine encephalitis, Mayaro, and, since 2016, Oropouche. It has two transmission cycles. In the jungle, the Oropouche virus’s reservoirs—the animals that keep the virus circulating, even if they themselves do not get sick—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, the disease is also known as “sloth fever.” It is not understood what role sloths and nonhuman primates play in the transmission cycle, says Navarro. “They are probably amplifying hosts”—meaning they likely allow the virus to rapidly reproduce to high concentrations in their bodies.
When there is an epidemic among humans, there is a second transmission cycle. In this, people are the amplifying hosts, and the virus is transmitted between them by blood-eating insects. The main vector that transfers the pathogen between humans is the midge Culicoides paraensis, which is the size of the head of a pin and is found from Argentina up 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, another type of mosquito.