outbreak in Oropouche The virus has been raging in the Amazon for decades, but historically the pathogen hasn’t bothered much of the rest of the world. But this seems to be changing. In 2024, the virus showed it could travel.

Most of the 11,000-plus cases this year have occurred in Brazil and Peru, where the virus is an old acquaintance, but it was also found in 2024 in Bolivia, Colombia, Ecuador, Guyana, Panama, and Cuba — the latter reported in 603 cases as well as in-country transmission for the first time. Infected travelers also bring 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 ones cases have been found throughout Spain, Italy, and Germany.

For those studying Oropouche and other arboviruses—the family of viruses transmitted by arthropods such as mosquitoes and ticks—the situation is worrisome. Although there are clues about its shipping cycle, there is not enough information to accurately predict Oropouche’s future behavior. “We have some pieces of the puzzle, but we are not completely sure what the role of each one is,” said Juan Carlos Navarro, director of research at SEK International University, where he led those who progressive 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 headache, 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. Oropouche infections are generally uncomplicated, if not benign, although for the first time this year Brazil recorded two deaths linked to the virus.

Where cases have occurred, researchers have increasingly noticed something that may explain why the virus emerged and spread: deforestation. Changing natural land to grow crops, drill for oil, or mine for resources “seems to be the main cause of outbreaks,” Navarro said. “It brings together three links: the virus, the vector, and the people.”

A Natural Cycle With Gaps

In 1955, a young charcoal burner fell ill after two weeks of 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, several outbreaks have been reported, most occurring 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 wild, the reservoirs of the Oropouche virus—the animals that circulate the virus, even if they themselves are not sick—are believed to be nonhuman primates such as neotropical marmosets and capuchin monkeys, sloths, rodents, and birds. The virus may have been isolated from these creatures or antibodies found in their systems. In fact, the disease is also known as “sloth fever.” It is not known what role sloths and nonhuman primates play in the transmission cycle, Navarro said. “Maybe they’re breeding hosts” – meaning they’re probably allowing the virus to multiply rapidly to high concentrations in their bodies.

When there is an epidemic in humans, there is a second cycle of transmission. In this, humans are the breeding hosts, and the virus is transmitted between them by blood-feeding 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 can be 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 venezueleensis, another type of mosquito.