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Oropouche epidemics The virus has thrived in the Amazon for decades, but historically the pathogen has rarely disturbed the rest of the world. But that seems to be changing. In 2024, the virus showed that it can travel.
Most of the more than 11,000 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 last report 603 cases as the transmission in the country for the first time. Infected travelers have 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 are were found in 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 is insufficient information to accurately predict Oropouche’s future behavior. “We have some pieces of the puzzle, but there is no total certainty of what role each one plays,” says Juan Carlos Navarro, director of research at SEK International University, where he heads the group of emerging diseases and epidemiology.
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 can occur, and in severe cases, meningitis or encephalitis – inflammation of the heart and its membranes – can develop. An Oropouche infection is usually uncomplicated, if unpleasant, although for the first time this year Brazil recorded two deaths related to the virus.
Wherever the cases have occurred, researchers are increasingly detecting something that can 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 driver of epidemics,” says Navarro. “It brings together three links: the virus, the vector and the human.”
A Natural Cycle with Gaps
In 1955, a young coal miner 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. It was the first documented case of Oropouche virus disease. Since then, dozens of fires have been reported, most of them 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 reservoirs of the Oropouche virus – the animals that keep the virus circulating, even if they are not sick – are believed to be non-human primates, such as Neotropical marmosets and capuchin monkeys, sloths, rodents and birds The virus was isolated from these creatures or antibodies were found in their systems. In fact, the disease is also known as “sloth fever”. It’s not understood what role sloths and nonhuman primates play in the transmission cycle, Navarro says. “They are probably host amplifiers” – meaning they probably allow the virus to reproduce rapidly at high concentrations in their bodies.
When there is an epidemic among humans, there is a second cycle of transmission. In this, people are the amplifying hosts, and the virus is transmitted between them by insects that eat blood. The main vector that transfers 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, another type of mosquito.
