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Oropouche epidemic The virus has raged in the Amazon for decades, but historically the pathogen has been of little concern to the rest of the world. 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 already known, but in 2024 Bolivia, Colombia, Ecuador, Guyana, Panama and Cuba were also detected – the latter reporting 603 cases as well as the first domestic transmission. Infected travelers have also carried the virus to North America and Europe: It has been detected twice in Canada this year and 94 times in the United States – 90 cases were reported in Florida, and 30 imported cases were found in Spain, Italy and Germany.
For those studying Oropouche and other arboviruses—a family of viruses transmitted by arthropods such as mosquitoes and ticks—the situation is troubling. Although there are clues about its transmission cycle, there is not enough information to accurately predict Oropouche’s future behavior. “We have some pieces of the puzzle, but we’re not entirely sure what role each one plays,” says Juan Carlos Navarro, director of research at SEK International University.
The first symptoms of the disease appear suddenly three to 12 days after the bite and usually last between four and six days. Symptoms include headaches, muscle and joint pain, tremors, 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 brain and its membranes – can develop. Although Oropuche infection is generally not severe and unpleasant, this year, for the first time, two deaths related to the virus have been reported in Brazil.
In the places where things are happening, researchers are increasingly finding something that could 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 acquisitions,” says Navarro. “It combines three connections: the virus, the vector, and humans.”
A Natural Cycle with Gaps
In 1955, a young coal stove fell ill after working and sleeping for two weeks in a forest near the Oropuche River in Trinidad and Tobago. He had a fever for three days. This was the first documented case of Oropuche virus. Since then, dozens of outbreaks have been reported, most of them in the Amazon basin.
Navarro has devoted 30 years to the study of arboviruses such as dengue, equine encephalitis, Mayaro and, since 2016, Oropuche. It has two transmission cycles. Reservoirs of Oropouche virus in the wild—animals that circulate the virus even if they don’t get sick themselves—are believed to be non-human primates such as Neotropical marmosets and capuchin monkeys, sloths, rodents and birds. The virus has either been isolated from these organisms, or antibodies have been detected in their systems. In fact, this disease is also known as “lazy fever”. What roles sloths and nonhuman primates play in the transmission cycle is not understood, Navarro says. “They probably overpower the host” — presumably allowing the virus to multiply rapidly to high concentrations in their bodies.
A second round of transmission occurs when there is an epidemic between people. In this, humans are the booster host and the virus is transmitted between them by blood-sucking 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 show that Culex and Aedes mosquitoes can also transmit Oropuche. In fact, the first isolation of the virus was from Trinidad and Tobago Coquillettidia venezuelensis, another type of mosquito.
