Oropouche fires the virus has been emerging in the Amazon for decades, but historically the pathogen has not bothered the rest of the world. But that seems to be changing. In 2024, the virus showed that it could travel.
Most of this year’s more than 11,000 cases occurred in Brazil and Peru, where the virus is an aged friend, but in 2024 it was also detected in Bolivia, Colombia, Ecuador, Guyana, Panama and Cuba – the latter of which had 603 cases and also national broadcast for the first time. Infected travelers also carried the virus to North America and Europe: It has been detected twice this year in Canada and 94 times in the United States – with 90 cases reported in Florida – and 30 imported cases detected 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 alarming. Despite having clues about the virus’s transmission cycle, there is not enough information to accurately predict Oropouche’s future behavior. “We have several pieces of the puzzle, but it’s not entirely clear what role each of them 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 three to 12 days after the bite and usually last four to six days. Symptoms include headaches, muscle and joint pain, chills, nausea, vomiting and photosensitivity. 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 infection is generally uncomplicated, if unpleasant, although for the first time this year two deaths linked to the virus have been reported in Brazil.
Where cases have occurred, researchers are increasingly detecting something that may explain the emergence and spread of the virus: deforestation. Conversion of natural land to grow crops, drill for oil or extract resources “seems to be the main cause of the epidemic,” Navarro says. “It connects three links: the virus, the vector and people.”
A natural cycle with gaps
In 1955, a adolescent 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. This was the first documented case of Oropouche viral disease. Since then, dozens of outbreaks have been reported, most of which occurred in the Amazon basin.
Navarro has spent 30 years studying arboviruses such as dengue, equine encephalitis, Mayaro and, as of 2016, Oropouche. It has two transmission cycles. In the jungle, non-human primates such as Neotropical marmosets and capuchin monkeys, sloths, rodents and birds are thought to be the reservoirs of Oropouche virus – animals that keep the virus circulating even if they do not become ill themselves. The virus was isolated from these creatures or antibodies were detected in their bodies. In fact, this disease is also called “lazy fever.” It’s unclear what role sloths and nonhuman primates play in the transmission cycle, Navarro says. They “likely amplify hosts” – meaning they likely allow the virus to reproduce rapidly at high concentrations in their bodies.
When an epidemic breaks out among humans, a second cycle of transmission occurs. In this case, the amplifying hosts are humans, and the virus is transmitted between them by blood-feeding insects. The main vector transmitting 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 research suggests 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.