This tropical virus is spreading from the Amazon to the US and Europe

Oropouche shoots The virus has raged in the Amazon for decades, but the pathogen has historically been of little concern to 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 more than 11,000 cases 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 603 cases as well as transmission within the country 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 (90 cases reported in Florida), while 30 imported cases 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 not enough information to accurately predict Oropouche’s future behavior. “We have some pieces of the puzzle, but there is no complete certainty about what role each one plays,” says Juan Carlos Navarro, research director at the SEK International University, where he directs the emerging diseases and epidemiology group.

The first symptoms of the disease appear suddenly between three and 12 days after the bite, and usually last between four and six days. Symptoms include headaches, muscle and joint pain, chills, nausea, vomiting, and sensitivity to light. 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, although unpleasant, although for the first time this year Brazil recorded two deaths related 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 for farming, drilling for oil or extracting resources “seems to be the main driver of outbreaks,” Navarro says. “It brings together three links: the virus, the vector and humans.”

A natural cycle with gaps

In 1955, a young chickadee 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 of them 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 rainforest, the reservoirs of the Oropouche virus (the animals that keep the virus circulating, even if they themselves do not become ill) are thought to be non-human primates such as neotropical marmosets and capuchin monkeys, sloths, rodents and birds. The virus has been isolated from these creatures or antibodies have been found in their systems. In fact, the disease is also known as “laziness fever.” It is not understood what role sloths and non-human primates play in the transmission cycle, Navarro says. “They are probably amplifying hosts,” meaning they probably allow the virus to reproduce rapidly until it reaches high concentrations in their bodies.

When there is an epidemic among humans, there is a second cycle of transmission. In this case, people are the amplifying hosts and the virus is transmitted between them through blood-eating insects. The main vector that transfers the pathogen between humans is the mosquito. Culicoides paraensis, which is the size of the head of a pin 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 in Coquillettidia venezuelensis, another type of mosquito.