This Tropical Virus Is Spreading Out of the Amazon to the US and Europe

Oropouche virus has posed little threat outside South America in the past, but land-use change, the climate crisis, and international travel all appear to be spreading this insect-borne disease to new places.

outbreaks of Oropouche virus have flared up in the Amazon for decades, but historically the pathogen has little troubled 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 11,000-plus cases occurred in Brazil and Peru, where the virus is an old acquaintance, but it has also been found in 2024 in Bolivia, Colombia, Ecuador, Guyana, Panama, and Cuba—the latter reporting 603 cases as well as in-country transmission 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—with 90 cases reported in Florida—while 30 imported cases were found across 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’s insufficient information to accurately predict Oropouche’s future behavior. “We have some pieces of the puzzle, but there is no total certainty as to what role each one 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 between three and 12 days after being bitten, and usually last between four and six days. Symptoms include headaches, 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. An Oropouche infection is generally uncomplicated, if unpleasant, though for the first time this year Brazil recorded two deaths linked 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 to grow crops, drill for oil, or mine for resources “seems to be the main driver of outbreaks,” says Navarro. “It brings together three links: the virus, the vector, and humans.”

A Natural Cycle With Gaps

In 1955, a young charcoal burner 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 occurring 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 Oropouche virus’s reservoirs—the animals that keep the virus circulating, even if they themselves do not get sick—are believed to be nonhuman primates such as neotropical marmosets and capuchin monkeys, sloths, rodents, and birds. The virus has either been isolated from these creatures or antibodies have been found in their systems. In fact, the disease is also known as “sloth fever.” It is not understood what role sloths and nonhuman primates play in the transmission cycle, says Navarro. “They are probably amplifying hosts”—meaning they likely allow the virus to rapidly reproduce to high concentrations in their bodies.

When there is an epidemic among humans, there is a second transmission cycle. In this, people are the amplifying hosts, and the virus is transmitted between them by blood-eating 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 is found from Argentina up 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.

But without a complete map of the virus’s reservoirs in the wild, the ecology of its vectors, and all their interactions, it is difficult to predict future scenarios. The midge Culicoides paraensis is associated with rural jungle areas, being found near bodies of water and banana crops, “but with new cases in urban areas, it is not known what role it plays,” says Navarro. Meanwhile, in Cuba, where transmission is now endemic, Culicoides paraensis has not been reported.

“If infected people are bitten by a competent mosquito, it could initiate a local cycle of transmission, as is happening with dengue in southern Europe,” Navarro says. “Before, this has happened with diseases that arrived in America: yellow fever, malaria, and Mayaro.”

A study by epidemiologist and ecologist Daniel Romero estimates that 5 million people could be at risk of Oropouche infection in the Americas, although the figure could be more, in light of the fact that several insects might be implicated in transmission. Travelers to Central and South America should identify sites with epidemic cycles. There are no vaccines for Oropouche and no specific antiviral treatments, but people can prevent bites with insect repellents and long-sleeved shirts.

Changing Landscape Is a Driver of Disease

Researchers are working to predict where the virus could spread by building what are known as “ecological niche models,” which contain data on cases, vectors, landscape, temperatures, and other factors to outline where the pathogen might be able to exist. “There are more cases in areas with recent deforestation,” says Navarro. Progress is being made every day in verifying the correlation between land-use change and greater transmission.

In outbreaks such as the current one, as well as one in Peru in 2016, researchers have found that badly affected areas lost more vegetation prior to the onset of the outbreak compared to regions without cases. In addition, 64 years ago, the first isolation of the virus in Brazil was in a sick sloth near the construction of the Belem-Brasilia highway. Navarro points out that human interference in nature seemingly driving disease outbreaks is not unique to Oropouche; years ago, the work of his colleague María Eugenia Grillet showed how the expansion of mining and deforestation reactivated malaria in Venezuela.

One possible explanation for this is the dilution effect, which suggests that the prevalence of a pathogen can increase at a site if species richness is reduced—with land-use change being an obvious cause of species loss. As an example, a study by the Borja Institute measured the transmission of arboviruses before, during, and after the construction of dams for the Panama Canal. It found that yellow fever decreased as monkeys disappeared due to the flooding of forested areas, but that equine encephalitis increased in areas near the dams due to the increase in waterfowl and mosquitoes.

Climate change could also be a contributing factor to the pathogen spreading. It is not known whether global warming affects the cycle of Culicoides paraensis, but it has been reported to accelerate the development of the mosquito Aedes aegypti—which is known to transmit other arboviruses—by reducing the time it takes for the mosquito to become an adult capable of biting. Inside the mosquito, viruses also replicate faster with heat (although excess heat damages them). Global warming also allows some vectors to thrive in regions where they could not before—which could lead to pathogens spreading to new habitats.

Destined to Change

The Oropouche virus genome is composed of three segments, unlike most insect-borne viruses, which have only one. This characteristic of the virus makes it prone to genetic reassortment, which occurs when two viruses infect a host at the same time. In the replication process inside the cell, they exchange segments of their genetic material, forming new, genetically unique combinations.

A reassortment containing Oropouche genetic material was found for the first time in Pará, Brazil, and was named Jatobal. A second was detected during an outbreak in Peru: 38 percent of the patients in that outbreak had respiratory problems, which is unusual for Oropouche, and so the virus may have picked up the capability to cause these through genetic material it gained from another virus. This reassortment was named Iquitos. Another reassorted strain, labeled Madre de Dios, has also been found in Peru. In 2012, yet another was isolated from a marmoset in Brazil, and named Perdões.

“That genetic process makes the virus more successful and diverse across other hosts. It is happening, and will happen more and more,” says Navarro. It appears that the current outbreak in the Brazilian Amazon coincides with the spread of a new reassorted lineage that emerged a decade ago in Brazil, which contains parts of viruses from the eastern Amazon, Peru, and Colombia. The worry with reassortment, Navarro explains, is that because of it you “can have variants with greater infection capacity or greater virulence.”

Next Strain, an international collaborative platform for real-time genomic surveillance, is available for monitoring such viral developments. It started out as a way of monitoring influenza viruses, before its use increased for tracking SARS-CoV-2. Genomic sequences of viruses are shared on this platform, and using genetic detective work, trees that trace the genetic characteristics and evolution of viruses are built. The Oropouche tree is currently under construction.

And there’s still much more to do besides this. Though outbreaks have been common, because they are usually quite small, studies into Oropouche have been limited. Navarro points out that countries also still need to work together to create rapid diagnostics, properly investigate the role of vectors in jungle, rural, and urban areas, as well as fully understand the transmission cycle. Research has also been slow because of a lack of funding—even though Oropouche is now the most common vector-borne disease after dengue in Brazil.

But now that the number of people at risk is increasing, and the World Health Organization is indicating a high risk to public health in the Americas, Navarro hopes that authorities will finally allocate more resources to study it. “A mistake in Latin America is that basic research is not given importance, because it is not seen as having an immediate application.”

This story originally appeared on WIRED en Español and has been translated from Spanish.

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