What we know about Oropouche fever, a rising vector-borne disease

Oropouche virus disease was a relatively rare illness for decades, lurking on the margins of tropical rainforests in the Caribbean and South America.

Sporadic reports of an infection causing fevers, coughs, chills, and body aches emerged among people living near or moving into the jungle. A tiny insect called a midge spreads the disease, and the earliest known case dates back to 1955 in a forest worker near a village called Vega de Oropouche in Trinidad. Since most people who were infected with the virus recovered on their own and since cases were so infrequent, it barely registered as a public health concern.

But a few years ago, something changed.

A major Oropouche fever outbreak beginning in 2023 infected at least 23,000 people across Bolivia, Brazil, Colombia, Cuba, the Dominican Republic, and Peru. It wasn’t just confined to remote wilderness areas but was spreading in metropolises like Rio de Janeiro. In some cases, travelers were infected and then brought the virus home: So far, Oropouche fever has sprung up in the US, Canada, and Europe in people returning from the afflicted region. The outbreak has killed at least five people.

The sudden rise of Oropouche disease startled scientists and health officials. Since its discovery, there have only been around 500,000 known cases. By contrast, there are upwards of 400 million dengue infections each year. It’s likely then that many more Oropouche infections have gone undetected, especially since its symptoms overlap with those from other diseases and there’s little active screening for the virus.

Now, researchers are looking back at the outbreak to try to find out what they missed and what lessons they can apply to get ahead of future epidemics. Oropouche virus is a critical case study in the complicated factors that drive vector-borne diseases. Dynamics like deforestation, urban sprawl, international travel, and gaps in surveillance are converging to drive up the dangers from infections spread by animals.

And as the climate changes, new regions are becoming more hospitable to the blood suckers that spread these diseases, increasing the chances of these seemingly-remote infections making it to the US and getting established. That means more people will face threats from illnesses that they may never have considered before.

“It’s very likely that these public health problems that people before called ‘tropical disease’ are not so tropical anymore and are basically everywhere,” said William de Souza, who studies arboviruses — viruses spread by arthropods like insects — at the University of Kentucky. “Vector-borne disease is not a local problem; this is a global problem.”

The rising specter of Oropouche fever comes at a time when the United States is cutting funding for research at universities, pulling back from studying vector-borne disease threats, and ending collaborations with other countries to limit their risk.

The Oropouche virus is a classic case study in how humans worsen vector-borne disease

The Oropouche virus belongs to the family of bunyaviruses. They appear as spheres under a microscope, and they encode their genomes in RNA, rather than DNA as human cells do. RNA viruses tend to have high mutation rates, making it harder to target them with vaccines and increasing the odds of reinfection. Oropuche’s relatives include the viruses behind Crimean-Congo hemorrhagic fever, spread by ticks, and Rift Valley fever, spread by mosquitoes.

Oropouche spreads mainly through the bites of a 1- to 3-millimeter-long insect called, appropriately, a biting midge (Culicoides paraensis). Midges are sometimes called sand flies or no-see-ums in the US, and they breed in damp soil, rotting vegetation, and standing water. Like mosquitoes, they feed on blood to drive their reproduction, but their minuscule bodies can easily slip through mosquito nets. When a midge bites an infected host, it can pass on the pathogen to a human during a subsequent bite. There’s also evidence that the virus may be sexually transmissible, but no such cases have been documented yet. The Centers for Disease Control and Prevention recommends that male travelers from regions where Oropouche is spreading should not have sex for six weeks if they show symptoms of the disease.

Vector-borne diseases like Oropouche continue to surprise us because there are so many variables that have to align in order to spread them — the pathogens, the vectors, the hosts, and the environment.

Unlike diseases like Covid-19 or influenza, vector-borne illnesses don’t spread directly from person to person. Instead, they require an animal, often arthropods like ticks, midges, and mosquitoes. The range, reproduction, and behavior of these organisms add another confounding factor in the spread of the diseases they carry. Globally, vector-borne diseases account for 17 percent of infectious diseases, leading to more than 700,000 deaths per year, according to the World Health Organization. But not every part of the world is equally vulnerable.

In cooler regions, vector-borne infections are often a minor public health concern, but in countries like Brazil, “it’s at the top,” said Tatiane Moraes de Sousa, a researcher at the Oswaldo Cruz Foundation (Fiocruz) in Rio de Janeiro. “Oropouche before 2024 was concentrated just in the Amazon. Last year, we saw the spreading of Oropuche in almost all Brazilian states.”

That gets to the first obstacle in tracking Oropouche: Which animals are the reservoirs for the virus and where are they? So far, researchers have detected the virus in animals including sloths, capuchin monkeys, marmosets, domestic birds, and rodents. These organisms form what’s known as the sylvatic, or forest, cycle of the virus. How the virus jumps between all these animals and which ones are most concerning for people is not known.

Additionally, it may be possible that other insects may be able to carry the Oropouche virus, but it’s not clear whether they can spread it to humans.

The pattern that does emerge is that when people spend more time inside and around the fringes of tropical rainforests, where the animals that harbor the virus and the insects that spread them reside, they’re more likely to get infected. With deforestation and development, more people are moving into areas where the disease naturally spreads.

“This is a classical example of how human behavior can lead to the emergence of a pathogen,” said Natasha Tilston, who studies Oropouche virus at the Indiana University School of Medicine.

People can travel great distances, and as people move back and forth from the wilderness to cities, they can unwittingly carry viruses like Oropouche. If enough of them gather in cities where vectors are present, they can trigger an urban epidemic cycle as the virus travels from person to midge to person. This was likely the pattern in the 2023–24 outbreak in major cities in South America.

It’s also true that more health workers were on guard for Oropouche and thus identified more infections. “The outbreak is probably a combination of one, there are more cases, and two, we’re also looking for a lot more than we did before,” Tilston said, noting that some past outbreaks of dengue may have actually been Oropouche as well.

What set off the outbreak?

One factor is that the virus likely evolved. Viruses mutate all the time, and most mutations are either inconsequential or detrimental to the virus, slowing or stopping its reproduction. But occasionally, a change can confer an advantage or make the pathogen more destructive. The Oropouche virus has a genome structure that makes it even more prone to a type of mutation called reassortment.

“Reassortment is when you have two similar viruses infect the same cell and they mix genomes,” explained University of Kentucky’s de Souza. “People previously infected by the old virus are now susceptible to new infection. This could help explain why the Amazon region, where this has been circulated for a long time, saw this emerge, because people were probably reinfected.”

The strain behind the outbreak appears to reproduce faster and cause more severe illness than prior varieties as well.

Part of the reason this outbreak racked up so many infected people is that health officials were starting to deploy the tools to identify on a wider scale. Particularly in the wake of the Covid-19 pandemic, more health departments across the region built up their tools to detect viruses.

But researchers still aren’t sure exactly what spurred the virus to spread so suddenly across so many countries. Travel restrictions imposed during the Covid-19 pandemic started relaxing in 2023 and made it easier for people to move back and forth from the rural areas where the virus is endemic to the cities where it became established.

The 2023 to 2024 outbreak also coincided with a powerful El Niño event that brought gargantuan amounts of rain and triggered unprecedented flooding across many parts of South America. These were also years that set new temperature records. Higher temperatures can speed up the reproduction of the virus inside midges. But scientists aren’t exactly sure how this heat and water affected the vectors, though Brazil has seen outbreaks of other infectious diseases in the aftermath of floods.

“El Niño and other climate phenomena have been associated with the change of the patterns of many different vector-borne diseases,” de Souza said. “For Oropouche specifically, we don’t have the answers yet, but the likelihood of impact is very high.”

On top of all this, there aren’t any specific ways to keep an outbreak in check once it ignites. There are no vaccines or treatments for Oropouche fever yet. So when all the factors align to spread the disease, there isn’t much people can do to target the disease, and when it reaches a new area, there aren’t as many people with immunity and few health workers who know what they’re dealing with.

How the US is preparing for diseases like the Oropouche virus

Fortunately, the Oropouche outbreak has died down, but a variety of infections are gaining a toehold in new places as infected people travel and as vectors move into new habitats, and the US is increasingly vulnerable. According to the CDC, the number of vector-borne disease cases per year has doubled in the US since 2001.

Last year, the US saw transmission of mosquito-borne diseases like Eastern equine encephalitis and West Nile virus. Malaria, a disease once eradicated across the country, saw the first local infections in 20 years in 2023 in Florida and Texas. Vectors like the Asian tiger mosquito are spreading further north as the climate changes and expands favorable conditions for its survival.

With travelers moving back and forth from regions where diseases are endemic, many will unwittingly bring back dangerous souvenirs, whether a stowaway insect in their luggage or an infection in their blood. And with midges, mosquitoes, and ticks spreading to new regions, dangerous pathogens are extending their reach.

There are ways to slow the spread of these diseases, however, and the US has managed to do so before. The US famously launched a successful campaign to eradicate malaria within its borders.

The first step is to simply acknowledge the threat. As Oropouche showed, there may be diseases lurking closer than we realized that we simply haven’t bothered to look for.

It’s fairly simple to do things like dump standing water where insects can breed or spray insecticides on midge breeding grounds. But some places are getting creative, working to build up habitats for fish, bats, birds, and dragonflies that are natural predators of mosquitoes and midges to limit their spread. Limiting the destruction and development in wilderness areas can reduce the likelihood of diseases spilling over from animals into humans.

Some regions are looking at even more drastic ways to stymie vectors. One measure that’s gaining traction is deploying sterile male mosquitoes. When they mate, they produce eggs that won’t hatch, thus reducing the population of the insect. Brazil recently inaugurated a factory that breeds mosquitoes to carry a bacterium known as Wolbachia that prevents the mosquitoes from reproducing easily, slowing the viruses that cause dengue, Zika, and chikungunya, a disease that can cause fever and joint pain, now established in the Americas. Hawaii is using these mosquitoes to arrest the spread of avian malaria.

Vaccines and treatments are critical tools for addressing the diseases directly. Many pathogens can be controlled with these measures, but because they more commonly spread in poorer countries, there is less investment in containing them. Many vector-borne diseases like Oropouche are considered “neglected,” and so when they do spread beyond their typical range, there isn’t much available to help those who get sick. But the growing burden of these diseases demands a new generation of tools that can target multiple threats. “We are seeing so many outbreaks that we need broad vaccines,” said Fiocruz’s Sousa.

Additionally, vector-borne diseases aren’t each waiting for their turns. Countries can have multiple outbreaks at the same time on top of all the other health concerns that emerge during severe weather like extreme heat or the healthcare disruptions in the wake of a disaster like a major storm. “We are seeing cumulative threats because we are seeing not just one vector-borne disease,” Sousa said. “In a lot of scenarios, we are also maintaining high levels of communicable diseases.”

Right now, some health departments are being proactive, keeping an eye out for sick travelers, collecting mosquitoes in the wild to see what kinds of germs they’re carrying, and coordinating with researchers across the country.

“We’ve been having bi-weekly meetings with CDC to talk about the potential for Oropouche coming into the US and spreading,” said Bethany Bolling, zoonotic virology group manager at the Texas Department of State Health Services. “We’ve seen in the past that Florida and Texas are some of the primary areas where these new viruses start to establish, so in Texas, we’re trying to be aware of Oropouche and what the vectors are.”

For the US, Brazil’s experience with Oropouche is an important lesson that could help health officials prepare and counter the disease when it inevitably arrives.

“There is a real threat to the United States,” Tilston said. “I think we have all the right settings, and I think it’s just a matter of everything being in the right place at the right time. With climate change, it’s just really a matter of when it’s going to happen.”