Oropouche virus 101: A guide to a little-known virus that's raising concern this year

The medical journal The Lancet calls it a “mysterious” and “emerging” threat.

PAHO, the Pan American Health Organization, has issued an epidemiological alert urging for increased prevention, surveillance and diagnosis.

The subject of these alerts is the Oropouche virus, named after a village in Trinidad where it was first identified in 1955. Like malaria, dengue and Zika, it’s spread by insect bites – mosquitoes as well as midges, biting flies. The symptoms are typical of many viruses and usually last a week: fever, rash, muscle aches, headache.

Here’s what we know about the virus (pronounced o-ro-push) – and why The Lancet uses those two concerning adjectives.

How do humans catch Oropouche? 

Oropouche is harbored in birds, monkeys, rodents and sloths – in fact it’s sometimes called “sloth fever.” It’s an arbovirus like dengue, Zika and Chikungunya – the term refers to any disease caused by insect bites. If a mosquito or a midge – a tiny biting fly – bites an infected critter and then goes for a blood meal from a human, the virus can take root and cause illness.

Where is it “emerging”?

Case counts used to be fairly low and confined to the Amazon Basin – the area around the river, with reports of Oropouche in Brazil, Panama and Peru. But in 2024, case counts, while still dwarfed by viruses like dengue, are definitely up. In Brazil, the number of Oropouche cases has jumped from 832 in 2023 to 7,284 so far in 2024.

And for the first time, there have been reports of Oropouche cases in Bolivia (with 356 cases) and Cuba (with 74 confirmed cases so far). In addition, travelers infected with the virus have been identified in Germany, Italy and Spain, although they are thought to have contracted the virus in Cuba. No local transmission in Europe has occurred.

Why is it popping up in new places? 

Climate change seems to be a key culprit, as heavier rainy seasons have led to more mosquitoes and midges. In addition, record rates of felling trees in the Amazon forest have displaced the usual animal targets of mosquitoes and midges, so they’re hungry for humans.

“If there are no monkeys, for example, [the midges] will find another animal to bite,” says Dr. Felipe Gomes Naveca, a public health researcher at the Oswaldo Cruz Foundation (Fiocruz) who works with emergent and re-emerging viruses.

And what makes it mysterious?

Simply put: There’s a lot that researchers don’t know.

There had been no reports of deaths from Oropouche in the last 69 years. But in 2024, two deaths of healthy young women have been confirmed in Brazil with a third death under investigation.

What’s more, there are 8 suspected cases of vertical (mother to fetus) transmissions. Four of the infected babies were born with congenital defects; the other four died.

“Although the disease has historically been described as mild, the geographic spread in transmission and the detection of more severe cases underscore the need for increased surveillance and characterization of possible more severe manifestations,” read PAHO’s statement on August 2.

More severe cases can cause encephalitis and meningitis — inflammation of the brain and its surrounding membrane that can cause it to swell.

How worried should we be?

NPR posed that question to Dr. Tulio de Oliveira, the director for the Center of Epidemic Response and Innovation at Stellenbosch University. He was in Brazil last week talking to public health officials and epidemiologists about Oropouche.

“The Brazilian public health officials and scientists are, in my humble opinion, the best ones in the world to deal with an arbovirus outbreak,” de Oliveira said. “I didn't see any panic. I saw the opposite. I saw a lot of action happen on the ground to try to understand and try to control these outbreaks so it doesn’t become widespread.”

But other countries that aren’t as used to dealing with a virus like Oropouche may have more difficulty curbing infections. Tulio de Oliveria says there is a potential for this virus to spread around the world.

The death of the two women in Brazil also gives pause to public health researcher Dr. Naveca. He adds that there may be other cases of more severe symptoms from Oropouche that were perhaps attributed to dengue fever.

And how do you know which virus you have? PCR tests for Oropouche are now widely available throughout Brazil, although that wasn’t the case earlier in the year. Naveca's additional concern: There is no vaccine for this virus, and creating one might be challenging because — like the flu — this virus is made up of several parts that can shift. If they do shift, it’s harder to target the virus with a shot.

Dr. de Oliveria, who led the team that first detected the omicron variant of COVID-19 in South Africa, said one big question is the interaction between Oropouche and other viral infections, such as dengue and Zika.

“The answer is that we do not know,” he says, “but what we do know is that many of these pathogens cause the worst clinical outcomes when they come in succession.”