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There have been plenty of reports of potentially concerning viruses this last year. Covid is still causing thousands of deaths, and bird flu appears set to make the jump to human-to-human transmission. Now there are new concerns over Oropouche, a virus largely spread by bites from insects called midges (sometimes called no-see-ums in the US).

There have been outbreaks of the Oropouche virus in Latin America for decades. But this one is different. The virus is being detected in all-new environments. It is turning up in countries that have never seen it before. The spread is being described as “unprecedented.”

It may also be causing more severe disease. People with Oropouche fever typically have a sudden fever, aches and pains, and nausea. Most cases are mild, but some people have developed encephalitis and meningitis. And this year, two otherwise healthy young women who caught the virus have died.

Oropouche can be passed from mother to fetus, and it has been linked to stillbirths and birth anomalies. There are no treatments. There are no vaccines, either. This week, let’s take a look at why Oropouche is spreading, and what we can do about it.

Oropouche virus was first identified in 1955, in a person and a pool of mosquitoes from the village of Vega de Oropouche in Trinidad and Tobago. It was found in a sloth in Brazil in 1960. Since then, there have been over 30 outbreaks—in those countries as well as Peru, Panama, Colombia, French Guiana, and Venezuela. At least 500,000 cases have been reported in South America, largely in areas close to forest.

That’s probably because of the way the virus is transmitted. Oropouche virus is thought to be carried by some populations of sloths, and potentially some nonhuman primates. These animals can host the virus, which can then spread to people via insect bites, usually from midges or some types of mosquitoes.

Since late 2023, outbreaks have been reported in a number of countries in South America, Central America, and the Caribbean, including Cuba, a first for the country. 

There has been an especially large surge of cases in Brazil. Since the beginning of this year, 10,275 cases of Oropouche have been confirmed in the Americas, according to a situation summary report published by the Pan American Health Organization (PAHO) earlier this week. And 8,258 of them were in Brazil. Travelers have also imported cases to the US and Europe for the first time—90 such cases have been reported in the US, and 30 in Europe.

Another change is that this time around, the virus has been infecting people in urban settings far from forests. It is not entirely clear why, but there are probably a few reasons. Climate change, for a start, has led to increased temperatures and rainfall, both of which can help create breeding grounds for the insects that transmit the virus. And deforestation and urbanization, both of which have caused people to encroach on the habitats of wild animals, have also raised the risk of transmission to people, says Ana Pereiro do Vale, a veterinarian and microbiologist at University College Dublin in Ireland.

The virus itself also appears to have changed, according to new research published this week. William de Souza at the University of Kentucky and his colleagues analyzed blood samples taken from people with an Oropouche diagnosis between 2015 and 2024, enabling them to compare the form of the virus that is currently circulating with a historical strain.

The team found evidence that the virus has swapped genetic material with a related one, creating a new “virus reassortment.” It is this new form of the virus that has spread since the end of 2023, the team says.

That’s not all. The genetic changes have endowed the virus with new features. The current reassortment appears to be better at replicating in mammalian cells. That might mean that infected people—and sloths—have more of the virus in their blood, making it easier for biting insects to pick it up and pass it on.

The new form of the virus also seems to be more virulent. The team’s lab tests suggest that compared with the historical strain, it appears to cause more damage to the cells it infects.

We are still getting to grips with how the virus can spread, too. We know midges and mosquitoes are responsible for spreading Oropouche, but the virus can also pass to a fetus during pregnancy, with potentially harmful consequences. According to the PAHO report, Brazil has reported “13 fetal deaths, three spontaneous miscarriages, and four cases of birth anomalies” linked to Oropouche infections.

In a separate study published earlier this week, Raimunda do Socorro da Silva Azevedo at the Evandro Chagas Institute in Ananindeua, Brazil, and her colleagues assessed 65 unexplained cases of microcephaly—a birth anomaly in which babies have an unexpectedly small head—that had been recorded in Brazil between 2015 and 2024. The team found evidence of an Oropouche infection in six of the babies—and in all three that had been born in 2024.

It’s still not clear whether or how the virus might affect fetuses and babies, and research is ongoing. But the US Centers for Disease Control and Prevention (CDC) recommends that pregnant travelers “reconsider non-essential travel” to Cuba. 

Some scientists worry that the virus might also spread via sex. In August, a 42-year-old Italian man who fell ill after returning from a trip to Cuba was found to have Oropouche virus in his semen. And it was still there 58 days later. The CDC currently recommends that men diagnosed with Oropouche should use condoms or not have sex for at least six weeks from the start of their symptoms. They should avoid donating semen, too, according to the organization.

There are a lot of unanswered questions when it comes to Oropouche. Some scientists have suggested that this is because outbreaks have historically been seen in poorer countries in the Global South.

“There is sufficient colonialism in disease research—if it doesn’t affect the industrial world and Western business interests, it’s not important,” Shahid Jameel, a virologist at the University of Oxford, told Gavi, an organization focused on global vaccination efforts. “Now that the virus has been found in Cuba—not far from Miami—the wheels of public health will turn.”

Let’s hope they get in gear quickly. As Vale says: “We don’t know what will happen with the virus, the mutation rate of the virus, or if the virus will jump to another host. We need to be careful and pay attention.”

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Oropouche infections can look similar to dengue—another viral disease, also spread by mosquitoes, that affects people in Brazil. The country is attempting to tackle the problem with bacteria-infected mosquitoes, Cassandra Willyard reported in March.

The spread of bird flu in dairy cattle in the US has virologists worried. The virus could stick around on US farms forever and is raising the risk of outbreaks in mammals—including humans—around the world.

Flu season is officially upon those of us in the Northern Hemisphere. This year, it could enable the creation of an all-new bird flu, too. 

Could gene editing help curb the spread of bird flu? Abdullahi Tsanni explored the possibility of using CRISPR to make chickens resistant to the virus.

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