Sloths are vulnerable to the Oropouche virus, which can spread via mosquitoes as well as tiny biting midges. Canva (2); Everyday Health
Key Takeaways
U.S. travelers returning from Cuba have come home infected with Oropouche virus, an illness known as "sloth fever."
The virus causes flu-like symptoms including fever, muscle aches, headaches, rash, diarrhea, and nausea or vomiting.
The virus is usually spread by biting midges, but can also be transmitted by mosquitoes.
No cases have been reported of the virus spreading between humans in the United States.
More than 20 people returning to the United States after traveling in Cuba have been infected with Oropouche virus disease, also known as "sloth fever" because sloths are susceptible to the virus.
The recent influx of cases in travelers returning to the United States prompted the Centers for Disease Control and Prevention to issue a health advisory recommending close surveillance of people returning from areas where the virus spreads, including Cuba, parts of South and Central America, and the Caribbean. The Pan American Health Organization, a regional arm of the World Health Organization, has issued an epidemiological alert for Oropouche virus and raised the public health risk level to "high" for the Americas region. The U.S. travelers with the virus had symptoms that included fever, muscle aches, and headaches, as well as diarrhea, nausea or vomiting, and a rash. A few had recurrent symptoms after the initial sickness, which is common in people with Oropouche virus disease.
Here's what you need to know about the illness and why health officials are concerned.
What Is Oropouche Virus?
"Historically, Oropouche virus has mainly circulated in South and Central America, including the Caribbean, where the first case was detected in Trinidad and Tobago in 1955," says Chantal Vogels, PhD , an assistant professor of epidemiology at Yale School of Public Health in New Haven, Connecticut.
The virus is mostly spread by biting midges, though some mosquitoes can also transmit it. Biting midges can sink their teeth into both humans and animals. The insects are small - less than 1/8 of an inch long - and only the females bite.
The Virus Is Similar to Those Causing Zika and Dengue Fever
Oropouche is very similar to the viruses causing Zika and dengue fever , which are primarily spread by mosquitoes, says Alex Greninger, MD, PhD , professor of laboratory medicine and pathology and director of retrovirology at UW Medicine in Seattle.
"Oropouche is a little different because it's more spread by midges, which is a unique aspect, but many of the symptoms - fever, headaches, nausea and vomiting - are the same," says Dr. Greninger.
Most people don't get severely ill, but in rare cases the disease can cause brain swelling.
Why Health Officials Are Sounding Alarms About Oropouche
While Oropouche is still rare in the United States, the current situation is concerning because of the large number of reported cases in other parts of the world, the spread of Oropouche to new areas, and reports of fatal cases in Brazil, says Dr. Vogels.
Beginning in late 2023, the virus was identified as the cause of large outbreaks in Amazon regions and endemic transmission in new areas in South America and the Caribbean. "Endemic transmission" means significant local spread rather than isolated cases involving travelers returning home.
In addition to 8,000 cases in the Americas, in July 2024 there were two Oropouche-caused deaths in Brazil, the first fatalities ever recorded since the virus was first identified nearly 70 years ago.
Also concerning are reports of the virus passing between mother and fetus, says Vogels.
How Does the Oropouche Virus Spread?
"To understand how the virus is spreading, we need to look at the transmission cycle, which consists of two main parts: the sylvatic-enzootic transmission cycle and the urban-epidemic transmission cycle," explains Vogels.
In the sylvatic cycle, the virus is spread between biting midges or mosquitoes and animal hosts, such as sloths and nonhuman primates, she says.
"The virus can spill over into humans via the bite of an infected insect, which can spark epidemic transmission where the humans are the hosts," says Vogels.
If one of the infected humans or insects travels to a new region, the virus could start spreading there, provided there are mosquitoes or midges to serve as vectors and the climate is suitable, she says.
For example, a person travels to the Amazon region, gets bitten by an infected insect, and then returns to their home country. Once home, if they get bitten by another mosquito or midge while still infected, that insect (which now has the virus) could then bite someone else and spread the disease in that country.
Will Oropouche Virus Start Spreading in the U.S.?
"All cases that have been reported in the United States are travel-related cases and there is currently no evidence for local transmission," says Vogels.
While it is possible that Oropouche could start spreading in areas of the United States where that type of midge is plentiful, the risk for spread in the United States is currently low, she says.
Access to healthcare and an abundance of air-conditioning and screened windows in the United States help further reduce the chances of ongoing transmission from travel-associated cases, adds Vogels.
How Do You Treat or Prevent Sloth Fever?
There are currently no specific treatments or vaccines for Oropouche, says Greninger. A person with the disease can cope with mild symptoms by resting, drinking fluids, and taking over-the-counter pain medicines, he says.
For more severe symptoms, seek medical care right away.
"Without the availability of vaccines or treatment, prevention of insect bites is currently our best strategy to avoid infection," says Vogels.
Use insect repellent. Apply EPA-registered insect repellents that are effective against biting midges and mosquitoes.
Wear protective clothing. Opt for long-sleeved shirts, pants, and socks to minimize skin exposure.
Avoid peak activity times. Mosquitoes are most active during dawn and dusk. Limiting outdoor activities during these times can reduce your risk.
Stay in screened or air-conditioned areas. This helps prevent contact with biting insects.
Reconsider certain kinds of travel if you are pregnant. "It is recommended that pregnant people reconsider nonessential travel to high-risk areas," says Vogels.
"Anyone who experiences symptoms should seek medical care and inform health professionals about their travel history," says Vogels.
Editorial Sources and Fact-Checking
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy . We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Becky Upham has been professionally involved in health and wellness for almost 20 years. She's been a race director, a recruiter for Team in Training for the Leukemia & Lymphoma Society, a salesperson for a major pharmaceutical company, a blogger for Moogfest, a communications manager for Mission Health, a fitness instructor, and a health coach.
She majored in English at the University of North Carolina and has a master's in English writing from Hollins University.
Upham enjoys teaching cycling classes, running, reading fiction, and making playlists.
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