In 1955, a 24-year-old who worked in the forests of Trinidad and Tobago, near the Oropouche River, became the first human on record to contract the fever-inducing ‘sloth virus’ or Oropouche virus.

Erem O, Biernath A, Gray R. Oropouche: The mysterious ‘sloth virus’ with no treatment. BBC. August 29, 2024.

Centers for Disease Control and Prevention. Clinical Overview of Oropouche Virus Disease. August 22, 2024.

While the virus is endemic to the Amazon basin, it is now spreading globally.

Centers for Disease Control and Prevention. Clinical Overview of Oropouche Virus Disease. August 22, 2024.

It’s worth noting that the moniker ‘sloth fever’ refers to the fact that the virus was initially found in a sloth, leading scientists to believe that the sloth played a key role in its spread between insects and animals.

Kekatos M. What you need to know about ‘sloth fever’ after 21 cases confirmed among US travelers. ABC News. August 29, 2024.

Suliman A. What to know about ‘sloth fever’ as U.S., Europe warn of imported cases. The Washington Post. August 28, 2024.

Over the years, outbreaks of the arthropod-borne virus have been seen in both humans and animals in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Haiti, Panama, and Peru.

Suliman A. What to know about ‘sloth fever’ as U.S., Europe warn of imported cases. The Washington Post. August 28, 2024.

Beginning in late 2023, large outbreaks occurred, impacting previously affected and new areas in South America and the Caribbean.

Centers for Disease Control and Prevention. Oropouche Virus Disease Among U.S. Travelers – United States, 2024. September 5, 2024.

Understanding the movement of the virus is tricky, however. Brian Labus, PhD, MPH, REHS, Assistant Professor in the Department of Epidemiology and Biostatistics at University of Nevada’sSchool of Public Health, tells MDLinx, “It's hard to say if it has spread to new areas or if we are just doing a better job looking for it. Oropouche virus has been overshadowed by other viruses that cause similar diseases, like dengue and chikungunya, that circulate in the same places—and our surveillance for it hasn't been very robust. Cases have been reported in the United States and Europe, but these have all been in people who were infected while traveling to endemic regions.”

Globally, the virus is thought to have affected over 500,000 people.

Zhang Y, Liu X, Wu Z, et al. Oropouche virus: A neglected global arboviral threat. Virus Research. 2024;341.

Case numbers are expected to rise in the coming years due to climate change and migration patterns of both humans and animals. Chad L. Cross, PhD, MFT, PStat(R), of the PARAVEC Laboratory at the University of Nevada’s School of Public Health, tells MDLinx that because of the virus’s relatively recent discovery and its episodic behavior, it’s considered both a neglected tropical disease and an emerging infectious disease. 

Patients are infected by the bite of a mosquito or midge (small fly) carrying the virus, not patient-to-patient transmission, Dr. Cross says. “If a human becomes infected and develops viral disease and then comes back to an urban area, then the urban cycle allows ready spread through vector feeding among humans,” he explains. For this reason, he says, anything that brings hosts, vectors, and pathogens together—like deforestation, wildfires, or long droughts—can bring about an epidemic. 

Oropouche virus symptoms

Dr. Labus says that the symptomatology of Oropouche virus looks like that of many other viruses. “About 60% of infected people develop symptoms, most typically fever, fatigue, joint and muscle pain, a rash, and vomiting or diarrhea,” he says. “These symptoms last for less than a week, although they can reoccur weeks later. About one in 20 people progress to a more serious disease that affects the nervous system, but death from the disease is very rare.” 

Researchers have also noted that there have been reports of patients with hemorrhagic symptoms or neurological complications associated with the virus.

Zhang Y, Liu X, Wu Z, et al. Oropouche virus: A neglected global arboviral threat. Virus Research. 2024;341.

The Centers for Disease Control and Prevention (CDC) says that sickness is usually self-limited, but there have been reports of two deaths in patients with Oropouche virus infection.

Centers for Disease Control and Prevention. Oropouche Virus Disease Among U.S. Travelers – United States, 2024. September 5, 2024.

More so, there are concerns over vertical transmission or transmission between mother and baby in utero.

Dr. Labus says that physicians can tell the Oropouche virus apart from other viruses by using a patient’s travel history as a diagnostic tool. “Oropouche can be considered in symptomatic patients who were in the areas with increased Oropouche virus activity—currently Brazil, Bolivia, Peru, Colombia, and Cuba—within two weeks of the onset of initial symptoms,” he says. But the disease does look very similar to other vector-borne diseases, like dengue, chikungunya, and zika, so laboratory testing will help differentiate. “The CDC recommends ruling out dengue virus infection as the two viruses circulate in the same areas,” Dr. Labus adds. 

Dr. Cross notes that blood serum or cerebrospinal fluid can be tested to confirm infection.