Sloth Fever (Oropouche) Virus Picks Up Speed

“Arboviral infections have hit South America heavily in the past decade…[T]he region is now facing the re-emergence of another little known arbovirus, Oropouche virus, on an unprecedented scale.”

“Oropouche fever, the mysterious threat,” The Lancet (Editorial), August 8, 2024

“Once considered a little-known pathogen largely confined to the Amazon region, the Oropouche virus has emerged as a growing public health threat across Latin America.”           

David Garcia Perez, Medscape News Global, July 24, 2026

You may have read about Oropouche (pronounced “oro-poosh”) virus, aka “sloth fever” virus (the three-toed sloth is the primary reservoir), in a previous Germ Gems post and forgotten all about this mysterious arbovirus. (Peterson, P., “Oropouche (‘Sloth Fever’) Virus Hits the U.S.: What You Need to Know,” Germ Gems, September 11, 2024). Unfortunately, this extremely nasty virus hasn’t forgotten about us.

Historically endemic to parts of South America, since late 2023, Oropouche virus fever has had a marked geographic expansion beyond the Amazon basin into other regions of South America and the Caribbean. And, although it is not endemic in either Europe or the U.S., “its rapid spread across Latin American has increased the likelihood of travel associated infections.”  (Perez, D.G., “Why Oropouche Virus is Gaining Global Attention,” Medscape News Global, July 24, 2026). Thus, I thought another post on Oropouche virus fever might keep you on your toes should you be planning a trip to an area where “sloth fever” is endemic.

A recap on Oropouche virus. Discovered in 1955 in the Caribbean nation of Trinidad and Tobago, Oropouche virus is a single-stranded RNA virus that belongs to the Peribunyaviridae family of arboviruses—a diverse group of viruses that are transmitted to susceptible hosts through the bite of hematophagous arthropods, i.e., mosquitoes, ticks, sandflies and midges. It causes Oropouche fever. A midge, Culicoides paraensis, is the primary vector for Oropouche virus. (Midges, aka  “no-see-ums,” are a group of very tiny insects—about the size of the period at the end of this sentence.)

Oropouche virus infection is a zoonotic infection. Monkeys, various marsupials and marmosets are primary hosts in the Amazon basin, and certain species of wild forest rodents and birds also sustain its cycle in nature. But, the three-toed sloth is its primary reservoir. (Even though the virus doesn’t cause illness in sloths, Oropouche virus fever has become popularized as “sloth fever.”)

Clinical picture. Symptoms of Oropouche virus infection include sudden onset of fever, severe headache, chills, muscle and joint pain, pain behind the eyes, nausea and vomiting. As there is a considerable overlap of symptoms with other arbovirus infections including dengue, West Nile fever, Zika, chikungunya and yellow fever, the best diagnostic test is a Reverse Transcription-Polymerase Chain Reaction of a blood sample.

Most cases of sloth fever are mild and the illness resolves after seven to eight days. But in minority of cases, the illness may evolve into more severe forms with neurological or hemorrhagic manifestations.

Treatment and Prevention. Antiviral options for Oropouche virus infections are currently unavailable. Similarly, no licensed vaccines exist.  Medical care relies entirely on relieving symptoms.

First-line pain relief with acetaminophen (Tylenol) can safely manage fever, headaches, and muscle pains. But official warnings are in effect against taking NSAIDS, such as aspirin and ibuprofen, until a doctor has ruled out dengue fever as NSAIDS can significantly increase the risk of severe, life-threatening bleeding if you have dengue.

Prevention depends on personal protection against insects (midges). Liberal use of insect repellents containing DEET, picardin, or oil of lemon eucalyptus is recommended as is wearing long-sleeved shirts and long pants. Since midges can bite through thin fabrics, treating outdoor clothing with permethrin is also recommended.

“Sloth fever” on the move. Since the discovery of Oropouche virus in 1955, 500,000 cases of Oropouche fever have been recorded overall. Since 2023, however, outbreaks have increased in frequency within endemic regions and expanded into previously unaffected areas. (Subissi, J., et al., “Oropouche virus: transmission, epidemiology, genetic diversity, and public health implications,” EClinicalMedicine, April 2026). “As of September 2025, cases have been reported in all 27 Brazilian federal units, and in Barbados, Bolivia, Colombia, Cuba, Ecuador, Guyana, Panama, Peru and Venezuela, with travel-associated cases in Italy, Spain, Germany, France, The Netherlands, the United States, Chile, Uruguay and Canada.” (In the U.S., a total of 110 cases of travel-associated Oropouche virus disease have been reported.)

Why is this virus spreading so quickly? A difficult-to-control vector together with “a virus with enhanced transmission potential have facilitated the rapid spread.” (Perez, D.G., “Why Oropoche Virus Is Gaining Global Attention,” Medscape News Global, July 24, 2026).

The Oropouche virus is transmitted mainly by the biting midge Culicoides paraenis. This midge breeds in moist soil rich in organic matter; it thrives in rural, forested and densely vegetated environments. “Higher temperatures and increased rainfall have created favorable conditions for this vector helping to explain why the disease is much more common in rural areas than in urban areas.”  C. paraenis is also very small—about one-third the size of a typical mosquito— making it less susceptible to conventional vector control measures such a fogging, eliminating standing water, and using mosquito nets.

In addition, Oropouche virus has a segmented RNA genome allowing it to easily swap pieces with other viruses, creating new variants with unknown traits. Researchers recently reported the emergence of a new viral variant that appears to replicate more efficiently and partially evade immunity acquired through previous infection.

The rapid surge of Oropouche fever in South America has made evident the importance of increased awareness and enhanced surveillance for the population living in endemic areas and for travelers. (“Opouche fever, the mysterious threat,” The Lancet (Editorial), August 8, 2024).  And, the Oropouche virus demonstrates yet again that a virus initially endemic to certain areas can rapidly become an emerging threat to global health even if its nicknamed “sloth fever” virus.