The illness nicknamed “sloth fever” is caused by the Oropouche virus (OROV), a mosquito-like virus transmitted mainly by the tiny biting midge Culicoides paraensis, also called “no-see-ums.” First identified in 1955 in the Amazon, it was largely confined to rainforest regions where midges spread the virus among primates, birds, and mammals like pale-throated sloths—hence the nickname.
Until recently, Oropouche caused only a few hundred cases annually, but new strains emerged around 2023. These mutated versions appear better at infecting humans and evading prior immunity. Since then, the virus has spread far outside the Amazon, with 12,000 confirmed cases across 11 countries in 2025 and the first-ever cases reported in the UK. Deaths remain rare but have included healthy young adults, as well as reports of fetal deaths and birth defects linked to maternal infection in Brazil.
How it spreads
- Primary vector: biting midges (“no-see-ums”), which are extremely small and leave tiny itchy welts.
- Possible secondary vectors: mosquitoes are being studied, but their role appears limited.
- Human travel also spreads the virus; people infected abroad can return home without symptoms and spark local cycles if midges are present.
Why it’s spreading
- Viral mutations making it more infectious.
- Deforestation, pushing animal–insect cycles into closer human contact.
- Climate change, expanding midge habitats through warmer, wetter environments.
- Human mobility, carrying the virus across continents.
Who is at risk?
Most U.S. cases reported so far have been tied to travel, especially from Cuban outbreaks, with Florida seeing the highest share due to its humid climate. Although Florida midges could theoretically spread Oropouche, sustained local transmission hasn’t been documented yet.
Symptoms
- Fever
- Chills
- Headaches
- Muscle and joint pain
Some cases progress to neurological conditions such as meningitis or encephalitis. Fatalities are rare, but confirmed.
Treatment & Prevention
There is no vaccine or antiviral treatment for Oropouche virus. Management is supportive: rest, fluids, and over-the-counter fever reducers.
Preventive steps:
- Wear long sleeves and pants.
- Use EPA-registered insect repellents.
- Use insect nets — but because midges are smaller than mosquitoes, not all nets provide protection.
Key takeaway: Oropouche, or “sloth fever,” is no longer confined to the Amazon and is spreading quickly across the Americas and beyond. While still rare in many parts of the world, scientists warn the combination of climate change, travel, and viral mutation could make it a growing vector-borne threat.
Would you like a clear comparison chart between Oropouche (“sloth fever”) and better-known viruses like dengue and Zika to understand how worried we should be about this new spread?
