Public health experts in South America and the Caribbean have raised the alarm as a rapidly spreading, little-known viral disease with no vaccine or treatment is gaining ground in the region. The disease, Oropouche fever, also known as sloth fever, is transmitted by bites from midges and mosquitoes and was first identified in 1955 in a forest worker in Trinidad and Tobago.
The disease is endemic in the Amazon basin and has recently been observed to be spreading rapidly to non-endemic regions. Before December 2023, most Oropouche cases were concentrated in South America and the Caribbean regions near the Amazon basin. By April 2025, over 20,000 cases of Oropouche had been reported across the region, according to the German Center for Infection Research (DZIF), which is twice the number of cases reported in 2024.
The ongoing Oropouche outbreak is the largest ever recorded, with scientists still unable to explain why it is spreading so widely and so quickly. An April 2025, study by The Lancet noted that the disease was of “unprecedented magnitude and spread” and called for urgent investigation.
Brazil badly hit
Brazil appears to be the epicenter of the disease. In 2023, the country reported 832 cases, with these being concentrated in the Amazon Basin. By August 2024, this number had surged to 7,284, out of a regional total of 8,078. In the first four weeks of 2025, Brazil had 3,678 cases of the 3,765 confirmed in Brazil, Canada, Cuba, Guyana, Panama and Peru, according to the Pan American Health Organization (PAHO).
The first-ever recorded deaths from Oropouche also occurred in Brazil. In July 2024, two previously healthy women aged 21 and 24 died in Bahia State which is thousands of kilometers away from Amazon.
“The virus, which used to circulate almost exclusively in the Amazon region, is now circulating in other regions, and this is very worrying,” Eurico Arruda, a Professor of Virology at the University of Sao Paulo School of Medicine, told SciDevNet.
Possible causes
Experts cite a combination of human and environmental factors behind the resurgence of the virus. According to PAHO, climate change has increased rainfall and temperatures, while deforestation and urbanization have changed the natural habitats of hosts and vectors by encouraging interactions between them and thus increasing the risk of transmission. Researchers have also discovered the virus can thrive in urban and forest locations thereby giving it the ability to spread to new regions. Global warming could also be expanding vector habitats to previously unaffected regions in other countries.
The aggressive re-emergence of Oropouche is also being blamed on mutation which makes the current strain replicate more efficiently in the cells of those infected. The 2023-2024 Oropouche outbreak replicated 100-fold higher in mammalian cells compared to the prototype strain, according to medical research.
Professor Arruda, who has studied Oropouche for three decades, speculated that the virus could have become deadlier when two of its strains infect the same cell.
“If two viruses infect the same cell it can result in a different virus. This apparently happened, which caused a change in its behaviour, making it more aggressive,” he told SciDevNet.
Symptoms
Oropouche symptoms are almost identical to those of Zika virus, Dengue fever and Chikungunya, which can lead to misdiagnoses. The symptoms appear four to eight days after an infectious bite and include a rapid fever, severe headache and weakness, joint and muscle pain, nausea, diarrhoea, chills and recurring vomiting. Although most patients recover within a week, some face a lengthy convalescence period, and about 60% of patients face a relapse weeks after recovery. In some cases, rare but serious complications occur such as aseptic meningitis during the second week of illness.
Oropouche cannot be vaccinated against or treated so medics advise the infected to relieve the symptoms by resting, keeping hydrated and taking pain and fever medication. Patients have to be closely monitored for complications such as low blood pressure, severe sweating which can lead to rapid dehydration, and nervous system complications such as meningitis, meningoencephalitis, and Guillain-Barré syndrome.
Warnings and precaution measures
The Centers for Disease Control and Prevention has issued travel alerts, particularly for visiting pregnant women, as there is evidence that Oropouche may lead to birth defects, stillbirths, and fetal deaths. While the virus has been detected in semen, sexual transmission has not yet been confirmed. Experts advise travelers to protect themselves against insect bites. Standard mosquito nets may not be helpful as the midges and mosquitoes that transmit Oropouche through bites are only up to 3 millimeters in size and can pass through these so therefore mosquito nets with fine mesh are advised.
“When it comes to protecting against the Oropouche virus, and also against other tropical viruses like Dengue and Zika, it’s (also) worth wearing long clothes and using insect repellent, such as DEET or Icaridin,” advised Professor Jan Felix Drexler, head of the Virus Epidemiology laboratory at the Institute of Virology at Charité Germany in an article published by DZIF.
