Sloth fever, also known as the Oropouche virus, is an emerging virus that was reported in the United States in 2024. The virus is spread by infected biting midges and mosquitos.
Common symptoms include a fever, headaches, chills, and muscle and joint pain. Americans should prevent insect bites while traveling, and pregnant people may want to defer travel to areas experiencing outbreaks of sloth fever.
The Oropouche virus — also called sloth fever — was identified in late 2023 as the cause of large outbreaks in the Amazon region. The virus was then found in more areas of South America and in the Caribbean. Between January and August 2024, more than 8,000 cases were reported.
First detected in 1955 in Trinidad and Tobago, sloth fever is endemic in the Amazon basin, meaning it's common in that region. The current outbreak is happening in both endemic areas of the Amazon basin and in other countries, including Brazil, Bolivia, Colombia, Cuba, and Peru.
In summer 2024, travel-associated cases of Oropouche virus were found in Europeans returning from Brazil and Cuba. In August 2024, 21 sloth fever cases were found in U.S. travelers returning from Cuba.
Although infection usually causes a fever that resolves on its own, two people have died from the virus. In addition, concerns have been raised for pregnant people as five cases of fetal death or congenital abnormalities have been reported from sloth fever.
The virus is thought to involve certain vertebrate hosts, such as birds, nonhuman primates, and sloths. Biting midges and possibly mosquitoes sting the infected vertebrates and then sting humans, passing the virus on through the bite.
There's no documented evidence that sloth fever is spread through direct contact with infected people. It's spread through the bites of small biting flies called midges and certain mosquitoes.
Risk factors for contracting sloth fever include:
The most serious complication of sloth fever is death. Two nonpregnant women died from their illnesses in the Brazil exposure. Other serious pregnancy complications were found in five cases, including fetal death and congenital abnormalities like microcephaly.
These deaths and associated adverse birth outcomes were the first reported serious complications of Oropouche virus.
You can reduce your risk of contracting Oropouche virus by taking precautions while traveling in South America and the Caribbean.
Protect yourself from insect bites by:
If you're pregnant, insect repellents containing DEET 50% are recommended at any stage of pregnancy if traveling to an outbreak area.
Oropouche virus symptoms are similar to those of the chikungunya, dengue, and zika viruses. The incubation period is typically between three and 10 days.
Although illness is usually mild, people may experience a sudden onset of symptoms. Symptoms and their prevalence in people who get sloth fever include:
Sloth fever virus symptoms may also include light sensitivity, chills, bloodshot eyes, and rarely neuroinvasive issues, such as meningitis and meningoencephalitis.
In addition, a recurrence of symptoms after the initial illness is common with sloth fever.
Sloth fever is diagnosed by testing blood serum in the pathology laboratory. People with symptoms of neuroinvasive disease may receive a spinal puncture to have their cerebrospinal fluid (CSF) tested.
These tests can be done at some public health labs and at the Centers for Disease Control and Prevention (CDC). Both are working to develop new diagnostic tests for Oropouche virus to limit its spread and create a U.S. response plan.
The goal of Oropouche virus treatment is to manage the symptoms. No specific vaccines exist to prevent sloth fever, and no medications exist to treat it.
The CDC is providing consultation and guidance to pregnant people and their health care providers. They're also tracking this and other emerging health threats to determine their impact on pregnant people and their newborns.
Of the 21 Oropouche virus cases reported among U.S. travelers returning from Cuba in August 2024, most recovered with limited illness. At least three people experienced recurrent symptoms after the initial illness had resolved.
Last reviewed by Christiane Hadi, MD on 2024-01-07.
