'Sloth fever': What to know about potentially deadly virus affecting US, Europe

Child runs into sloth while zip lining in Costa Rica

A child zooming on a zip line through a rainforest in La Fortuna, Costa Rica collided into a sloth clinging to the cable on March 13. (Go.Adventure Arenal Park via Storyful)

A potentially deadly virus known as "sloth fever" has health officials on alert, following a significant rise in cases across the United States and Europe.

The Centers for Disease Control and Prevention (CDC) issued a critical Health Alert Network advisory after more than 8,000 cases of Oropouche virus, or sloth fever, were reported from January 1 to August 1, 2024, with two deaths and five instances of transmission resulting in fetal death or congenital abnormalities.

In Florida, 20 travel-related cases of the virus were reported, nine of which were reported last week.

According to the CDC, the virus is spread primarily through the bite of infected midges, which are tiny biting insects. Mosquitoes can also carry the virus.

Three-Toed Sloth, Amazon, Brazil, South America (Credit: Hoberman Collection/Universal Images Group via Getty Images)

It is often referred to as "sloth fever" because transmission of the Oropouche virus usually occurs in forested areas between the insects and non-human hosts like sloths, birds and rodents.

Oropouche virus cases

Countries reporting cases include Brazil, Bolivia, Peru, Colombia, and Cuba.

Travel-associated cases have also been confirmed in the United States and Europe, with travelers returning from Cuba and Brazil testing positive for the virus.

The CDC said that reports of cases from additional countries are expected.

What is Oropouche virus?

According to the CDC, Oropouche virus was first detected in 1955 in Trinidad and Tobago and is endemic in the Amazon basin. Previous outbreaks have been described in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Panama, and Peru. One child was infected in Haiti in 2014.

The CDC said the current 2024 outbreak is occurring in endemic areas and new areas outside the Amazon basin. Although travel-associated cases have been identified in the United States, no evidence of local transmission currently exists within the U.S. or its territories.

Humans can become infected while visiting forested areas and are likely responsible for introducing the virus into urban environments.

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Although people exposed to midges or mosquitoes infected with the virus are most at risk for developing disease, the risk factors for more severe Oropouche virus disease are not well-defined.

Earlier this year, Brazil reported two deaths in otherwise healthy non-pregnant women, and five cases in pregnant people with evidence of vertical transmission of the virus to the fetus associated with fetal death or congenital abnormalities.

The CDC issued a warning about the possibility for transmission from a mother to the fetus during pregnancy.

Travelers to areas with Oropouche virus transmission should use prevention measures to avoid midge and mosquito exposure during travel and for three weeks after travel, or if infected during the first week of illness, to mitigate additional spread of the virus and potential importation into unaffected areas in the United States.

Symptoms of ‘sloth fever’ virus

Approximately 60% of people infected with Oropouche virus become symptomatic, according to the health agency. The incubation period is typically 3–10 days.

Initial clinical presentation is similar to diseases caused by dengue, Zika, and chikungunya viruses, with acute onset of fever, chills, headache, myalgia, and arthralgia.

Other symptoms can include eye pain, light sensitivity, nausea, vomiting, diarrhea, fatigue, ash, conjunctival injection and abdominal pain.

Initial symptoms typically resolve after a few days, but a high proportion (about 70%) may experience recurrent symptoms days to weeks after resolution of their initial illness.

Although illness is typically mild, it is estimated less than 5% of patients can develop intense occipital pain, dizziness, confusion, lethargy, photophobia, nausea, vomiting, nuchal rigidity, and nystagmus.

People at risk for more severe disease likely include those at risk for severe disease with other viral infections transmitted by vectors, such as people aged 65 years or older, or those with underlying medical conditions, such as immune suppression, hypertension, diabetes or cardiovascular disease.

No specific treatments or vaccines are currently available for Oropouche virus disease.

Patients who develop more severe symptoms should be hospitalized for close observation and supportive treatment. Pregnant people with laboratory evidence of Oropouche virus infection should be monitored during pregnancy and live-born infants should be carefully evaluated, the CDC noted.