US Reports First Sloth Fever Cases After 21 Travellers Return From Cuba

The United States has recorded its first cases of 'sloth fever' after 21 travellers returned from Cuba infected with the oropouche (pronounced or-oh-poosh) virus earlier in August.

Officials at the US Centers for Disease Control and Prevention (CDC) say most patients have made a full recovery from the insect-borne illness, though three have developed recurring symptoms.

There is currently no evidence that the oropouche virus can pass directly between people. The pathogen is believed to circulate in tropical insects in Central and South America, which infect forest-dwelling animals – mainly sloths, as well as certain primates, rodents and birds.

Oropouche virus cases and the risk of spread

Although the likelihood of local transmission in the US is considered low, the Pan American Health Organization (PAHO) has recently issued an advisory calling on member states to reinforce disease surveillance. Cases are rising for unexplained reasons across Central and South America and the Caribbean.

PAHO officials have expressed particular concern about the oropouche virus appearing in places where it was not formerly considered endemic, including Bolivia and Cuba. Public health specialists believe climate change, deforestation and unplanned urbanisation may be helping the disease to spread.

No medicines are currently available to treat or prevent the ongoing outbreak. By July, the World Health Organization had recorded more than 8,000 cases across Bolivia, Brazil, Colombia, Cuba and Peru.

The oropouche virus is transmitted by at least two insects – a mosquito and a midge – and was first identified in a three-toed sloth, giving rise to its informal name. In people, the infection can readily be confused with dengue fever, chikungunya, Zika or malaria. Symptoms may include sudden fever, intense headache, diarrhoea, nausea, vomiting, chills, muscle pain or joint pain. In uncommon cases, the illness can be fatal.

Sloth fever cases among travellers from Cuba

The Florida Department of Health (FLDOH) identified 20 of the travellers who returned to the US with sloth fever after all members of the group tested negative for dengue. They had recently travelled to Cuba, where the oropouche virus has been circulating since June.

The remaining sloth fever case was found in a traveller who had returned to New York state from Cuba.

"Oropouche and dengue viruses can co-circulate and cause similar symptoms" explain officials at the CDC; "patients with clinically suspected dengue should be managed according to dengue clinical management recommendations until dengue is ruled out."

European health authorities recently released comparable advice after detecting 19 sloth fever cases in travellers returning from South America.

Oropouche virus deaths and pregnancy concerns

This is not the first oropouche virus outbreak to affect South and Central America. Since it was first identified in the Caribbean in 1955, about half a million people have been infected following bites from the midge species Culicoides paraensis or a mosquito known as Culex quinquefasciatus. Both species occur in tropical areas.

However, this is the first occasion on which human deaths have been linked to the virus. Sloth fever has so far killed a 21-year-old woman and a 24-year-old woman in Brazil.

Experts are also concerned that the oropouche pathogen may have a fatal effect on fetuses, similar to the Zika virus. Antibodies to oropouche were recently detected in newborns in South America with microcephaly, a congenital condition that severely affects brain development.

"Acknowledging that these observations are still in the initial stages of investigation and the true trajectory is unknown, the risk level for the region has been upgraded to high," PAHO warned on 3 August 2024.

It remains unknown whether insects found in other regions can spread the pathogen. However, a recent study found evidence that a midge species in the US and Canada can transmit oropouche if it becomes infected.

This may provide a pathway for the disease to begin spreading on a new continent.

The CDC says it is working with partners including PAHO to understand more clearly how the disease is spilling over into humans and why an outbreak is now occurring. Potential pregnancy-related risks have led officials to advise pregnant people to postpone travel to areas affected by outbreaks.

Avoiding mosquito bites while travelling in these areas is essential.