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 carrying the oropouche (pronounced or-oh-poosh) virus earlier in August.

The US Centers for Disease Control and Prevention (CDC) says that the majority of those affected have made a full recovery from the insect-borne illness, though three people have had symptoms return.

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

Oropouche virus surveillance and outbreak concerns

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

PAHO officials are particularly worried about the oropouche virus appearing in places where it was not previously thought to be endemic, including Bolivia and Cuba. Public health specialists believe that influences such as climate change, deforestation and unplanned urbanisation may be helping the disease to spread.

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

Sloth fever symptoms and US cases

At least two insects - a mosquito and a midge - can transmit the oropouche virus. It was initially identified in a three-toed sloth, which is why it gained its informal ‘sloth fever’ label. In people, the infection can easily be confused with dengue fever, chikungunya, Zika or malaria. It may cause a sudden fever, intense headache, diarrhoea, nausea, vomiting, chills, muscle pain or joint pain. On rare occasions, the illness can be fatal.

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

The remaining sloth fever case involved a traveller who returned from Cuba to New York state.

“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 provided comparable advice after finding 19 sloth fever cases in travellers returning from South America.

Fatal cases and pregnancy risks

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

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

Experts also fear that the oropouche pathogen may harm fetuses in a way similar to the Zika virus. Antibodies to oropouche were recently detected in South American newborns with microcephaly, a congenital condition that seriously 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 is not yet known whether insects found in other regions can spread the pathogen. However, a recent study found that a midge species present in the US and Canada could transmit oropouche if it became 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 gain a clearer understanding of why the disease is spilling over into humans and what is driving the present outbreak. Because of the possible risks during pregnancy, officials advise pregnant people to postpone travel to areas with outbreaks.

Avoiding mosquito bites while travelling in those locations is essential.