From UF/IFAS
In Summary:
Oropouche virus, often called “sloth fever,” drew public health concern after infected travelers returning from Cuba were identified in Florida in 2024. Because the insect typically responsible for spreading the virus is not found in Cuba, researchers worried mosquitoes could act as alternative vectors, increasing the risk of local transmission. A new University of Florida study funded by the CDC found that the most common disease-spreading mosquitoes in the Southeast are unlikely to transmit the virus, significantly reducing the risk of widespread outbreaks.
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In 2024, “sloth fever” hit the news cycle as a new public health threat: The virus was found in people who’d recently traveled to Cuba, and the disease's serious health risks prompted concern that it could spread and become established in the U.S.
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Sloth fever is the popularized name for the illness caused by the Oropouche virus, which is endemic in countries such as Brazil, Bolivia, Colombia, Cuba, and Peru. It is primarily spread from person to person by a species of no-see-um. In 2024, about 105 travel-related cases of Oropouche virus infection were spotted in the U.S. – about 103 of which arrived in Florida, according to the U.S. Centers for Disease Control and Prevention. The infected people had recently traveled to Cuba. Because the species of no-see-um that transmits Oropouche virus had never been found in Cuba, researchers suspected that mosquitoes were transmitting the virus from person to person. If so, the virus would be much more likely to establish itself in Florida and elsewhere in the U.S.
In general, the Oropouche virus causes fever, chills, headache, muscle and joint aches, and a bumpy rash. But the biggest concern is that it could cause fetal birth defects, both cognitive and developmental, if a pregnant woman is infected, harkening back to concerns like that of the Zika outbreak in 2016-2017.
But researchers have good news: The main two types of disease-spreading mosquitoes in the Southeastern U.S. are incredibly bad at spreading the Oropouche virus, so even if travel cases arrive, and a mosquito bites that person, it is unlikely the virus will spread to the next person bitten, said Barry Alto, associate professor at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS). The research was funded by a grant from the CDC and was published last week in the peer-reviewed journal PLOS Neglected Tropical Diseases.
Dongmin Kim, research scientist at UF/IFAS Florida Medical Entomology Laboratory (FMEL), said the findings provide important reassurance because Oropouche virus has been linked to possible negative outcomes in pregnancy, and limited emerging evidence suggests the possibility of sexual transmission.
“We were concerned about an outbreak because the disease, like Zika, can cause serious birth defects,” he said. “It could have been really serious.”
The study looked at Aedes aegypti, also called the “yellow fever mosquito,” and Culex quinquefasciatus, the “southern house mosquito,” which are the most common disease-spreading mosquitoes in the South.
Aedes aegypti are well-known for spreading other pathogens associated with disease in the U.S. and internationally, such as dengue, Zika, yellow fever, and chikungunya. The southern house mosquito spreads West Nile virus, St. Louis encephalitis virus, and Western equine encephalitis virus.
“These are the ones that really bite humans regularly,” Alto said.
As the researchers completed their large infection study, they tested over 2,000 mosquitoes to determine how well each spread the Oropouche virus. They found that few had the infection inside them at all, and even fewer had the virus in their saliva, so they would be unable to spread the virus to the next person they bit, said Nathan Burkett-Cadena, associate professor at UF/IFAS FMEL.
“We feel it’s very unlikely the virus will become established and transmitted locally,” he said.
The next steps for this research will be to investigate whether no-see-ums in the U.S., also called biting midges, can transmit the Oropouche virus and whether mosquitoes can transmit it to their offspring.
Key Points:
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About 105 travel-related Oropouche virus cases were identified in the U.S. in 2024, with most reported in Florida.
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The virus can cause fever, headache, muscle aches, rash, and may pose risks during pregnancy.
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UF researchers studied Aedes aegypti and Culex quinquefasciatus, the most common disease-carrying mosquitoes in the South.
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Testing of more than 2,000 mosquitoes showed very limited infection and almost no ability to spread the virus.
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Scientists say it is very unlikely the virus will become established or spread locally in the U.S.
