In 2024, public health officials were on high alert as a wave of “sloth fever” cases appeared in travelers returning to the United States from Cuba. With the vast majority of the nation’s 105 cases recorded in Florida, fears mounted that the virus could establish a permanent foothold in the Southeast and threaten pregnancies much like the Zika virus did nearly a decade ago.
However, a new study released last week by the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) offers a significant sigh of relief: The region’s most aggressive mosquitoes appear unable to spread the disease effectively.
Sloth fever, scientifically known as Oropouche virus, is typically transmitted by a specific species of “no-see-um” (biting midge) endemic to countries including Brazil, Bolivia, Colombia, and Peru.
When travelers began returning from Cuba with the virus last year, researchers grew concerned because the specific midge known to carry the virus had never been found on the island. This led to a troubling hypothesis: local mosquitoes might be transmitting the virus from person to person.
If mosquitoes were indeed capable vectors, the risk of the virus becoming established in Florida—and spreading across the U.S.—would have skyrocketed.
To test this, researchers at the UF/IFAS Florida Medical Entomology Laboratory (FMEL) launched a major study funded by the Centers for Disease Control and Prevention (CDC). They tested over 2,000 mosquitoes, focusing on the two most common disease-spreading species in the South: Aedes aegypti (the yellow fever mosquito) and Culex quinquefasciatus (the southern house mosquito).
These species are notorious for transmitting pathogens like dengue, West Nile, and Zika. Yet, when it came to Oropouche, the mosquitoes hit a wall.
“These are the ones that really bite humans on a regular basis,” said Barry Alto, an associate professor at UF/IFAS.
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The study, published in the journal PLOS Neglected Tropical Diseases, found that the local mosquitoes are “incredibly bad” at transmitting the virus. Very few mosquitoes developed the infection after exposure, and even fewer carried the virus in their saliva—a necessary step to pass it on to a human victim.
“We feel it’s very unlikely the virus will become established and transmitted locally,” said Nathan Burkett-Cadena, an associate professor at UF/IFAS FMEL.
The findings provide critical reassurance regarding public health, particularly for pregnant women. Like Zika, Oropouche virus has been linked to severe fetal birth defects, including cognitive and developmental issues.
“We were concerned about an outbreak because the disease, like Zika, can cause serious birth defects,” said Dongmin Kim, a research scientist at the laboratory. “It could have been really serious.”
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While the virus causes uncomfortable symptoms in the general population—including fever, chills, headaches, joint pain, and a bumpy rash—the potential for birth defects and limited evidence suggesting sexual transmission made the prospect of a local outbreak particularly alarming.
With the primary mosquito threat largely ruled out, the research team says the next steps involve investigating whether no-see-ums native to the U.S. could potentially carry the virus, or if mosquitoes can transmit the virus to their offspring.
For now, however, the data suggests that while travelers may continue to bring the virus home, the buzzing pests in their backyards are unlikely to start a new epidemic.
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