According to a story from Axios, 21 cases of Oropouche virus disease have been detected in US citizens returning to the country after spending time in Cuba. As a result, health officials have decided to release an official warning about the little-known illness.

The virus is implicated in thousands of cases across the Caribbean and South America. A recent CDC report about the disease highlights the following:

“Recent reports of outbreaks in areas without previous endemic transmission, fatal cases, and vertical transmission associated with adverse pregnancy outcomes have raised concerns about human health risks.”

The Oropouche virus is spread by the bite of certain mosquito and midge species. There is currently no vaccine, with treatment being entirely supportive. The virus was first identified from Trinidad and Tobago in 1955. It has also been found in sloths, leading to its other name: sloth fever. 8,000 cases have been reported as of Aug. 1 in Brazil since the start of the year, with two recorded deaths.

Cases of vertical transmission, in which the virus is spread from mother to fetus, have also been documented, much like the more famous Zika virus. This spread is associated with congenital abnormalities or death of the child. As a result, this illness is of special concern to pregnant people. Symptoms include fever, nausea, muscle/joint pain, rash, headache, diarrhea, and vomiting. More rare and potentially deadly complications include meningitis and encephalitis.

None of the US cases have resulted in death and there’s also no sign of local spread in the country for now. While most individuals recovered on their own, there were three instances in which recurrence of symptoms was reported after an initial period of recovery. This pattern of recurrence appears to be unique to Oropouche virus disease.

Ultimately, the best way to avoid the illness is to avoid areas with lots of mosquitoes and to use effective prevention methods, such as door/window screens and insect repellents.