The CDC said that a total of 21 Oropouche virus disease cases among U.S. travelers returning from Cuba have been reported as of Aug. 16.

The Oropouche virus is spread primarily through bites from midges as well as from some mosquitoes. Symptoms of the disease include headache, fever and muscle aches and joint pains, but the virus is rarely fatal.

Most patients returning from Cuba reported their symptoms between May and July.

Overall, three patients were hospitalized, and no deaths were reported, the agency said in its Morbidity and Mortality Weekly Report.

A dire economic crisis in Cuba has made it difficult to control the spread of Oropouche. Frequent power outages mean many sleep with windows open during the hot Caribbean summer. Few Cubans have access to insect repellent, and fumigation efforts have been stymied by fuel shortages.

Health officials in Cuba first began to report cases of the virus in May and say it has spread quickly from rural areas to urban centers, including the capital Havana, a top tourist destination.

Cuba's health ministry said last week it had registered more than 400 cases of Oropouche on the island this year, though reports around Havana suggest a marked increase in recent weeks.

Earlier this month, the CDC issued a health alert to notify clinicians and public health authorities of an increase in the disease in the Americas region that has caused two deaths so far this year.

Authorities in Bolivia, Brazil, Colombia, Cuba and Peru have reported at least 8,000 locally acquired cases since late last year.

Currently, there are no specific treatments or vaccines available for the disease.