Brazil, which has recently suffered serious outbreaks of Zika virus and yellow fever, now faces a new threat, according to reports from local scientists: Oropouche fever.

The Oropouche virus, named for a river in Trinidad, where it was first isolated in 1955, circulates in monkeys and sloths in the Amazon jungle. The virus has caused occasional outbreaks, short but intense, in towns in tropical areas of Brazil, Peru and Panama, and on some Caribbean islands.

But in the last few years, Oropouche cases have turned up more often in urban areas, including some in northeast Brazil, where Zika began its explosive spread in this hemisphere.

Oropouche causes symptoms resembling those of dengue: high fever, headaches and joint pain, nausea and malaise. The infection is not normally fatal, although it can cause meningitis — dangerous swelling of protective tissue around the brain — if it reaches the spinal fluid. There is no vaccine.