Oropouche fever is a tropical disease caused by infection with Oropouche virus. It is a zoonosis transmitted by biting midges and mosquitoes, from a natural reservoir which includes sloths, non-human primates, and birds. The disease is named after the region where it was first discovered and isolated in 1955, by the Oropouche River in Trinidad and Tobago.

Oropouche fever is endemic to the Amazon basin, with some evidence that its range may be spreading more widely in South and Central America. Since its discovery in 1955, there have been more than 30 epidemics of OROV in countries including Brazil, Peru, and Panama, with over half a million diagnosed cases in total. It has also been detected in-between epidemics, indicating that it may spread silently.

The signs and symptoms of Oropouche fever are similar to those of dengue, chikungunya, and Zika. Symptoms are often mild and typically begin three to eight days after infection. Fever, headache, and muscle and joint pains are most common; a skin rash, unusual sensitivity to light, and nausea and vomiting may also occur. Most cases are self-limited, with recovery in two to seven days. In severe illness, however, the central nervous system may be affected, with symptoms of meningitis and encephalitis, and a tendency to excessive bleeding has been reported in up to 15% of cases.

Oropouche fever has been recognized as among the most neglected of tropical diseases and as an emerging infectious disease. Little is known about its epidemiology, pathogenesis, and natural history, and there is no specific treatment or vaccine.

Oropouche fever is characterized as an acute febrile illness, meaning that it begins with a sudden onset of a fever followed by other clinical symptoms. It typically takes four to eight days (the incubation period) from the bite of the infected mosquito or midge to the first signs of infection.

Fever is the most common symptom, occurring in nearly all cases, with temperatures as high as 40 °C (104 °F). Other symptoms include chills, headache, muscle and joint pain (myalgia and arthralgia), dizziness, photophobia, nausea, vomiting, epigastric pain, and rashes. As in dengue, a skin rash resembling rubella, conjunctival injection, and pain behind the eyes may occur. The initial febrile episode typically resolves within seven days, but a reccurrence of symptoms with a lesser intensity is common, typically in about 60% of cases. Fatigue and weakness may also persist for up to a month after infection.

In serious cases, particularly in large outbreaks, the central nervous system (CNS) may be affected with symptoms of meningitis and encephalitis, including severe headache, dizziness, neck stiffness, double vision, darting of the eyes, uncoordinated movements, and evidence of viral infection in the cerebrospinal fluid (CSF). A tendency to abnormal bleeding has been reported in up to 15% of cases.

in July 2024, the Brazilian Ministry of Health released a report of four cases of microcephaly in newborns of infected mothers. Fetal deaths were observed possibly associated with vertical transmission, i.e. from mother to child.