After an incubation period of 3-8 days, the symptoms of OHF begin suddenly with chills, fever, headache, and severe muscle pain with vomiting, gastrointestinal symptoms and bleeding problems occurring 3-4 days after initial symptom onset. Patients may experience abnormally low blood pressure and low platelet, red blood cell, and white blood cell counts.
After 1-2 weeks of symptoms, some patients recover without complication. However, the illness is biphasic for a subset of patients who experience a second wave of symptoms at the beginning of the third week. These symptoms include fever and encephalitis (inflammation of the brain).
The case fatality rate of OHF is low (0.5% to 3%).