The majority of cases are asymptomatic or may result in a nonspecific flu-like syndrome. The incubation periods vary slightly from three to fifteen days depending on the specific virus. Onset may be insidious or sudden with fever, headache, muscle aches, tiredness and possibly prostration. Some cases can progress to encephalitis with focal paralysis, seizures, and coma with a fatal outcome or permanent neurologic sequelae. Only a small proportion of infected persons progress to the encephalitis stage.
Diagnosis is confirmed by laboratory tests on blood, serum, or spinal fluid.
Some of the tests require two blood samples taken about two weeks apart. Treatment is supportive, dealing with problems caused by the virus in the brain. Antibiotics are not effective because the infectious agent is a virus. There are no vaccines approved for humans in the United States. There are vaccines available for use in horses.