Infection can cause a range of illnesses, from no symptoms in the majority of cases to a fatal disease rarely seen. People with mild illness often have only a headache and sometimes fever. People with more severe disease can have sudden high fever, headache, drowsiness, irritability, nausea, and vomiting, followed by confusion, weakness, and coma. Young infants often suffer seizures. Symptoms usually appear in 5 to 10 days after the bite of an infected mosquito.
Diagnosis is based on tests of blood or spinal fluid.
Recovery is generally complete with rare instances of permanent neurological symptoms. There is no specific treatment for western equine encephalitis. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Care of patients centres on treatment of symptoms and complications. Western equine encephalitis is a relatively rare disease in humans that can occur in isolated cases or in epidemics.