Infection can cause a range of illnesses, from no symptoms to fatal disease. 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.
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 centers 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. Fewer than 5 cases are reported each year.