Most people (about 99%) infected with SLEV do not develop any symptoms.

Some people (about 1%) develop a fever and other symptoms. For people with symptoms, the time from infected mosquito bite to feeling sick (incubation period) ranges from 4 to 14 days. The disease is generally milder in children and young adults than in older adults.

• Symptoms usually start abruptly, with fever, headache, dizziness, nausea, and generalized weakness.
• Signs and symptoms usually get worse over a period of several days to a week.
• Some patients recover after this period.
• Others develop signs of central nervous system (CNS) infections, including inflammation of the brain (encephalitis) or the membranes around the brain and spinal cord (meningitis).
  • Symptoms can include stiff neck, confusion, disorientation, dizziness, tremors, and unsteadiness.
  • Coma can develop in severe cases.
  • Among patients diagnosed with St. Louis encephalitis, 5 to 20% die.
  • The risk of dying from SLEV CNS infections increases with age.

Diagnosis of SLEV disease is based on the patient’s signs and symptoms and appropriate laboratory testing. If you think you have SLEV disease, contact your healthcare provider.

No specific medication is available to treat SLEV disease. People with severe illness usually require hospitalization, supportive care, and/or rehabilitation.

The best way to prevent SLEV disease is to avoid mosquito bites. Please see our mosquito prevention page for more information.

For more information about St. Louis encephalitis virus, visit the CDC SLEV webpage.

SLEV disease is rare in Indiana with no cases reported in the past 10 years. For more information about infectious diseases in Indiana, please visit:

• Indiana Annual Report of Infectious Diseases

For maps showing recent mosquito-borne disease detections in people, horses, and mosquitoes, click here.

National statistics for SLEV disease can be found at the CDC SLEV Statistics and Maps webpage.