Symptoms, Diagnosis, & Treatment


Most people infected with St. Louis Encephalitis (SLE) virus do not have symptoms.

For people with symptoms, the time from infected mosquito bite to feeling sick ranges from 4 to 14 days. Symptoms usually start abruptly, with fever, headache, dizziness, nausea, and generalized weakness. They typically get worse over a period of several days to a week. Some patients recover after this period. Others develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors, and unsteadiness. Coma can develop in severe cases.

The disease is generally milder in children and young adults than in older adults. Among patients diagnosed with SLE, 5 to 20% die. The risk of dying increases with age.


  • See your healthcare provider if you develop the symptoms described above.
  • Your healthcare provider can order tests to look for SLE virus infection.
  • To learn more about testing, visit our Healthcare Providers page.


  • No vaccine or specific antiviral treatment for SLE is available.
  • Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms.
  • In severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.
  • If you think you or a family member might have SLE, talk with your healthcare provider.
  • To learn more about treatment, visit our Healthcare Providers page.