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 (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.
    • 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 infection 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 SLE, 5 to 20% die.
    • The risk of dying increases with age.


If you think you or a family member might have SLE, talk with your healthcare provider.

  • Healthcare providers diagnose SLE virus infection based on:
    • Signs and symptoms
    • History of living in or traveling to an area where SLE virus is known to circulate
    • History of possible exposure to the mosquitoes that can carry SLE virus
    • Laboratory testing of blood or spinal fluid
  • Your healthcare provider can order tests to look for SLE virus infection and other infections that can cause similar symptoms.

To learn more about testing, visit our Healthcare Providers page.


  • There are no medications to prevent or treat SLE. Antibiotics are not effective against viruses, including SLE virus.
  • Rest, fluids, and over-the-counter pain medications may 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.

To learn more about treatment, visit our Healthcare Providers page.