Symptoms, Diagnosis, & Treatment


Most people infected with La Crosse (LAC) virus do not have symptoms. For people with symptoms, the time from infected mosquito bite to feeling sick (incubation period) ranges from 5 to 15 days.

  • Initial symptoms can include fever (usually lasting 2-3 days), headache, nausea, vomiting, fatigue (tiredness), and lethargy (reduced activity or alertness).
  • LAC virus can cause severe disease, such as infection of the brain (encephalitis).
    • Symptoms of severe disease include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, seizures, muscle weakness, vision loss, numbness and paralysis.
    • Severe disease occurs most frequently in children under the age of 16 years.
    • Recovery time from severe illness varies and some effects to the central nervous system can persist over time. These can include recurrent seizures and cognitive or behavioral impairments.
    • Most patient will recover, though death does occur rarely (<1%).


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

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

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


  • There are no medications to prevent or treat LAC virus disease. Antibiotics do not treat viruses.
  • Rest, fluids, and over-the-counter pain medications may relieve some symptoms.
  • Patients with severe disease often need to be hospitalized to receive support for breathing, staying hydrated, or reducing swelling in the brain.

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