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 ranges from 5 to 15 days. Symptoms of LAC include fever (usually lasting 2-3 days), headache, nausea, vomiting, fatigue (tiredness), and lethargy (reduced activity or alertness). Severe neuroinvasive disease (disease affecting the nervous system) occurs most frequently in children under the age of 16 years.

Although seizures are common, death is rare (<1%) and most patients seem to recover completely. Recovery time from severe illness varies and some effects to the central nervous system can persist over time. These can include recurrent seizures, hemiparesis (partial paralysis of one side of the body), and cognitive or behavioral impairments.


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


No vaccine or specific medicines are available for LAC virus disease. Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms. Patients with severe illness should 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 LAC virus disease, talk with your healthcare provider. To learn more about treatment, visit our Healthcare Providers page.