Clinical Signs and Symptoms of La Crosse Virus Disease

Key Points

  • La Crosse virus disease can present as an acute febrile illness or neuroinvasive disease.
  • The incubation period for La Crosse virus disease ranges from 5–15 days.
  • Most patients with La Crosse virus neuroinvasive disease survive; approximately 1% of cases are fatal and 6–15% have neurologic sequelae.
Doctor looking at a brain image on a computer

Clinical presentation

La Crosse virus disease should be considered in any person with an acute febrile or neurologic illness who has had recent exposure to mosquitoes, especially during the summer months in endemic areas of the United States. La Crosse virus disease is most common in children under the age of 16 years.

Other causes of encephalitis and aseptic meningitis should also be considered, as appropriate (e.g., herpes simplex viruses, enteroviruses, West Nile virus, St. Louis encephalitis virus, Jamestown Canyon virus, Powassan virus).

Signs and symptoms

Most La Crosse virus infections are clinically inapparent, and the vast majority of infections remain undiagnosed. The incubation period for La Crosse virus disease ranges from 5 to 15 days.

Initial symptoms include fever, headache, nausea, vomiting, fatigue, and lethargy. The disease can progress to encephalitis, meningoencephalitis, or aseptic meningitis. Symptoms of encephalitis can include altered mental status, seizures, speech problems (aphasia, dysarthria), paresis or paralysis, movement disorders, and cranial nerve palsies. La Crosse virus infection has not been associated with acute flaccid paralysis.

Clinical assessment

In acute La Crosse virus neuroinvasive disease cases, cerebrospinal fluid (CSF) examination typically shows a mildly elevated white blood cell count and normal glucose; CSF protein is elevated in about one third of cases. The peripheral white blood cell count is usually elevated. Computed tomography (CT) brain scans are usually normal, while electroencephalographic (EEG) abnormalities are more common. EEG results often resemble those seen in cases of herpes simplex encephalitis.


Approximately 1% of La Crosse virus neuroinvasive disease cases are fatal. Neurologic sequelae, including epilepsy, hemiparesis, and cognitive and neurobehavioral abnormalities, have been reported in 6%−15% of cases.