Symptoms and Treatment


The incubation period for La Crosse virus (LACV) disease (the time from infected mosquito bite to onset of illness) ranges from 5 to 15 days. LACV disease is usually characterized by 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.

Although seizures during the acute illness are common, fatal cases are rare (<1%) and most patients seem to recover completely. Neurologic sequelae (conditions resulting from the initial disease) of varying duration have been reported in some cases. These may include recurrent seizures, hemiparesis (partial paralysis of one side of the body), and cognitive and neurobehavioral abnormalities.


No vaccine against LACV infection or specific antiviral treatment for clinical LACV infection is available. Patients with suspected LAC encephalitis should be hospitalized, appropriate serologic and other diagnostic tests ordered, and supportive treatment (including seizure control) provided.

Clinical Evaluation (for Health Care Providers)

In acute LACV 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.

LACV is difficult to isolate from clinical samples, and almost all isolates (and positive PCR results) have come from brain tissue or CSF. In the absence of a sensitive and non-invasive virus detection method, serologic testing remains the primary method for diagnosing LACV infection. Combined with a consistent clinical presentation in an endemic area, a rapid and accurate diagnosis of acute neuroinvasive LACV disease can be made by the detection of LACV-specific IgM antibody in serum or CSF. LACV IgM tests are available commercially, in some state health department laboratories, and at CDC. A positive LACV IgM test result should be confirmed by neutralizing antibody testing of acute- and convalescent-phase serum specimens at a state public health laboratory or CDC. To submit specimens for testing at CDC, please contact your state health department.

All LACV disease cases should be reported to local public health authorities. Reporting can assist local, state and national authorities to recognize outbreaks of this relatively rare disease and to institute control measures to limit future infections.