LaCrosse viral encephalitis mimics herpes simplex viral encephalitis.

Temporal lobe abnormalities, findings commonly associated with herpes simplex virus encephalitis, were observed in a male 10 years of age found to have LaCrosse virus encephalitis. Diagnostic features included magnetic resonance imaging revealing abnormal signal intensity in the bilateral frontotemporal regions, and left-sided periodic lateralizing epileptiform discharges. LaCrosse virus encephalitis should be included in the differential diagnosis of viral encephalitis associated with structural and electrographic temporal lobe lesions, represented by periodic lateralizing epileptiform discharges. The recently developed LaCrosse RNA polymerase chain reaction for cerebrospinal fluid may enable rapid diagnosis, prevent the need for treatment with acyclovir, and give parents an encouraging prognosis.