La Crosse virus is a mosquito-borne bunyavirus that has been neglected as a cause of pediatric central nervous system (CNS) infection. The disease recurs every summer in endemic foci in the midwestern and mid-Atlantic United States in areas forested with hardwood trees, which provide breeding sites for the treehole-dwelling mosquito vector, Aedes triseriatus. During hyperendemic years, the prevalence of disease in some of these areas can be remarkably high, exceeding that of bacterial meningitis. Clinical manifestations in symptomatic cases of La Crosse encephalitis (LE) tend to cluster into a mild form or a severe form of the disease.3-10  The usual clinical course (80% to 90%) is the mild form in which headache, fever, and vomiting frequently occur on days 1 to 3. Lethargy, behavioral changes, and/or brief seizures may occur on days 3 and 4, followed by improvement over a 7- to 8-day period. The...

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