INTRODUCTION

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.¹ During hyperendemic years, the prevalence of disease in some of these areas can be remarkably high, exceeding that of bacterial meningitis.^1,2 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 less common (10% to 20%), more severe presentation includes abrupt fever and headache, disorientation and seizures, occurring within the first 8 to 24 hours of symptomatology, sometimes progressing to deep coma, associated in rare cases with cerebral edema. Overall, symptomatic children present with seizures in about 50% of cases and status epilepticus occurs in 10% to 15%. Focal signs are present in 16% to 25% of individuals with LE. Studies of neurologic outcome of LE indicate residual seizures in 6% to 13%, persistent hemiparesis in roughly 1%, and cognitive morbidity in a small, ill-defined number of cases (see "Discussion"). The case fatality rate approaches 1% of symptomatic cases.

Mild cases of LE resemble enteroviral meningoencephalitis (also common in the summertime) but LE has no rash, less often shows meningeal signs, and is much more often complicated by seizures. Severe cases of LE may mimic herpes simplex encephalitis (HSE),^1,3 presenting with fever, focal signs, and/or progression to deep coma but the outcome . . . [Full Text of this Article]