St. Louis Encephalitis and West Nile Virus Encephalitis.

St. Louis encephalitis virus is a major cause of epidemic arboviral encephalitis in the US. Transmitted by a mosquito vector, this virus is an annual public health concern during the late summer and early fall in much of the midwest and southeast. The characteristic epidemic features of this viral encephalitis coupled with public health surveillance and vector monitoring programs have made the diagnosis readily accessible during the past decade. Recently, however, the arboviral landscape in the US changed dramatically with the emergence and persistence of West Nile virus and associated human neurologic illness in New York and the Northeast. In its New York presentation, West Nile virus encephalitis exhibited clinical and laboratory similarities to St. Louis encephalitis. Not surprisingly, this led to initial confusion in establishing the diagnosis. In anticipation of the potential geographic spread of West Nile virus beyond the northeastern US, neurologists must now consider West Nile virus along with St. Louis encephalitis when diagnosing patients with suspected epidemic mosquito-borne viral encephalitis or meningoencephalitis. Although no specific antiviral agents are yet available, patients will benefit from close monitoring during the initial phase of illness, supportive critical care, and appropriate rehabilitation.