Western equine encephalitis mimicking herpes simplex encephalitis.

A patient with severe encephalitis had focal findings suggesting herpes simplex encephalitis. After brain biopsy and administration of vidarabine, the patient's condition improved but was ultimately diagnosed as a case of western equine encephalitis. Further complicating laboratory diagnosis, an endogenous murine coronavirus was isolated from newborn mice inoculated with the patient's brain biopsy tissue. This case illustrates the need for caution in attributing therapeutic responses to antiviral agents or in attributing human illness to viruses that are actually endogenous to animals and tissue culture used in diagnostic virology.