Herpes simplex virus encephalitis. A case with dysplastic plasma cell infiltration.

A case of herpes simplex virus (HSV) encephalitis was diagnosed by means of a brain biopsy specimen on the 31st day of the illness. The infiltrate showed dense plasma cells, with dysplastic features that mimicked plasma cell neoplasm. The diagnosis of HSV encephalitis was substantiated by the finding of intranuclear virus particles and by seroconversion with HSV-specific enzyme-linked immunosorbent assay tests in blood serum and cerebrospinal fluid. This histologic appearance correlated temporally with the time of peak seroconversion. Extensive plasma cell infiltrate may be an unrecognized variant of the histopathology of HSV encephalitis in its subacute phase, and only careful electron microscopic study will establish the diagnosis.