Rabies and Herpes simplex virus encephalitis

Summary A retrospective study on the frequency, site and distribution of rabies and Herpes simplex virus (HSV) type 1 antigens by means of immunofluorescence (IF) and immunoperoxidase (IP) techniques was performed on routinely processed (formol-fixed, paraffin-embedded) brain autopsy material stored for up to 25 years. In 2 animal and 2 human rabies cases, inclusion bodies in neuronal cytoplasm and processes were brilliantly stained for rabies antigens by IF but were much less prominent or absent in usual histological stains. In 33 cases of histopathologically diagnosed necrotizing encephalitis, HSV antigens were demonstrated in 18 of 26 acute cases; 7 subacute cases (course longer than 4 weeks) were all negative for HSV antigens. Neuronal cytoplasm and nuclear membranes were the main sites of HSV antigens; nuclear inclusion bodies were inconstantly stained. Since most of HSV antigen negative cases also showed intranuclear inclusion bodies in HE stains, such nuclear inclusions are no reliable criterion for an HSV aetiology. On the other hand, their absence does not rule out a herpetic aetiology, but such a constellation is rare (only one of 18 HSV positive cases). Distribution of cells showing a positive reaction for HSV antigens may be patchy and irregular; therefore, a false negative result must be expected if very small tissue samples are examined (e.g, in needle biopsies from temporal lobe). In the leptomeninges, HSV antigen positive cells were found inconstantly and only in small numbers; this finding makes unlikely the possibility of an intravital diagnosis of HSV encephalitis by immunostaining of cerebrospinal fluid (CSF) cell preparations. Both immunohistological techniques applied in this study (IF and IP) gave the same results. Imprint preparations are useful when quick diagnosis is necessary.Immunohistological investigations are a simple and effective means to demonstrate a viral aetiology even in routinely processed material; the use of such material rules out hazards in laboratories which are not designed to handle highly infectious fresh material.