Secondary syphilis: a histologic and immunohistochemical evaluation.

The usual method for detecting spirochetes in tissue sections is the silver stain; however, they are often difficult to detect due to marked background staining commonly seen with this technique. In certain clinical settings, such as neurosyphilis, congenital syphilis, and immunosuppressive conditions including human immunodeficiency virus (HIV) infection, a better method of detecting spirochetes in tissue sections is needed. We compare immunohistochemistry (IHC) with a monoclonal antibody to Treponema pallidum to silver staining in 19 biopsies from 17 patients with serologic evidence of secondary syphilis. IHC demonstrated a sensitivity of 71%, which was superior to the 41% sensitivity of the silver stain (p = 0.084). Furthermore, specificity was improved with IHC, as background artifacts were markedly reduced. Dermal spirochetes were visualized in all 12 positive cases, while epidermal organisms were seen in only eight cases. This finding lies contrary to accepted teaching that organisms are most commonly seen at the dermal epidermal junction. Of interest, perineural plasmacellular infiltrates were frequently seen in our cases (74%). Spirochetes were not seen in any of 14 control cases with similar histopathologic patterns. Although serologic studies remain the gold standard, IHC is more sensitive and specific than silver stain for detecting T. pallidum in biopsies of secondary syphilis.