Skin homing of Treponema pallidum in early syphilis: an immunohistochemical study

The incidence of syphilis is increasing in most parts of the world including some major European cities. The lesions developing during secondary syphilis may be difficult to diagnose clinically. Similarly, the histopathologic changes do not always fulfill the typical diagnostic criteria. The objective of this study was to assess the contribution of immunohistochemistry for the identification and localization of Treponema pallidum on the skin. We retrieved from our files 12 paraffin-embedded biopsies of skin lesions, which had posed diagnostic problems in the past. Only a serologic test had proven that the patients had syphilis. Controls consisted of lichenoid dermatoses unrelated to syphilis and borreliosis. Immunohistochemistry using an antispirochete (T. pallidum and Borrelia) antibody was performed retrospectively. In all samples from primary and secondary syphilis, T. pallidum was highlighted, but none of the control lesions unrelated to syphilis showed positivity. Interestingly enough, T. pallidum present in the lower mid-part of the epidermis often outnumbered that in the dermis. This difference was more striking in secondary syphilis compared with primary syphilis. Immunohistochemistry for T. pallidum considerably increased the sensitivity and the specificity of the histologic diagnosis. The strong epidermal homing of T. pallidum is highlighted in early syphilis.