Unusual clinical manifestation and challenging serological interpretation of syphilis: insights from a case report

In 2020, a 52-year old HIV-positive bisexual male patient was admitted to our department with a 4-month history of moderately itchy cutaneous lesions localized at his neck, trunk and arms. In 2013, the patient presented with a classic syphilitic roseola of the trunk and a secondary syphilis was diagnosed, with increased levels of rapid plasma reagin (RPR), Treponema pallidum hemagglutination assay (TPHA), anti-Treponema pallidum IgM and IgG Index. A second episode occurred in 2018, as a primary syphilis with multiple ulcerative lesions of the penis, and increased levels of RPR, IgG and IgM. In 2019, a further episode of secondary syphilis was treated with Doxycycline. In 2020, erythematous and papular lesions with vesicular components and urticarial erythema multiforme (EM)-like lesions were present at the neck, trunk and arms. Serological tests and Nucleic Acid Amplification Test (NAAT) for Treponema Pallidum were performed, as well as a cutaneous biopsy with histological and immunohistochemical evaluation of one lesion. NAAT was negative for T. pallidum. Serological test results were discordant with a new syphilis infection, showing only increased levels of RPR and anti-Treponema IgG. The cutaneous biopsy revealed a non specific histological pattern, while the immunohistochemical evaluation with anti-spirochetal antibodies was mandatory for the diagnosis of recent syphilis, showing clusters of rod-shaped elements, some of which with spiral form, focally present at the epidermis and adnexal structures.