The challenging diagnosis of overlapping oral primary/secondary syphilis with nonreactive serology.

The prevalence of oral syphilis, known as "the great imitator" because of its diagnostic complexity and varied clinical manifestations, is increasing worldwide, particularly in people living with HIV (PLWH), who could present false-negative serological results. Although some studies have described the variable presentation of oral syphilis in the context of HIV infection, the difficulty in distinguishing between the primary and secondary stages, clinically and histopathologically, underscores the need to describe atypical cases. We report the case of a 28-year-old HIV-positive man presenting with a 3-month history of painless white/red ulcerated lesion on the soft palate. Physical examination revealed an ulcerated lesion with local signs of inflammation. Initial biopsy revealed a nonspecific inflammatory process and immunohistochemistry (IHC) using anti-Treponema pallidum antibodies showed negative results. The results of serological tests for syphilis (Venereal Disease Research Laboratory and fluorescent treponemal antibody-absorption test) were negative on repeated occasions. Nonetheless, polymerase chain reaction (PCR) assay and subsequent IHC for T. pallidum showed positive results, confirming the diagnosis of oral syphilis. This case illustrates that the diagnosis of oral syphilis is challenging in the absence of serological evidence, and specific tests such as PCR and IHC are useful complementary diagnostic tools.