Background Syphilis incidence has exponentially increased in recent decades, particularly among men who have sex with men (MSM). Primary syphilis is characterised by a chancre appearing at the site of Treponema pallidum (TP) inoculation. Atypical morphological variants of syphilitic chancre are frequent. Clinical suspicion must be confirmed either by the demonstration of TP within the lesion through direct tests, such as dark field microscopy (DFM) or T. pallidum nucleic acid amplification technique (TP-NAAT), or by serological tests.

Objectives To analyse the clinical features, the sexual behaviour and the role of diagnostic tests in a cohort of men with primary syphilis in Milan.

Methods Epidemiological, clinical and laboratory data of male patients with primary syphilis seen at the STI Center of the University of Milan between 2015 and 2019 were retrospectively evaluated. Diagnosis was confirmed by at least one positive diagnostic test of either DFM, TP-NAAT or serology.

Results Among a total of 244 patients, 160 (65.6%) were MSM and 32 (13.1%) were living with HIV. One hundred twenty-four (50.8%) patients had a clinically atypical chancre. Chancres were exclusively extragenital in 30 (12.3%) patients, with MSM being more commonly affected (MSM vs heterosexuals: 16.3% vs 4.8%, respectively; p=0.012), and anal region the most frequently involved site. Chancres were multiple in 68/242 (28.1%) patients and morphologically atypical in 76/244 (31.1%). Diagnosis was obtained by (1) both serology and direct methods in 158/244 patients (64.7%), (2) serology solely in 47/244 (19.3%) and (3) direct methods solely in 39/244 (16%). DFM yielded positive results in 83/139 (59.7%) patients, while TP-NAAT gave positive results in 114/121 (94.2%) patients.

Conclusions Patients with primary syphilis frequently present with morphologically atypical chancres. Furthermore, MSM commonly exhibit extragenital involvement. A combined diagnostic approach including both direct and indirect tests is needed.