Background The incidence of syphilis has globally increased over the last decade, particularly among men who have sex with men coinfected with the human immunodeficiency virus (HIV). HIV infection may make the clinical symptoms and seroreactivity of syphilis atypical, which requires careful consideration in terms of diagnoses and treatments by clinicians. Syphilis is known as a great imitator, and is often difficult to be diagnosed or it can be overlooked if clinicians depend only on its symptoms or signs. It is also highly contagious and could be transmitted without sexual intercourse, and reinfection is common. Guidelines recommend that all HIV-infected persons be provided with STD screening, including syphilis, at least annually. However, to our knowledge, there are no published data on the actual frequency of testing and instances of syphilis among HIV-infected persons in Japan. Materials and Methods We collected data from HIV infected male patients who had sex with men (MSM) at Tokyo Medical University Hospital from June 2011 to June 2012. Data from the patients, who had been tested with the rapid plasma reagin assay (RPR) at least once during the study period, were retrospectively obtained from clinical records and were analyzed. Results Among 1000 patients with HIV infection, 935 patients were MSM. 723 patients (77.4%) were tested using the Treponema pallidum latex agglutination test (TPLA) and RPR more than once during the study period. Out of the 723 patients, 443 patients (61.3%) were reactive for TPLA and 238 patients (32.9%) had reactive tests for RPR. All patients who were reactive for RPR were reactive for TPLA. Among the patients who were reactive for RPR, 93 patients (12.9%) were considered newly diagnosed or with a repeat infection. In this cohort, all patients were MSM with a median age of 37 years, and a median CD4+T-lymphocyte cell count of 465/uL. A total of 76 patients had been prescribed antiretroviral therapy, and 61 patients had a documented HIV-1 RNA viral load of
