This study is part of the Search for a Treponema pallidum Antigen Test (SeTPAT) project to study the proteomic, immunological, serological and clinical changes associated with pre- and post-treatment syphilis infection in a way that could ultimately lead to the development of a new ELISA and rapid diagnostic test of T. pallidum antigenaemia.

The general aim of this prospecive observational cohort study is thus to quantify a set of target proteins with the highest diagnostic potential for the diagnosis of initial T. pallidum infection and T. pallidum persistence. A test which could directly detect the presence of T. pallidum antigens could represent a considerable advance over currently used tests in the diagnosis of initial syphilis infection, its response to therapy and in the diagnosis of syphilis reinfections. This prospective observational cohort study of HIV-positive patients with a new diagnosis of syphilis infection will be conducted at the HIV/Sexually Transmitted Infections (STI) Clinic at the Institute of Tropical Medicine, Antwerp.

This cohort study is part of the Search for a Treponema pallidum Antigen Test (SeTPAT) project to study the proteomic, immunological, serological and clinical changes associated with pre- and post-treatment syphilis infection in a way that will ultimately lead to the development of a new ELISA and rapid diagnostic test of T. pallidum antigenaemia. The general aim of this observational cohort study is thus to quantify a set of target proteins with the highest diagnostic potential for the diagnosis of initial T. pallidum infection and T. pallidum persistence. A test which could directly detect the presence of T. pallidum antigens could represent a considerable advance over currently used tests in the diagnosis of initial syphilis infection, its response to therapy and in the diagnosis of syphilis reinfections.

This prospective observational cohort study of HIV-positive patients with a new diagnosis of syphilis infection will be conducted at the HIV/Sexually Transmitted Infections (STI) Clinic at the Institute of Tropical Medicine, Antwerp (ITM). 120 patients with a new diagnosis of syphilis infection who receive treatment for their syphilis at the ITM and give informed consent, will be recruited; based on the current rate of syphilis diagnosis this should take approximately 18 months. Participants will receive a detailed, standardized clinical assessment, fill out a brief behavioural questionnaire and then be treated with therapy for syphilis according to their stage of disease. In addition to the routine tests performed, if and after they consent to participate in this study, they will have their serum analyzed for a number of other immunological markers. A control group will be recruited, consisting of 30 convenience selected patients who are HIV infected but have no evidence of syphilis (Rapid Plasma Reagin, T. pallidum antigenaemia , or clinical evidence that could be suggestive of syphilis infection) are attending the HIV clinic regularly, who were seen in the same month as one of the acute syphilis cases and who consent to take part in the study. The control group will only undergo the investigations planned for the baseline visit.

Noteworthy, it is hoped that the results of the study will also allow to develop and evaluate the utility of a composite diagnostic rule for the diagnosis of syphilis reinfection.

Inclusion Criteria:

• Willingness to provide written consent
• Prepared to follow the study schedule
• EITHER a diagnosis of a new episode of syphilis - initial or repeat syphilis (Patients group), OR no evidence of past or present syphilis, defined as no clinical or serological evidence of syphilis (Control group).

Exclusion Criteria:

• Use of doxycycline, a macrolide antibiotic or a Beta-lactam antibiotic in the preceding 28 days
• Not willing to give informed consent

Patients with a new diagnosis of syphilis infection who receive treatment for their syphilis at the Institute of Tropical Medicine (ITM), and a control group , consisting of patients who are HIV infected but have no evidence of syphilis