Prevalence and correlates of active syphilis diagnosed using serological assays and the molecular research-use-only Aptima Treponema pallidum assay among pregnant women in Zambia, 2023

Introduction Syphilis is a leading cause of adverse pregnancy outcomes, excessively affecting sub-Saharan Africa. Early diagnosis and treatment are crucial to prevent mother-to-child transmission. The aims of the present study were to assess prevalence and epidemiological and clinical correlates of active syphilis diagnosed using serological [treponemal and non-treponemal (lipoidal)] assays and the research-use-only (RUO) Aptima Treponema pallidum assay [Hologic; transcription-mediated amplification (TMA) assay] among pregnant women attending four antenatal care facilities in Nchelenge, Zambia in 2023.Methods Syphilis serology was performed using rapid diagnostic test [RDT; Core tests (R) ONE STEP Syphilis Test Kit (Core Technology, Atlanta, USA)]; positive samples were further evaluated by rapid plasma reagin [RPR; RPR Carbon Antigen Reagent (Eurocarb Products Ltd., Bristol, United Kingdom)] test. Active syphilis was defined as a positive RDT plus a positive RPR test, or a positive RUO Aptima T. pallidum assay. T. pallidum and other non-viral STIs (Chlamydia trachomatis, Neisseria gonorrhoeae, Mycoplasma genitalium, and Trichomonas vaginalis) were detected in clinician-collected vaginal swab samples using Aptima assays on the Panther system (Hologic). HIV testing was performed with the Determine HIV Test Kit, and bacterial vaginosis (BV) was diagnosed using Nugent's score. Active syphilis correlates were identified using univariable and multivariable logistic regression.Results In total, 996 pregnant women were included in the analysis, of whom 136 women (13.7%) had an active syphilis infection. Of these women with active syphilis, 117 (86.0%) were positive in serology only, 15 (11.0%) in serology plus RUO Aptima T. pallidum assay, and four (2.9%) in RUO Aptima T. pallidum assay only. Rates of other current STIs and reproductive tract infections included: BV (23.3%), T. vaginalis (22.7%), M. genitalium (12.7%), HIV (8.7%), N. gonorrhoeae (8.4%), and C. trachomatis (7.4%). Secundigravidity, history of stillbirth, current concomitant M. genitalium, N. gonorrhoeae, T. vaginalis, and HIV infections were significant risk factors of active syphilis. Advanced gestational age (from 28 to 39 weeks) at enrolment was associated with significantly lower risk of syphilis positivity.Discussion A remarkably high burden of syphilis and high co-infection rates with HIV and other non-viral STIs in pregnant women in Zambia underscore the urgent need for integrating syphilis and HIV universal screening programs, as well as to consider concurrent testing for selected other non-viral STIs. Where feasible due to the increased cost, supplementing serological screening with a highly sensitive and specific molecular test such as the RUO Aptima T. pallidum assay for suspected early syphilis can ensure timely detection of both seroconverted and seronegative early syphilis cases, early treatment and effective prevention of mother-to-child transmission.