Introduction Syphilis infection has been increasing among men who have sex with men (MSM) in Thailand. We assessed syphilis incidence and associated risk factors in the Bangkok MSM Cohort Study (BMCS).
Methods We enrolled Thai MSM age ≥18 years old in the BMCS from April 2006 to January 2008 (Period1) and September 2009 to December 2010 (Period2), and followed participants every 4 months for up to 60 months. Treponema pallidum (TP) screening was performed at enrollment, annually and at any unscheduled study visits if indicated, using the rapid plasma reagin (RPR). If the RPR was reactive, we confirmed with a TP-specific antibody test. We defined incident syphilis as a RPR titer ≥1:8 and a reactive treponemal test. We calculated risk factors for incident syphilis in all participants using a Cox proportional hazard model, adjusting for baseline demographic and behavioural characteristics, and prevalent HIV infection.
Results Among 1,502 participants who had no syphilis at baseline and had more than one follow-up visit, 54.7% reported unprotected anal intercourse (UAI). The overall syphilis incidence was 2.7 (95% CI 2.3—3.1) per 100 person-years (/100PY). There was a significant difference in syphilis incidence in the two periods (2.5/100PY in Period1 vs. 3.3/100PY in Period2, p = 0.003). Factors independently associated with incident syphilis were enrollment in Period2 (Adjusted HR [AHR] 1.7; 95% CI 1.2—2.4), UAI (AHR 1.4; 95% CI 1.0—2.0), amyl nitrate use (AHR 1.9; 95% CI 1.2—2.8), Viagra use (AHR 1.6; 95% CI 1.1—2.5), and prevalent HIV infection (AHR 2.3; 95% CI 1.6—3.1).
Conclusion In the BMCS, statistically significant differences in incident syphilis by enrollment period may be due to recent increases in syphilis among urban Thai MSM. Syphilis screening identifies MSM at high risk for HIV, and syphilis screening and treatment are necessary for comprehensive HIV VCT services.
Disclosure of interest statement The Bangkok MSM Cohort Study (BMCS) is funded by the Thailand Minister of Public Health (MOPH) – US Centres for Disease Control and Prevention (CDC) Collaboration. No pharmaceutical grants were received in the development of this study.