title: Outbreaks of syphilis among men who have sex with men attending STI clinics between 2007 and 2015 in the Netherlands: a space-time clustering study.
abstract: Infectious syphilis (syphilis) is diagnosed predominantly among men who have sex with men (MSM) in the Netherlands and is a strong indicator for sexual risk behaviour. Therefore, an increase in syphilis can be an early indicator of resurgence of other STIs, including HIV. National and worldwide outbreaks of syphilis, as well as potential changes in sexual networks were reason to explore syphilis trends and clusters in more depth.
  
  
  
  National STI/HIV surveillance data were used, containing epidemiological, behavioural and clinical data from STI clinics. We examined syphilis positivity rates stratified by HIV status and year. Additionally, we performed space-time cluster analysis on municipality level between 2007 and 2015, using SaTScan to evaluate whether or not there was a higher than expected syphilis incidence in a certain area and time period, using the maximum likelihood ratio test statistic.
  
  
  
  Among HIV-positive MSM, the syphilis positivity rate decreased between 2007 (12.3%) and 2011 (4.5%), followed by an increasing trend (2015: 8.0%). Among HIV-negative MSM, the positivity rate decreased between 2007 (2.8%) and 2011 also (1.4%) and started to increase from 2013 onwards (2015: 1.8%). In addition, we identified three geospatial clusters. The first cluster consisted of MSM sex workers in the South of the Netherlands (July 2009-September 2010, n=10, p<0.001). The second cluster were mostly HIV-positive MSM (58.5%) (Amsterdam; July 2011-December 2015; n=1123, p<0.001), although the proportion of HIV-negative MSM increased over time. The third cluster was large in space (predominantly the city of Rotterdam; April-September 2015, n=72, p=0.014) and were mostly HIV-negative MSM (62.5%).
  
  
  
  Using SaTScan analysis, we observed several not yet recognised outbreaks and a rapid resurgence of syphilis among known HIV-positive MSM first, but more recently, also among HIV-negative MSM. The three identified clusters revealed locations, periods and specific characteristics of the involved MSM that could be used when developing targeted interventions.
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: F
authors.initials: F
authors.lastname: van Aar
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: C
authors.initials: C
authors.lastname: den Daas
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.|Julius Center for Health Sciences and Primary Care, University Medical Center, Utrecht, The Netherlands.
authors.forename: M A B
authors.initials: MAB
authors.lastname: van der Sande
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.|Department of Medical Statistics, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: L C
authors.initials: LC
authors.lastname: Soetens
authors.affiliation: STI Outpatient Clinic, Public Health Service of Amsterdam (GGD Amsterdam), Amsterdam, The Netherlands.|Department of Dermatology, Academic Medical Center (AMC), University of Amsterdam, Amsterdam, The Netherlands.|Center for Infection and Immunology Amsterdam (CINIMA), Academic Medical Center (AMC), University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: H J C
authors.initials: HJC
authors.lastname: de Vries
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: B H B
authors.initials: BHB
authors.lastname: van Benthem
mesh_terms: D000328:Adult; D000554:Ambulatory Care Facilities / Q000706:statistics & numerical data*; D060085:Coinfection / Q000453:epidemiology* / Q000517:prevention & control; D004196:Disease Outbreaks / Q000517:prevention & control; D015658:HIV Infections / Q000453:epidemiology*; D018451:Homosexuality, Male; D006801:Humans; D008297:Male; D008875:Middle Aged; D009426:Netherlands / Q000453:epidemiology; D010342:Patient Acceptance of Health Care; D012309:Risk-Taking; D018571:Sentinel Surveillance; D012747:Sexual Partners; D013029:Space-Time Clustering; D013587:Syphilis / Q000175:diagnosis / Q000453:epidemiology* / Q000517:prevention & control
keywords: GAY MEN; HIV; SYPHILIS