Introduction Syphilis incidence among men who have sex with men (MSM) has been on the rise globally during the last years. Especially in western countries, sharp increases in numbers of syphilis infections were observed [1-4]. In Europe, the syphilis incidence was 5.1 cases/100,000 inhabitants overall, with distinct differences between countries, probably due to the differences in the notification systems, completeness of data and healthcare structures [3]. Since 2009, the syphilis incidence increased in Europe in men, especially in western European countries, while the incidence decreased in women concurrently. In Germany, the number of reported syphilis cases doubled between 2001 and 2004 to over 3,000 per year and remained mainly stable until 2009. Between 2010 and 2014, the number increased between 11% and 22% per year [5]. High rates of bacterial sexually-transmitted infections (STIs) including syphilis are reported for MSM coinfected with HIV from many countries, e.g. Australia, Canada, England, Germany, and Spain [6-10]. We discuss reasons for the increasing syphilis incidence in MSM, in particular the increase in risky sexual behaviour, such as a higher frequency of condomless sexual intercourse, while applying HIV serostatus knowledge-based risk management strategies, particularly HIV-serosorting [11-16]. Syphilis is a STI caused by Treponema pallidum. It has different stages of disease (primary, secondary, latent, and tertiary syphilis), of which especially the first and second stages are highly infectious. Syphilis can lead to severe sequelae such as serious cardiovascular or neurological impairments and also death, and increases the risk of HIV acquisition and transmission [17,18]. As congenital syphilis, T. pallidum can also be transmitted to a fetus during pregnancy and can cause severe health impairments of the newborn, including premature delivery and stillbirth. Syphilis can still be treated effectively with penicillin [17]. To assess the epidemiological dynamics of syphilis in Germany and to shape appropriate public health interventions, we analysed data of the mandatory syphilis notification system reported between 2001 and 2015. Additionally, we analysed data of four waves of a behavioural survey among MSM in Germany to assess potential changes in relevant sexual behaviours. Methods Mandatory syphilis notification In Germany, syphilis diagnoses have been notified anonymously on the basis of the Protection against Infection Act in Germany [19] since 2001 by laboratories, with physicians inserting relevant clinical information. Syphilis cases are defined as cases diagnosed by direct detection of T. pallidum by microscopic or histological examination OR a positive screening test and a confirmation test (a combination of T. pallidum particle agglutination test (TPPA), T. pallidum haemoagglutination test (TPHA), Immuno- Assay, fluorescence Treponema antibody absorption test (FTA-ABS), Immunoblot) AND venereal disease research laboratory (VDRL)/rapid plasma reagin (RPR) activity or IgM antibodies OR clinical information consistent with syphilis [17]. Potential double notifications were identified by comparing cases by demographic data, diagnosis date, antibody titres, and clinical information. We analysed syphilis cases by year of diagnosis, age, sex, area of residence, and transmission group. Behavioural surveys Self-reported data on sexual risk behaviours and diagnoses of HIV and syphilis among MSM were collected during four waves of a behavioural MSM survey in 2003, 2007, 2010, and 2013. Survey participants were recruited exclusively online in the 2010 and 2013 surveys, and by a combination of print questionnaires distributed by gay magazines and online questionnaires in the 2003 and 2007 surveys. The methods and the results of this survey have been published elsewhere in German [20-23]. These surveys are part of the HIV behavioural surveillance in Germany implemented in the late 1980s [22]. Although they use the same or very similar questions, their comparability is restricted, mainly due to the different recruitment methods. Recruitment bias affected information on age, city size, and sexual identity. This is why we restricted the analysis to a subgroup of men self- identified as gay, aged 30–44 years, and living in cities with more than 500,000 inhabitants (in descending order according to the number of inhabitants: Berlin, Hamburg, Munich, Cologne, Frankfurt am Main, Stuttgart, Dusseldorf, Dortmund, Essen, Bremen, Leipzig, Dresden, Hanover, Nuremberg). This subgroup is less affected by the change in recruitment methods. The sample sizes of the surveys were: 4,750 in 2003, 8,170 in 2007, 54,387 in 2010, and 16,734 in 2013. The subgroup of gay-identified men aged 30–44 years and living in cities with more than 500,000 inhabitants consisted of 1,039 (22%) men in 2003, 1,315 (16%) in 2007, 8,242 (15%) in 2010, and 1,547 (9.2%) in 2013. We analysed trends in condomless anal intercourse (cAI) with steady and non-steady partners in the previous 12 months (scAI respectively nscAI), and with partners of unknown HIV status (ucAI), also stratified by HIV status, as well as the proportion of MSM getting tested for HIV in the previous 12 months, to explore the increasing syphilis transmission among MSM. Data on syphilis testing were only collected in 2010 and 2013 in the behavioural surveys. Data were analysed using descriptive statistics. Results Data from mandatory syphilis notification As at 1 March 2016, 54,747 newly diagnosed cases of syphilis had been notified in Germany between 1 January 2001 and 31 December 2015, with cases increasing since 2010 (Figure 1). In 2015, 6,834 cases were reported, corresponding to a 19.4% increase compared with 2014. Incidence was 8.5 per 100,000 inhabitants overall, with highest incidences above 20.0 mainly in larger German cities such as Berlin (39.0), Cologne (35.6), Munich (30.0), Frankfurt am Main (29.5), Dusseldorf (26.6), Leipzig (23.7), Hamburg (21.4) and Stuttgart (20.4). They were especially high in Berlin inner city areas with 62.8–117.8/100,000 inhabitants. Notified cases increased in 14 of 16 German federal states in 2015.
