Background In 2016, a syphilis outbreak was detected among young heterosexuals in the South of the Netherlands. Syphilis in young Dutch heterosexuals is very rare since many years. Here, we describe the outbreak investigation.

Methods A case was defined as a laboratory confirmed sexually transmitted Treponema pallidum diagnosis between April 2016-December 2017 in individuals reporting heterosexual behavior only, and who were tested in this geographical area. Men having sex with men (MSM), commercial sex workers and swingers were excluded. Outbreak management included raising awareness among GPs and hospitals, extended STI screening, intensified partner notification and enhanced laboratory surveillance. Patients medical records and questionnaires for partner notification were used for analyses.

Results From 2016–2017, 18 cases were identified. Fourteen cases were <25 years of whom 9 were <20 years, 10 were women. Five cases consulted their GP months prior to diagnosis with symptoms indicating infectious syphilis, but were not diagnosed or adequately treated, causing doctors delay. Two cases were not traceable after diagnosis and not treated. The cases reported 84 sex partners for partner notification, of whom 71% were aged <25. 46%(39/84) of these sex partners were indeed notified, of whom 30%(25/84) were tested, 5 were diagnosed with syphilis. 54%(45/84) of the sex partners could not be notified due to lack of contact information. 10 cases could not be linked to other cases and the remaining cases comprised 4 heterosexual couples. No clear source of infection was identified for 14 cases.

Conclusion This unusual outbreak among young heterosexuals indicates a larger epidemic. Continuous transmission is to be expected because most cases could not be linked or their source of infection was not found; only one third of notifiable sex partners were tested despite intensified efforts of partner notification; doctor and patient delay probably caused prolongation of transmission.

Disclosure No significant relationships.