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Background: Syphilis re-infections comprise 5-50% of annual diagnoses across syphilis outbreaks in men who have sex with men (MSM) in Canada. We used mathematical modeling to evaluate if previous infection could be used as a marker of risk in a targeted screening intervention.
Methods: We parameterized a risk-stratified compartmental model of syphilis transmission in MSM to reflect the epidemiologic characteristics of an ongoing syphilis outbreak in Winnipeg. We evaluated the impact of screening 75% of men with a prior syphilis infection every 3 months, and compared this to distributing equivalent numbers of tests each year to all MSM or men with the highest rate of partner change. We estimated incident and diagnosed early syphilis cases averted over a 10-year time horizon and prevalence at the end of the intervention period, relative to a base case of 30% of MSM screened annually.
Results: In the base case, re-infections accounted for 16% of diagnosed infections within 10 years. Targeted screening of men with a prior infection was expected to avert 48% of incident infections, 40% of diagnosed cases, and reduce early syphilis prevalence at the end of the intervention period by 83%, relative to the base case. General screening was projected to have the least impact, while targeted screening in men with high rates of partner change was projected to be most effective (averting 55% of incident cases). When frequency of enhanced screening was reduced to semi-annually or annually, focusing on men prior infection was more effective than allocating tests to men with high rates of partner change.
Conclusions: Targeted screening in MSM with a prior syphilis infection is a readily actionable intervention and is expected to reduce infection burden in the population, especially when identification of high-risk men via self-reported partner change and high-frequency screening among high-risk men are difficult to achieve.