SCREEN MORE OR SCREEN MORE OFTEN? USING MATHEMATICAL MODELS TO INFORM SYPHILIS CONTROL STRATEGIES

Purpose: We created a mathematical model of syphilis transmission dynamics to inform optimal syphilis screening strategies in urban areas in Ontario, Canada.

Method: Given that the syphilis resurgence among men who have sex with men (MSM) continues despite attempts at heightened screening and testing, we developed an agent-based dynamic model representing a core population of 2,000 MSM, forming a network of sexual contacts along which syphilis transmission can occur. Model parameters describing the epidemiology of the current epidemic and syphilis disease natural history were drawn from Ontario surveillance data supplemented by literature-derived estimates. Model outputs for the pre-intervention period were compared to surveillance data to identify credible simulations. A total of 380 to 405 well-calibrated simulations were used for the analysis of each intervention. Evaluated strategies included: (i) increased frequency of syphilis screening; (ii) increasing coverage of annual syphilis screening; or (iii) a combination of (i) and (ii). Intervention impact was measured as the cumulative incidence of detected and total infectious syphilis cases per year over a 5-year time period.

Result:   Model outputs indicated that increasing frequency of syphilis screening to every three months was most effective in reducing reported and total infectious syphilis infections. By contrast, increasing test numbers by increasing the fraction of individuals tested, without increasing test frequency, resulted in no appreciable change in syphilis incidence, as the reduction in the number of infectious individuals, due to treatment, was counterbalanced by increased infectious syphilis in individuals who had previously had latent (non-infectious) infection.

Conclusion:   Our model reproduced the (counterintuitive) persistence of elevated syphilis incidence that has been noted empirically in the face of screening “blitzes” targeting MSM at high risk of infectious syphilis.  By contrast, strategies that focus on higher frequency of testing in smaller fractions of the population were more effective in reducing syphilis incidence in a simulated MSM population. These findings highlight how treatment-induced loss of protective immunity creates nuances in screening-based control strategies.