Intersecting Risk Factors Associated With High Syphilis Seroprevalence Among a Street-Involved Population in Canada.

Background: Syphilis has reemerged as a global public health concern. In Ontario, Canada's most populous province, a 340% increase in infectious syphilis cases was observed between 2013 and 2023. This surge was accompanied by a demographic shift, with women emerging as the fastest-growing at-risk group. We examined intersecting risk factors associated with syphilis seropositivity among a street-involved population. Methods: Data were collected from the Syphilis Point of Care Rapid Test and Immediate Treatment Evaluation (SPRITE) study-an outreach model of care implemented by 8 public health units (PHUs) across Ontario between 2023 and 2024. Reactive treponemal antibodies defined syphilis seroprevalence. A mixed-effects regression with a log-binomial distribution was used to evaluate the association between risk factors and seropositivity. Adjusted prevalence ratio (aPR) controlled for age and sex and clustering by PHUs. Results: A total of 630 participants, 42% women, with a median age of 38, were included; 19.1% of participants reported having sexual risk factors, using illicit drugs, and being un(der)housed. Overall, syphilis seroprevalence was 7.6% (95% confidence interval 5.5-9.7), with significant heterogeneity across the province and higher among those reporting 3 risk factors (19.2% [11.2-29.7]) compared with 1 risk factor (4.8% [1.8-10.1]). Seropositivity was higher among women (aPR 1.62 [.94-2.80]) and people who use illicit drugs (aPR 2.30 [.93-5.50]), particularly those who use crystal methamphetamine (aPR 2.88 [1.31-6.33]). Conclusions: Syphilis is heightened at the intersection of sexual risk factors, illicit drug use, and housing instability among equity-deserving populations. Targeted outreach models of care are necessary to reach this emerging at-risk population.