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. 2025 Aug 7;12(8):ofaf472.
doi: 10.1093/ofid/ofaf472. eCollection 2025 Aug.

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

Lucy Mackrell  1 , Jillian P Antoun  1 , Megan Carter  1   2 , Natasha Larkin  1   3 , Jessica Burnside  1 , Maggie Hoover  1 , Farhan Khandakar  1 , Patrick O'Byrne  4 , Casey Cassan  5 , Jennifer Burbidge  6 , Stephanie Vance  7 , Kandace Belanger  8 , Sahar Saeed  1 ; SPRITE study
Collaborators, Affiliations

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

Lucy Mackrell et al. Open Forum Infect Dis. .

Abstract

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.

Keywords: people who are unhoused; people who use drugs; seroprevalence; street-involved population; syndemic; syphilis; women.

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Conflict of interest statement

Potential conflicts of interest. S. S. has a perceived conflict of interest; she received an honorarium from Novo Nordisk as an advisory board member (unrelated to this study). All other authors declare: no support from any organization for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.

Figures

Figure 1.
Figure 1.
Dataset creation flowchart. For participants with repeat observations, we retained the last observation with a syphilis serology result.
Figure 2.
Figure 2.
Map of Ontario segmented by Public Health Units (PHUs), highlighting the locations of participating PHUs and their respective study seroprevalence.
Figure 3.
Figure 3.
A, The number (N) of participants by self-reported risk factors, reflected by circle size. B, Syphilis seroprevalence (%) by risk factor(s).
See this image and copyright information in PMC

References

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