The population-level impact of introducing rapid diagnostic tests on syphilis transmission in Canadian arctic communities - a mathematical modeling study

Abstract

Background: Canadian Arctic communities have experienced sustained syphilis transmission, with diagnoses rates 18-times higher than the national average. Remoteness from laboratory facilities leads to delays between syphilis screening and treatment, contributing to onward transmission. Rapid diagnostic tests can eliminate treatment delays via testing at the point-of-care. This study aims to describe syphilis diagnostic gaps and to estimate the impact of introducing rapid diagnostic tests at the point-of-care on syphilis transmission.

Methods: To assess the population-level impact of deploying rapid diagnostic tests, an individual-based model was developed using detailed surveillance data, population surveys, and a prospective diagnostic accuracy field study. The model was calibrated to syphilis diagnoses (2017-2022) from a community of approximately 1,050 sexually active individuals. The impacts of implementing rapid diagnostic tests using whole blood (sensitivity: 92% for infectious and 81% for non-infectious syphilis; specificity: 99%) from 2023 onward was calculated using the annual median fraction of cumulative new syphilis infections averted over 2023-2032.

Findings: The median modeled syphilis incidence among sexually active individuals was 44 per 1,000 in 2023. Males aged 16-30 years exhibited a 51% lower testing rate than that of their female counterparts. Maintaining all interventions constant at their 2022 levels, implementing rapid diagnostic tests could avert a cumulative 33% (90% credible intervals: 18-43%) and 37% (21-46%) of new syphilis infections over 5 and 10 years, respectively. Increasing testing rates and contact tracing may enhance the effect of rapid diagnostic tests.

Interpretation: Implementing rapid diagnostic tests for syphilis in Arctic communities could reduce infections and enhance control of epidemics. Such effective diagnostic tools could enable rapid outbreak responses by providing same-day testing and treatment at the point-of-care.

Funding: Canadian Institutes of Health Research.

Keywords: Inuit health; Mathematical modelling; Rapid diagnostic tests; Sexual health; Syphilis.

Fig. 1

Schematic diagram of the individual-based model's structure for the natural history of syphilis and its testing and treatment cascade. Where ${\mu }_a$ is the age-specific all-cause mortality rate; ${\lambda }_t$ is the individual's probability of infection; $p^{import}$ is the probability of importing syphilis from another community; ${\sigma }_{i\in (1,2,..,7)}$ is duration of each syphilis disease stage; ${\varphi }^{recurrent}$ is the proportion of early latent cases that progress through the recurrent stage; ${\psi }_{a,t,s}^{test}$ is the probability of testing for syphilis, which varies by age, time, and sex; ${\varsigma }_k$ is the sensitivity of the conventional syphilis testing assay for each disease stage k; ${\pi }^{treatment}$ is the probability that treatment is successful; and ${\tau }_{i\in (1,2)}$ is the duration of treatment for patients initiated at different syphilis disease stage.

Fig. 2

Observed (red dots) and modeled (lines) number of syphilis diagnoses (1st row) overall, by sex, and (2nd row) by broad age groups in the study community located in Nunavik (Québec, Canada). The observed number of cases are from the Nunavik syphilis surveillance database and are used as a calibration outcome. The lines correspond to the median of the simulations and the shaded areas to the 90% credible intervals.

Fig. 3

Number of diagnosed syphilis cases by sex, broad age groups, and year (2017–2022) in the study community located in Nunavik (Québec, Canada). Data source: Nunavik syphilis surveillance database.

Fig. 4

Annual fractions of cumulative syphilis infections averted 2023–2032 by implementing rapid diagnostic tests since 2023 in the study community located in Nunavik (Québec, Canada). Dots represent the median and the error bars the 90% credible intervals.

Fig. 5

Differences between fractions of cumulative syphilis infections averted under different scenarios with varying contact tracing rate and testing rate, as compared to the fraction with the same testing rates and probability of contact tracing in 2022. The x-axis represents the absolute variation (in percentage-points) in the averted fraction of syphilis infections over 5 years.