The prevalence of gestational syphilis in Malawi between 2014 and 2022: spatiotemporal modeling of population-level factors

Abstract

Background: Mother-to-child transmission of syphilis remains high especially in the WHO AFRO region with a prevalence of 1.62%, resulting in a congenital syphilis rate of 1,119 per 100,000 live births. Elimination efforts can be supported by an understanding of the spatial and temporal changes in disease over time, which can identify priority areas for targeted interventions aimed at reducing transmission.

Methods: We collated routine surveillance data from health facilities and covariate data from demographic and health surveys conducted in Malawi between 2014 and 2022. We fitted a Bayesian hierarchical mixed model with spatial and temporally structured random effects to model the district-level monthly counts of maternal syphilis notifications as a function of individual- and district-level predictors. We then generated district-level spatiotemporally explicit risk profiles to estimate the effect of individual- and district-level covariates on maternal syphilis notifications and to identify hotspot areas.

Results: Overall, the national prevalence of maternal syphilis increased from 0.28% (95% CI: 0.27-0.29%) in 2014 to peaking in 2021 at 1.92% (95% CI: 1.89-1.96%). Between 2020 and 2022, there was a decline in prevalence, with the most significant decline seen in Zomba District (1.40, 95% CI: 1.12-1.66%). In regression models, a one percentage point increase in district-level antenatal HIV prevalence was associated with increased maternal syphilis (prevalence ratio [PR]: 1.15, 95% credible interval [CrI]: 1.10-1.21). There was also an increased prevalence of maternal syphilis associated with an increased district-level mean number of sex partners (PR: 1.05, 95% CrI: 0.80-1.37). The number of districts with a high prevalence of maternal syphilis also increased between 2014 and 2022, especially in the southern region, where most had a high probability (approaching 100%) of having high maternal syphilis (defined as relative risk >1 compared to the standard population of women aged 15-49 years) in 2022.

Conclusion: Maternal syphilis prevalence in Malawi shows an increasing upward trend, with an estimated six times relative increase between 2014 and 2022 (0.28% to 1.73%) and strong associations with higher district-level HIV prevalence. Controlling syphilis depends on reaching vulnerable populations at the sub-national level, which may be disproportionately affected. Our findings support the move to integrate the elimination of mother-to-child transmission (EMTCT) of syphilis programs with existing prevention of mother-to-child transmission (PMTCT) of HIV programs.

Keywords: HIV; eMTCT; maternal syphilis; spatio-temporal model; syphilis prevalence.

Figure 1

Empirical average monthly maternal syphilis prevalence among pregnant women attending ANC who completed testing for syphilis and were recorded in HMIS over the entire study period from 2014 to 2022. The prevalence was calculated by dividing the number of positive syphilis cases with the population of women registered for ANC.

Figure 2

District-level summary of maternal syphilis empirical prevalence for the period of 2014–2022. The prevalence was calculated by dividing the number of positive syphilis cases with the population of women registered for ANC.

Figure 3

Standardized incidence ratio (SIR) estimates for maternal syphilis for the period of 2014–2022. Values SIR >1 indicate increased risk, while SIR <1 indicates decreasing risk of maternal syphilis.

Figure 4

Predicted district maternal syphilis prevalence ratio (PR) over the period of 2014–2022. The predicted values are obtained from the fitted model. Darker colors on the map indicate districts with a high predicted prevalence ratio, while lighter colors indicate districts with a lower predicted prevalence ratio.

Figure 5

Average yearly maternal syphilis risk for the entire country between 2014 and 2022 relative to the standard population of women aged 15–49 years in the corresponding year. Values greater than 1 indicate a higher risk compared to the standard population.

Figure 6

Posterior exceedance probabilities that the risk is greater than 1 between 2014 and 2022. The probabilities were calculated from the model. Darker colors on the map indicate higher probabilities with values close to 1, especially in the southern region.