Estimating prevalence trends in adult gonorrhoea and syphilis in low- and middle-income countries with the Spectrum-STI model: results for Zimbabwe and Morocco from 1995 to 2016

Abstract

Objective: To develop a tool for estimating national trends in adult prevalence of sexually transmitted infections by low- and middle-income countries, using standardised, routinely collected programme indicator data.

Methods: The Spectrum-STI model fits time trends in the prevalence of active syphilis through logistic regression on prevalence data from antenatal clinic-based surveys, routine antenatal screening and general population surveys where available, weighting data by their national coverage and representativeness. Gonorrhoea prevalence was fitted as a moving average on population surveys (from the country, neighbouring countries and historic regional estimates), with trends informed additionally by urethral discharge case reports, where these were considered to have reasonably stable completeness. Prevalence data were adjusted for diagnostic test performance, high-risk populations not sampled, urban/rural and male/female prevalence ratios, using WHO's assumptions from latest global and regional-level estimations. Uncertainty intervals were obtained by bootstrap resampling.

Results: Estimated syphilis prevalence (in men and women) declined from 1.9% (95% CI 1.1% to 3.4%) in 2000 to 1.5% (1.3% to 1.8%) in 2016 in Zimbabwe, and from 1.5% (0.76% to 1.9%) to 0.55% (0.30% to 0.93%) in Morocco. At these time points, gonorrhoea estimates for women aged 15-49 years were 2.5% (95% CI 1.1% to 4.6%) and 3.8% (1.8% to 6.7%) in Zimbabwe; and 0.6% (0.3% to 1.1%) and 0.36% (0.1% to 1.0%) in Morocco, with male gonorrhoea prevalences 14% lower than female prevalence.

Conclusions: This epidemiological framework facilitates data review, validation and strategic analysis, prioritisation of data collection needs and surveillance strengthening by national experts. We estimated ongoing syphilis declines in both Zimbabwe and Morocco. For gonorrhoea, time trends were less certain, lacking recent population-based surveys.

Keywords: AFRICA; GONORRHOEA; MATHEMATICAL MODEL; SURVEILLANCE; SYPHILIS.

Figure 1

Spectrum-estimated national adult syphilis prevalences. Data are shown after adjustments for diagnostic test performance and missing high-risk populations, as described in the ‘Methods’ section. Regional estimates—shown for reference, but not used in the estimation process—are those by WHO for 2012 for both countries, and those from a meta-analysis of antenatal care (ANC) surveys in sub-Sahara African countries for 1990–1999 (put at 1996) and 2000–2011 (put at 2005) for Zimbabwe. Dashed lines represent 95% CIs around the Spectrum estimate. Data shown are for women aged 15–49 years; the estimated prevalence applies to both women and men 15–49 years.

Figure 2

Spectrum-estimated national gonorrhoea prevalence in women aged 15–49 years. Data are shown after adjustments for diagnostic test performance, urban versus rural prevalence ratios and missing high-risk populations, as described in the ‘Methods’ section. Regional estimates (included in the estimation at a weight of 1% relative to national studies) are from the WHO for 2012, and for Zimbabwe from a meta-analysis of antenatal care (ANC) surveys in sub-Sahara African countries for 1990–1999 (put at 1996) and 2000–2011 (put at 2005). Data and estimates shown are for women 15–49 years; for Zimbabwe, some additional data included in the estimation are not shown in the graph: three male survey data points from Zimbabwe; one male survey data point from South Africa; and 1 female data point from Mozambique in year 2000 at 26.8% test-adjusted prevalence; see online supplementary file 2c).