Abstract
BackgroundThe current World Health Organization (WHO) target for the three major soil-transmitted helminth (STH) infections is to reduce prevalence of moderate-to-heavy infections to below 1% by 2020. In terms of monitoring and evaluation (M&E), the current WHO guidelines for control of STHs recommend evaluation of infection levels in school-age children (SAC) after five to six years of preventive chemotherapy (PC), using the standard Kato-Katz faecal smear. Here, we assess the predictive performance of various sampling designs for the evaluation of the morbidity target.Methodology/Principal findingsUsing two mathematical models for STH transmission and control, we simulate how the number of villages and SAC sampled affect the ability of survey results in sentinel villages to predict the achievement of the morbidity target in PC implementation units (e.g. districts). As PC is stopped when the prevalence of infection in SAC in sentinel villages is less than 1%, we estimate the positive predictive value (PPV) of this indicator for meeting the morbidity target in the whole district. The PPV varies by species and PC strategy, and it is generally higher in areas with lower pre-control prevalence. Sampling a fixed number of SAC spread out over 10 instead of 5 sentinel villages may increase the PPV by up to 20 percentage points. If every SAC in a village is tested, a higher number of villages may increase the PPV by up to 80 percentage points. Increasing the proportion of SAC tested per village does not result in a relevant increase of PPV.Conclusions/SignificanceAlthough the WHO guidelines provide a combined strategy to control the three STH species, the efficacy of PC strategies clearly differs by species. There is added value in considering more villages within implementation units for M&E of morbidity targets, the extent varying by STH species. A better understanding of pre- and post-control local STH prevalence levels is essential for an adequate M&E strategy including the definition of morbidity targets at the appropriate geographical scale.
Highlights
1.5 billion people are infected with soil-transmitted helminths (STHs) worldwide [1]
We find that the efficacy of preventive chemotherapy (PC) strategies differs significantly by species and pre-control infection levels
Achieving 1% prevalence of infection in sentinel villages may still imply a prevalence of moderate-to-heavy infections >1% in the entire PC implementation unit
Summary
1.5 billion people are infected with soil-transmitted helminths (STHs) worldwide [1]. The World Health Organization (WHO) global target for STHs is to eliminate morbidity in high-risk groups by 2020, defined as achieving less than 1% prevalence of moderate-to-heavy infections among pre-school-age children (preSAC, age 2–5), school-age children (SAC, age 5–14), and women of childbearing age [2]. Current WHO treatment guidelines recommend annual or semi-annual preventive chemotherapy (PC) using single-dose albendazole or mebendazole with a coverage of at least 75% of the aforementioned risk populations [1]. The current World Health Organization (WHO) target for the three major soil-transmitted helminth (STH) infections is to reduce prevalence of moderate-to-heavy infections to below 1% by 2020. In terms of monitoring and evaluation (M&E), the current WHO guidelines for control of STHs recommend evaluation of infection levels in school-age children (SAC) after five to six years of preventive chemotherapy (PC), using the standard Kato-Katz faecal smear. We assess the predictive performance of various sampling designs for the evaluation of the morbidity target.
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