Abstract
Starting and stopping preventive chemotherapy (PC) for soil-transmitted helminthiasis is typically based on the prevalence of infection as measured by Kato-Katz (KK) fecal smears. Kato-Katz-based egg counts can vary highly over repeated stool samples and smears. Consequentially, the sensitivity of KK-based surveys depends on the number of stool samples per person and the number of smears per sample. Given finite resources, collecting multiple samples and/or smears means screening fewer individuals, thereby lowering the statistical precision of prevalence estimates. Using population-level data from various epidemiological settings, we assessed the performance of different sampling schemes executed within the confines of the same budget. We recommend the use of single-slide KK for determining prevalence of moderate-to-heavy intensity infection and policy decisions for starting and continuing PC; more sensitive sampling schemes may be required for policy decisions involving stopping PC. Our findings highlight that guidelines should include specific guidance on sampling schemes.
Highlights
Soil-transmitted helminthiases (STHs), caused by Ascaris lumbricoides, Trichuris trichiura, and hookworms, affect approximately 1.5 billion people worldwide
We show that when designing surveys for assessing the epidemiological situation of STH within a finite budget, a trade-off exists between the sensitivity of the survey to detect infection and the statistical precision of survey results
Because prevalence estimates depend on the choice of sampling scheme, it is important that threshold values for policy decision regarding starting, changing, or stopping preventive chemotherapy (PC) are defined in conjunction with a clearly specified survey strategy, ie, with explicit mention of the numbers of samples per person and slides per sample tested
Summary
Soil-transmitted helminthiases (STHs), caused by Ascaris lumbricoides, Trichuris trichiura, and hookworms, affect approximately 1.5 billion people worldwide. The WHO further recommends that decisions on starting, continuing, and stopping PC are based on estimates of STH prevalence (any intensity) in school-age children as measured by the classic parasitological Kato-Katz (KK) fecal smear [1,2,3]. Kato-Katz-based egg counts vary highly within individuals over repeated stools samples from different days and between repeated slides based on the same stool sample. Surveys based on multiple stool samples per individual and/or multiple slides per stool sample typically yield higher prevalence estimates than when examining 1 slide per person [4]. We do so by bootstrap analysis of existing datasets from different epidemiological settings where multiple stool samples were collected and/or stool samples were examined multiple times
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