A general framework to support cost-efficient survey design choices for the control of soil-transmitted helminths when deploying Kato-Katz thick smear
Authors: Kazienga Adama , Levecke Bruno , Leta Gemechu Tadesse , de Vlas Sake J. , Coffeng Luc E. 
Published: February 2023

Background

To monitor and evaluate soil-transmitted helminth (STH) control programs, the World Health Organization (WHO) recommends screening stools from 250 children across 5 schools, deploying Kato-Katz thick smear (KK). However, it remains unclear whether these recommendations are sufficient to make adequate decisions about stopping preventive chemotherapy (PC) (prevalence of infection <2%) or declaring elimination of STH as a public health problem (prevalence of moderate-to-heavy intensity (MHI) infections <2%).

Methodology

We developed a simulation framework to determine the effectiveness and cost of survey designs for decision-making in STH control programs, capturing the operational resources to perform surveys, the variation in egg counts across STH species, across schools, between and within individuals, and between repeated smears. Using this framework and a lot quality assurance sampling approach, we determined the most cost-efficient survey designs (number of schools, subjects, stool samples per subject, and smears per stool sample) for decision-making.

Principal findings

For all species, employing duplicate KK (sampling 4 to 6 schools and 64 to 70 subjects per school) was the most cost-efficient survey design to assess whether prevalence of any infection intensity was above or under 2%. For prevalence of MHI infections, single KK was the most cost-efficient (sampling 11 to 25 schools and 52 to 84 children per school).

Conclusions/Significance

KK is valuable for monitoring and evaluation of STH control programs, though we recommend to deploy a duplicate KK on a single stool sample to stop PC, and a single KK to declare the elimination of STH as a public health problem.

Author summary

Worldwide large-scale deworming programs are implemented to reduce the morbidity attributable to intestinal worms in school children. To monitor and evaluate the progress towards the programs goals, Word Health Organization (WHO) has developed both a survey design and a corresponding decision tree based on the prevalence and intensity of infections. However, these programs operate in resource-constrained countries, and hence it is crucial to minimize the operational costs to survey worm infections while ensuring the correctness of the program decision. To further support WHO in more evidence-based recommendations for cost-efficient decision-making, we developed a general framework that captures both the operational resources to perform surveys and the variation in test results when deploying the current diagnostic standard. Subsequently, we determined the most cost-efficient survey design to decide to stop the deworming programs and to verify whether the morbidity attributable to intestinal worms has been eliminated as a public health problem. Generally, we found that the current WHO-recommended survey design may not allow for optimal decision making. Based on our results, we proposed alternative survey designs for each of the worm species and program targets.