Development, field-testing and optimization of tools to quantify soil-transmitted helminths in fecal sludge from school pit latrines in Ethiopia

Abstract

BackgroundSurveys to monitor large-scale deworming programs against soil-transmitted helminthiases (STH) involve examination of stool samples from schoolchildren. These surveys are resource demanding and impact school activities. A potentially cost-saving alternative that does not involve children is to process fecal sludge samples from school pit latrines. To provide a proof-of-principle of latrine-based monitoring of STH programs, we optimized tools to collect fecal sludge and to quantify STH eggs in the samples. MethodsFirst, we designed, developed and field-tested three locally made fecal sludge sampling prototypes. Second, we developed a modified egg-counting method and conducted spiking experiments to explore its analytical performance. Third, we estimated the variation in egg counts in fecal sludge samples collected from six primary schools in Ethiopia at different pit latrine depths used by boys and girls and by repeatedly examining samples. Finally, field data were used to inform an egg count simulation model to quantify this variation in egg counts and to determine the sampling and analysis strategies that resulted in surveys as precise as a stool-based survey. ResultsThe modified fecal sludge sampling prototypes were generally successful, except for a few pit latrines with dried/solid sludge types and insufficient sludge volume. The egg-counting method had moderately high analytical sensitivity that varied across the consistency of the samples. The variation in egg counts was mainly explained by differences between squat holes followed by repeated fecal sludge sample processing. Latrine-based surveys were as precise as stool-based surveys only for Ascaris and when the intensity of infections was low. ConclusionsWe developed a sampling and diagnostic strategy that we will use in a follow-up study. This study will be conducted in Jimma Zone across 25 schools (52 children per school) and will compare the mean fecal sludge egg counts at school level with the STH prevalence in children. Author summaryProgress of large-scale deworming programs are currently monitored through screening individual stool samples of schoolchildren. A latrine-based program monitoring, without active participation of schoolchildren and interruption of routine school activities, is a potentially cost-effective alternative. As a proof-of-concept, we developed a method to quantify worm eggs in fecal sludge samples, conducted spiking experiments to determine its analytical performance and applied the method across schools in Ethiopia to explore variation in egg counts (e.g., squat holes, depth of sample collection, and repeated analyses). Based on our findings, we then determined the sample collection and analysis strategy that results a latrine-based survey as precise as a survey based on screening individual stool samples. The analytical performance was moderately high but varied across the consistency of the samples. The variation in egg counts was mainly driven by variation between squat holes on the same pit latrine, and thus it is better to sample more squat holes (at least 3) than repeatedly processing the same fecal sludge sample (one examination is sufficient). We will now apply this sample collection and analysis across 25 schools (52 children per school) in Ethiopia and compare it with a survey based on screening individual stool samples.