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

BACKGROUND: Surveys to monitor large-scale deworming programs against soil-transmitted helminthiases (STH) involve collection and examination of stool samples from schoolchildren. These surveys are resource demanding and interrupt school activities. A potentially cost-saving alternative that does not involve children is to process fecal sludge samples from school pit latrines. As a first step towards 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. METHODS: First, we 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 that were collected from six primary schools in Ethiopia at different pit latrine depths used by boys and girls and that were repeatedly examined. Finally, field data was used to inform an egg count simulation model to quantify the variation in egg counts and to determine the sampling and analysis strategies that resulted in surveys as precise as stool-based surveys. RESULTS: The modified fecal sludge sampling prototypes were generally successful, except for a few pit latrines with dried/solid fecal sludge and insufficient sludge volume. The egg counting method had moderately high analytical sensitivity that varied across the consistency of the samples and examination effort. 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. CONCLUSIONS: We developed a sampling and diagnostic strategy that we will use in a follow-up study which 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.