Community-level epidemiology of soil-transmitted helminths in the context of school-based deworming: Baseline results of a cluster randomised trial on the coast of Kenya

Most epidemiological studies of soil-transmitted helminth (STH) infections focus on school-going children. The majority of large-scale cross-sectional and longitudinal community-based studies have been conducted prior to the implementation of wide-scale mass drug administration (MDA). This study investigates age-related patterns in prevalence and intensity of STH infection, and associated risk factors, in a region of south coastal Kenya that had previously received three consecutive years of school-based deworming (2012–14) and four rounds of community-based MDA for lymphatic filariasis between 2003 and 2014. Between March and May 2015, a cross-sectional survey was conducted in 120 community clusters as a baseline for a cluster randomised trial. Individuals aged two years and above were randomly selected during household surveys and requested to provide stool samples, which were assessed for presence and intensity of STH using the duplicate Kato-Katz thick smear method. Species-specific predictors of presence and intensity were investigated through multilevel logistic regression and zero-inflated negative binomial regression models. Of the 19,684 individuals who provided a stool sample, 21.5% were infected with at least one STH. Hookworm was the predominant species, with an overall prevalence of 19.1%; Trichuris trichiura prevalence was 3.6% and Ascaris lumbricoides was negligible (0.4% prevalence). The vast majority were light intensity infections. Prevalence and intensity of hookworm infection were significantly higher in adults and males, and were associated with environmental conditions, low socio-economic status, household flooring, individual and household water, sanitation and hygiene (WASH) characteristics and behaviours, previous treatment, lack of shoe-wearing and not attending school. In contrast, T. trichiura was more commonly found in school-aged boys and those living in communities close to the coast, with reduced infection in the least poor individuals with private latrine access. Overall, results show that despite several years of school-based deworming, hookworm infection remains common among untreated adults in this population, suggesting that this strategy alone is insufficient to reduce community-wide hookworm infection and in the longer term to eliminate transmission. Trial registration ClinicalTrials.gov NCT02397772
Author summary Soil-transmitted helminth (STH) infections, including Trichuris trichiura, Ascaris lumbricoides and the hookworms Ancylostoma duodenale and Necator americanus, remain endemic in many regions of sub-Saharan Africa (SSA), including parts of Kenya. The current WHO-recommended treatment strategy focuses on morbidity control, and comprises periodic deworming of population groups at particular risk, including pre-school and school–age children. Consequently, the majority of epidemiological descriptions of STH have focused on infection in this age group, and are conducted using a school platform. There is therefore a notable lack of age-stratified data (that includes adults) from communities. We present data from a community-wide, cross-sectional survey of STH infection across 19,684 individuals and investigate associated risk factors at the household and individual level. We demonstrate highest prevalence and intensity of hookworm (the predominant species in this setting) in adults and males—a group not routinely included in deworming activities. There was marked geographic variation in infection risk across the study area, and an enduring relationship between infection risk and factors associated with poor access to sanitation and hygiene. Conducted after three years of ongoing, annual school-based deworming, the findings presented here are likely to be representative of many regions of SSA, with similar epidemiological and demographic profiles, implementing the current WHO-recommended STH control strategy.