Investigating the Effectiveness of Current and Modified World Health Organization Guidelines for the Control of Soil-Transmitted Helminth Infections

Abstract

Background: Considerable efforts have been made to better understand the effectiveness of large-scale preventive chemotherapy therapy for the control of morbidity caused by infection with soil-transmitted helminths (STHs): Ascaris lumbricoides, Trichuris trichiura, and the 2 hookworm species, Necator americanus and Ancylostoma duodenale. Current World Health Organization (WHO) guidelines for STH control include mass drug administration (MDA) programs based on prevalence measurements, aiming at reducing morbidity in pre-school-aged children (pre-SAC) and school-aged children (SAC) by lowering the prevalence of moderate- to heavy-intensity infections to <1%.

Methods: We project the likely impact of following the current WHO guidelines and assess whether the WHO morbidity goals will be achieved across a range of transmission settings. We also investigate modifications that could be made to the current WHO treatment guidelines, and project their potential impacts in achieving morbidity and transmission control.

Results: While the standard guidelines are sufficient at low transmission levels, community-wide treatment (ie, involving pre-SAC, SAC, and adults) is essential if WHO morbidity goals are to be met in moderate- to high-transmission settings. Moreover, removing the recommendation of decreasing the treatment frequency at midline (5-6 years after the start of MDA) further improves the likelihood of achieving morbidity control in SAC.

Conclusions: We meld analyses based on 2 mathematical models of parasite transmission and control by MDA for the dominant STH species, to generate a unified treatment approach applicable across all settings, regardless of which STH infection is most common. We recommend clearly defined changes to the current WHO guidelines.

Figure 1.

World Health Organization (WHO) decision tree showing the current WHO guidelines to achieve morbidity control in school-aged children (SAC) using 75% coverage in SAC (black and orange boxes). The blue boxes represent the modified guidelines assuming 75% community-wide coverage (pre-SAC, SAC, and adults) to replace the orange boxes (adapted from [1]).

Figure 2.

Prevalence of moderate to heavy soil-based helminth infection in school-aged children (SAC) after 10 years of treatment under current guidelines assuming 75% coverage in SAC, highlighted by achievement (green) or otherwise (red) of 1% prevalence goal, or varying predictions between models (yellow), in various transmission settings. Results in each cell have Imperial College London results at the top and Erasmus Medical Center Rotterdam below. Figures in parentheses are the probability of measuring a prevalence of medium- and high-intensity infections <1% from a sample of all SAC. Decisions are based on measured prevalence of infection and correspond with World Health Organization recommendations [1]. Abbreviation: MDA, mass drug administration.

Figure 3.

Prevalence of moderate to heavy soil-transmitted helminth infection in school-aged children (SAC) after 10 years of treatment, under recommended updates to treatment guidelines of 75% community-wide coverage. Results in each cell have Imperial College London (ICL) results at the top and Erasmus University Medical Center (EMC) below. Figures in parentheses are the probability of measuring a prevalence of medium- and high-intensity infections <1% from a sample of all SAC. Highlighted by achievement (green) or otherwise (red) of 1% prevalence goal, or varying predictions between models (yellow), in various transmission settings. Results in each cell have ICL results at the top and EMC below. Figures in brackets are the probability of measuring a prevalence of medium- and high-intensity infections <1% from a sample of all SAC. Decisions are based on measured prevalence of infection and correspond with World Health Organization recommendations [1]. The gray box indicates that no villages fell into those categories in either EMC or ICL models. Abbreviation: MDA, mass drug administration.