Abstract
Soil-transmitted helminths (STHs) are a group of parasitic worms that infect humans, causing a wide spectrum of disease, notably anaemia, growth retardation, and delayed cognitive development. The three main STHs are Ascaris lumbricoides, Trichuris trichiura and hookworm ( Necator americanus and Ancylostoma duodenale). Approximately 1.5 billion people are infected with STHs worldwide. The World Health Organization goal for 2030 is morbidity control, defined as reaching <2% prevalence of medium-to-high intensity infections in preschool-age children and school-age children (SAC). Treatment guidelines for achieving this goal have been recommended. The Neglected Tropical Diseases Modelling Consortium has developed mathematical and statistical models to quantify, predict, and evaluate the impact of control measures on STHs. These models show that the morbidity target can be achieved following current guidelines in moderate prevalence settings (20-50% in SAC). In high prevalence settings, semi-annual preventive chemotherapy (PC) ideally including adults, or at least women of reproductive age, is required. For T. trichiura, dual therapy with albendazole and ivermectin is required. In general, stopping PC is not possible without infection resurgence, unless effective measures for improved access to water, hygiene, and sanitation have been implemented, or elimination of transmission has been achieved. Current diagnostic methods are based on egg counts in stool samples, but these are known to have poor sensitivity at low prevalence levels. A target threshold for novel, more sensitive diagnostics should be defined relative to currently preferred diagnostics (Kato-Katz). Our analyses identify the extent of systematic non-access to treatment and the individual patterns of compliance over multiple rounds of treatment as the biggest unknowns and the main impediment to reaching the target. Moreover, the link between morbidity and infection intensity has not been fully elucidated. By providing more insights on all the above, we aim to inform discussions on the goals and treatment guidelines for STHs.
Highlights
Soil-transmitted helminth (STH) infections are caused by several species of parasitic worms that are transmitted by eggs present in human faeces, which contaminate the soil in areas with poor sanitation
The morbidity target is defined in terms of the prevalence of medium-to-high intensity (M&HI)
Infection intensity does not necessarily reflect morbidity accurately, as light infections can be associated with non-negligible morbidity and the severity of symptoms associated with M&HI is highly variable[29]
Summary
Soil-transmitted helminth (STH) infections are caused by several species of parasitic worms that are transmitted by eggs present in human faeces, which contaminate the soil in areas with poor sanitation. STHs cause some of the most common infections, with about 1.5 billion people infected worldwide[1]. The three main STHs are roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura) and hookworm (Necator americanus and Ancylostoma duodenale). STHs reduce the nutritional status of infected individuals[1]. Infected children can be affected by reduced physical fitness and impaired growth and cognitive development[1]. Infections with A. lumbricoides and hookworms can be treated effectively with benzimidazole drugs (albendazole, mebendazole). Benzimidazoles are less effective against T. trichiura. Dual treatment with albendazole and ivermectin increases treatment efficacy for T. trichiura[2,3,4]. Albendazole and mebendazole are donated to the World Health Organization (WHO) for distribution to affected populations
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