Soil-transmitted helminthiasis

Abstract

Soil-transmitted helminthiasis (STH) is a group of infections caused by Ascaris lumbricoides, Trichuris trichiura and Necator americanus and Ancylostoma duodenale. The risk of acquiring STH is significantly influenced by climatic and socio-economic factors including temperature, rainfall, occupation and availability of safe water and sanitation facilities. STH are transmitted through the parasites' eggs excreted in faeces of infected humans that enter the human body by ingestion or skin penetration. More than a billion people living in tropical and subtropical countries are currently infected with STH, with over 300 million of them suffering from morbidity. The epidemiology of STH is characterized by overdispersed distribution in the population; heavy-intensity infections are the major source of morbidity and these infections usually occur in children and women; treatment administered at regular intervals to these groups at risk will periodically decrease the worm burden of infected individuals and control morbidity. Soil-transmitted helminths can affect the nutritional status of their host by feeding on the content of the host's intestine or on host tissues (e.g. blood), impairing digestion or absorption of nutrients, causing an inflammatory response that leads to the production of substances affecting appetite, intake, metabolism and storage of micronutrients and eliciting fever and immune response, thus increasing consumption of energy. The burden of disease associated with STH is mainly attributable to their chronic and insidious impact on nutritional status, physical development and quality of life of those infected rather than to the overt morbidity or mortality they cause. The severity of the clinical manifestations mainly depends on the intensity of infection and on the underlying nutritional condition of the human host. Eggs of intestinal helminths are generally easy to identify because of their large size and distinctive morphology and their identification in stool specimens remains the most commonly used means of diagnosis, especially in developing countries. The Kato-Katz technique direct smears in saline or concentration techniques (such as formol-ether) can be alternatively used. WHO-recommended treatment options include albendazole, mebendazole, levamisole and pyrantel. All the mentioned medicines are to be administered orally. These drugs are effective, safe and extremely low cost. For these reasons, in area of high endemicity WHO recommends preventive chemotherapy; this measure consists on the large-scale distribution of anthelmintic drugs to population groups at risk. It has been demonstrated that treatment at regular intervals, when started early in life, protects children from the worst consequences of STH and prevents the development of complications associated with heavy-intensity infections. One of the risks of this approach is the development of resistance to anthelmintic drugs in targeted parasites; as of today, however, scientific investigations have not demonstrated any presence of resistance among helminths that infect humans. To further increase cost efficacy of the control activities WHO supports and endorses the development of multi-disease national plans of action (e.g. addressing also the control of schistosomiasis and lymphatic filariasis) implemented in integrated way and complemented by other public health interventions such as the provision of health education and the improvement of the sanitation infrastructure. WHO envisages a world free of childhood morbidity due to STH. The goal is to reduce morbidity from STH among preschool-age children (aged 1-4 years) and school-age children (5-14 years) to a level below which it would not be considered a public health problem, i.e. when the level of prevalence of STH infections of moderate and high intensity among school-age children is <1%. This target is achievable; in 2012 over 260 million of children have been treated with preventive chemotherapy for the control of STH and the large drug donations by pharmaceutical companies and the support provided by international donors are sustaining the efforts of endemic countries.