Soil-transmitted helminths: Ascaris, Trichuris and hookworm infections.

The soil‐transmitted helminths (STH) are a group of parasitic worms that require soil for development and share humans as a definitive host. They are comprised of Ascaris lumbricoides, Trichuris trichiura, and the human hookworms, which include Necator americanus, Ancylostoma duodenale, and the increasingly recognized Ancylostoma ceylanicum. STH infections can be asymptomatic or cause a wide range of acute and chronic clinical symptoms. These include rash, diarrhea, abdominal pain, and, rarely, intestinal perforation or death. They also include more insidious sequelae of chronic infection such as neurologic and physical stunting, anemia, and malnutrition. Symptoms and severity are determined by both pathogen and host factors including the type and number of worms, existence of co‐infections, and the nutritional and immune status of the host.

An estimated 1.5 billion people are infected with at least one STH globally, with distribution heavily influenced by climate, geography, and the underlying economies and sanitation infrastructure in endemic countries. STHs disproportionately affect those living in low‐ and middle‐income countries lacking improved sewage and water systems, especially in tropical climates (which are the most suitable for parasite survival in the environment). A warming planet threatens to change the geographical habitat of STH and expose new populations to disease.

Optimal treatment regimens for the different soil‐transmitted helminths vary, but primarily involve oral albendazole or mebendazole. Single‐dose regimens have the highest success rates for Ascaris infections, while they are much less effective for Trichuris infections (which are much more effectively cured by longer treatment courses and combination antiparasitic therapy). Diagnosis is primarily by stool examination for characteristic eggs. In many cases, however, treatment is empiric, especially in endemic settings and during mass‐drug administration (MDA) campaigns. MDA campaigns are the mainstay of global efforts to control STH infections and have largely focused on treating school‐aged children who are the most heavily affected by STH. MDA decreases worm burden and individual studies have shown decreased morbidity. However, systematic reviews have not found robust morbidity benefits and there has been increasing interest for expanding MDA beyond school‐aged children, integrating STH control with other campaigns and concomitant investment in improving water, sanitation, and hygiene.