Soil-transmitted helminthiasis

摘要

Causative agents. There are four main nematode species of human soil-transmitted helminths (STHs): Ascaris lumbricoides (round-worm), Trichuris trichiura (whipworm), Ancylostoma duodenale and Necator americanus (hookworm). After infective larvae are ingested with the eggs (A. lumbricoides and T. trichiura) or penetrate the skin (hookworms), they moult and develop into adult worms directly in the large intestine (T. trichiura) or in the small intestine after migration through the respiratory tract and subsequent swallowing (A. lumbricoides and hookworms). The adult worms mate and produce large numbers of eggs (A. lumbricoides produce up to 200,000 per day, T. trichiura and hookworms produce 5,000-20,000 per day) after 4-6 weeks. A. duodenale larvae can undergo a hypobiosis state in the human body under certain conditions for several months. Eggs can remain viable in the soil for several months (A. lumbricoides and T. trichiura) and larvae several weeks (hookworms) in appropriate conditions. Adult worms in infected hosts feed on nutrients, proteins and blood (hookworms) and can cause intestinal obstruction (A. lumbricoides). STHs contribute to malnutrition, iron-deficiency anaemia and impaired cognitive performance. They do not multiply in the host and the parasite density is dependent on the time and extent of exposure. Morbidity is strongly associated with the intensity of infection, which peaks in school-age years and is then reduced by lower exposure and increased immune response.