The soil-transmitted helminths (STH, also called geohelminths) are a group of intestinal parasites belonging to the class Nematoda that are transmitted primarily through contaminated soil. They are so called because they have a direct life cycle which requires no intermediate hosts or vectors, and the parasitic infection occurs through faecal contamination of soil, foodstuffs and water supplies. The adult forms are essentially parasites of humans, causing soil-transmitted helminthiasis, but also infect domesticated mammals. The juveniles are the infective forms and they undergo tissue-migratory stages during which they invade vital organs such as lungs and liver. Thus the disease manifestations can be both local and systemic. The geohelminths together present an enormous infection burden on humanity, amounting to 135,000 deaths every year, and persistent infection of more than two billion people.
STHs are caused by four groups of nematodes, namely
- ascaris (Ascaris lumbricoides)
- whipworm (Trichuris trichiura)
- hookworms (Ancylostoma duodenale and Necator americanus)
- pinworm/threadworm Strongyloides stercoralis
Soil-transmitted helminthiasis (STH) is a collective name for the diseases caused by ascaris, whipworm and hookworms in humans. It includes species-specific diseases such as
- ascariasis, which is caused by A. lumbricoides
- hookworm diseases (ancylostomiasis and necatoriasis), which are caused by N. americanus and A. duodenale
- trichuriasis, which is caused by T. trichiura
This is caused by S. stercoralis. Even though the disease is principally a STH, the infection being mediated through soil, it is however generally omitted in clinical practices and control programmes because of its (allegedly) relatively less significant influence on health and socio-economic conditions. Also it is not restricted to humans, as it is common in pets. But there is an emerging hyperinfection syndrome caused by S. stercoralis, which exhibits a high mortality rate (15% to 87%).
Geohelminth infection is a major health problem particularly in rural areas of developing countries like Subsaharan Africa, India and other Southeast Asian countries. It is an important cause of morbidity in school age children who harbour the highest intensity of worm infestation. Some of the significant morbidity attributed to intestinal helminthiasis are malnutrition, growth retardation, anaemia, vitamin A deficiency and impaired intellectual performance.
Interventions and chemotherapy
Prevention of infection is mainly based on sanitation, such as clean water, personal hygiene, and avoiding the use of uncomposted human faeces as fertilizer. The most important strategy of intervention involves mass drug administration, especially among school-age children. For this purpose, broad-spectrum benzimidazoles such as mebendazole and albendazole are the drugs of choice recommended by WHO. For stongyloidiasis, ivermectin is the recommended anthelmintic.
- STH is also used to abbreviate soil-transmitted helminthiasis.
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- London Declaration (2012) (30 January 2012). "London Declaration on Neglected Tropical Diseases". Retrieved 2013-03-26.
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- Centers for Disease Control and Prevention
- WHO information page
- WHO information on strongyloidiasis
- Strongyloidiasis at UpToDate (Wolters Kluwer Health
- ICEH Resources Information Sheet on strongyloidiasis
- USAID's NTD Program
- London Declaration Uniting to Combat NTDs
- Information at Right Diagnosis
- USAID's Neglected Tropical Diseases Programme
- Types of soil-transmitted helminths at Shinpoong