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The soil-transmitted helminths (also called geohelminths) are a group of intestinal parasites belonging to the phylum 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 (STH), 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.^[1]^[2]

Species

Soil-transmitted helminths are typically from the following families of nematodes, namely:^{[citation needed]}

Roundworms (family Ascarididae), e.g. Ascaris lumbricoides
Whipworms (family Trichuridae), e.g. Trichuris trichiura
Hookworms (family Ancylostomatidae), e.g. Ancylostoma duodenale and Necator americanus
Threadworms (family Strongyloididae), e.g. Strongyloides stercoralis^[3]

Diseases

Soil-transmitted helminthiasis

Soil-transmitted helminthiasis is a collective name for the diseases caused by ascaris, whipworm and hookworms in humans. It includes species-specific diseases such as^{[citation needed]}

Ascariasis, which is caused by Ascaris lumbricoides
Hookworm diseases (ancylostomiasis and necatoriasis), which are caused by Necator americanus and Ancylostoma duodenale
Trichuriasis, which is caused by Trichuris trichiura

Soil-transmitted helminthiasis is classified as one of the neglected tropical diseases projected to be controlled/eradicated by 2020 through the London Declaration on Neglected Tropical Diseases.^[4]

Strongyloidiasis

This is caused by Strongyloides stercoralis. Even though the disease is principally a soil-transmitted helminthiasis, the infection being mediated through contaminated 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%).^[5]^[6]

General impact

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.^[2]^[7]

Control activities

The World Health Organization is promoting the control of soil transmitted helminthiasis with large scale, regular distribution of anthelminthics (albendazole or mebendazole) to the population groups more at risk of morbidity (children and women of reproductive age). The number of school-age children treated with PC has progressed steadily from less than 120 million in 2008 to over 450 million in 2018 and is in contiuous espansion.^[8]

References

^ Holland CV, Kennedy MW (Eds.) (2002). The Geohelminths: Ascaris, Trichuris and Hookworm. Springer. pp. 1–352. ISBN 978-0-7923-7557-9.
^ ^a ^b Bethony J, Brooker S, Albonico M, Geiger SM, Loukas A, Diemert D, Hotez PJ (2006). "Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm". The Lancet. 367 (9521): 1521–1532. doi:10.1016/S0140-6736(06)68653-4. PMID 16679166. S2CID 8425278.
^ In the UK and Australia, however, the term threadworm can also refer to nematodes of the genus Enterobius, otherwise known as pinworms. Vide Vanderkooi, M. (2000). Village Medical Manual (5th ed.). Pasadena: William Carey Library. ISBN 0878087788.
^ London Declaration (2012) (30 January 2012). "London Declaration on Neglected Tropical Diseases" (PDF). Retrieved 2013-03-26.{{cite web}}: CS1 maint: numeric names: authors list (link)
^ Marcos LA, Terashima A, Dupont HL, Gotuzzo E (2013). "Strongyloides hyperinfection syndrome: an emerging global infectious disease". Trans R Soc Trop Med Hyg. 102 (4): 314–318. doi:10.1016/j.trstmh.2008.01.020. PMID 18321548.
^ Buonfrate D, Requena-Mendez A, Angheben A, Muñoz J, Gobbi F, Van Den Ende J, Bisoffi Z (2013). "Severe strongyloidiasis: a systematic review of case reports". BMC Infect Dis. 13: 78. doi:10.1186/1471-2334-13-78. PMC 3598958. PMID 23394259.
^ WHO (2012). Eliminating Soil-transmitted Helminthiasis as a Public Health Problem in Children: Progress Report 2001–2010 and Strategic Plan 2011–2020 (PDF). WHO Press, World Health Organization, Geneva, Switzerland. pp. 1–78. ISBN 978-92-4-150312-9.
^ Montresor, Antonio; Mupfasoni, Denise; Mikhailov, Alexei; Mwinzi, Pauline; Lucianez, Ana; Jamsheed, Mohamed; Gasimov, Elkan; Warusavithana, Supriya; Yajima, Aya; Bisoffi, Zeno; Buonfrate, Dora; Steinmann, Peter; Utzinger, Jürg; Levecke, Bruno; Vlaminck, Johnny (2020-08-10). Babu, Subash (ed.). "The global progress of soil-transmitted helminthiases control in 2020 and World Health Organization targets for 2030". PLOS Neglected Tropical Diseases. 14 (8): e0008505. doi:10.1371/journal.pntd.0008505. ISSN 1935-2735. PMC 7446869. PMID 32776942.