Soil-transmitted Helminths
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The soil-transmitted helminths (also called geohelminths) are a group of intestinal parasites belonging to the
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
Nematoda that are transmitted primarily through contaminated soil. They are so called because they have a direct life cycle which requires no
intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
s 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.


Types

Soil-transmitted helminths are typically from the following families of
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
s, namely: * Roundworms (family Ascarididae), e.g. '' Ascaris lumbricoides'' * Whipworms (family
Trichuridae The roundworm family Trichuridae includes the type genus ''Trichuris'' and some less widely known members. They are (after the abolishment of the artificial "Adenophorea" assemblage) placed in subclass Dorylaimia of the class Enoplea; however, th ...
), e.g. '' Trichuris trichiura'' * Hookworms (family
Ancylostomatidae The Ancylostomatidae are a family of worms that includes the hookworms. Genera of Ancylostomatidae * '' Agriostomum'' * ''Ancylostoma'' * ''Bunostomum ''Bunostomum'' is a genus of nematodes of the small intestine of ruminants and camelids. ...
), e.g. ''
Ancylostoma duodenale ''Ancylostoma'' is a genus of nematodes that includes some species of hookworms. Species include: : '' Ancylostoma braziliense'', commonly infects cats, popularly known in Brazil as ''bicho-geográfico'' : '' Ancylostoma caninum'', commonly infe ...
'' and '' Necator americanus'' * Threadworms (family
Strongyloididae Strongyloididae is a family of nematodes belonging to the order Rhabditida. Genera: * '' Leipernema'' Narayan Singh, 1976 * '' Parastrongyloides'' Morgan, 1928 * ''Strongyloides ''Strongyloides'' (from Greek ''strongylos'', round, + ''eidos'' ...
), e.g. ''
Strongyloides stercoralis ''Strongyloides stercoralis'' is a human pathogenic parasitic roundworm causing the disease strongyloidiasis. Its common name in the US is threadworm. In the UK and Australia, however, the term ''threadworm'' can also refer to nematodes of the ...
'')


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 *
Ascariasis Ascariasis is a disease caused by the parasitic roundworm ''Ascaris lumbricoides''. Infections have no symptoms in more than 85% of cases, especially if the number of worms is small. Symptoms increase with the number of worms present and may in ...
, which is caused by '' Ascaris lumbricoides'' * Hookworm diseases ( ancylostomiasis and necatoriasis), which are caused by '' Necator americanus'' and ''
Ancylostoma duodenale ''Ancylostoma'' is a genus of nematodes that includes some species of hookworms. Species include: : '' Ancylostoma braziliense'', commonly infects cats, popularly known in Brazil as ''bicho-geográfico'' : '' Ancylostoma caninum'', commonly infe ...
'' * Trichuriasis, which is caused by '' Trichuris trichiura'' Soil-transmitted helminthiasis is classified as one of the
neglected tropical diseases Neglected tropical diseases (NTDs) are a diverse group of tropical disease, tropical infections that are common in low-income populations in Developing country, developing regions of Africa, Asia, and the Americas. They are caused by a variety ...
projected to be controlled/eradicated by 2020 through the '' London Declaration on Neglected Tropical Diseases''.


Strongyloidiasis

This is caused by ''
Strongyloides stercoralis ''Strongyloides stercoralis'' is a human pathogenic parasitic roundworm causing the disease strongyloidiasis. Its common name in the US is threadworm. In the UK and Australia, however, the term ''threadworm'' can also refer to nematodes of the ...
''. 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%).


General impact

Geohelminth infection is a major health problem particularly in rural areas of developing countries like
Subsaharan Africa Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the African ...
, 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.


References


External links


WHO information page

USAID's Soil-Transmitted Helminth Program

World Health Organisation (WHO) topic page on helminthiasis
{{DEFAULTSORT:Soil-transmitted helminths Soil Tropical diseases Intestinal infectious diseases Parasitic diseases Helminthiases Foodborne illnesses Conditions diagnosed by stool test