The soil-transmitted helminths (also called geohelminths) are a group of

intestinal parasite An intestinal parasite infection is a condition in which a parasite infects the gastro-intestinal tract of humans and other animals. Such parasites can live anywhere in the body, but most prefer the intestinal wall. Routes of exposure and inf ...

s 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 Nomenclatu ...

Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

that are transmitted primarily through contaminated soil. They are so called because they have a direct

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia * Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesi ...

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 A parasitic disease, also known as parasitosis, is an infectious disease caused by parasites. Parasites are organisms which derive sustenance from its host while causing it harm. The study of parasites and parasitic diseases is known as parasitolo ...

occurs through faecal contamination of soil, foodstuffs and water supplies. The adult forms are essentially

parasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

of humans, causing

soil-transmitted helminthiasis Soil-transmitted helminthiasis is a type of helminth infection (helminthiasis) caused by different species of roundworms. It is caused specifically by those worms which are transmitted through soil contaminated with faecal matter and are theref ...

(STH), but also infect

domesticated Domestication is a sustained multi-generational relationship in which humans assume a significant degree of control over the reproduction and care of another group of organisms to secure a more predictable supply of resources from that group. A ...

mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...

. The juveniles are the infective forms and they undergo tissue-migratory stages during which they invade vital organs such as

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...

and

liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...

. 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 Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

of nematodes, namely: * Roundworms (family

Ascarididae The Ascarididae are a family of the large intestinal roundworms. Members of the family are intestinal parasites, infecting all classes of vertebrates. It includes a number of genera,Anderson RC (2000)''Nematode Parasites of Vertebrates. Their D ...

), e.g. ''

Ascaris lumbricoides ''Ascaris lumbricoides'' is a large parasitic worm that causes ascariasis in humans. A roundworm of genus '' Ascaris'', it is the most common parasitic worm in humans. An estimated one-sixth of the human population is at some point infected by a ...

'' * Whipworms (family Trichuridae), e.g. '' Trichuris trichiura'' * Hookworms (family Ancylostomatidae), 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 ''Necator americanus'' is a species of hookworm (a type of helminth) commonly known as the New World hookworm. Like other hookworms, it is a member of the phylum Nematoda. It is an obligatory parasitic nematode that lives in the small intestin ...

'' * Threadworms (family Strongyloididae), 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 ...

, which is caused by ''

'' * Hookworm diseases ( ancylostomiasis and

necatoriasis Necatoriasis is the condition of infection by '' Necator'' hookworms, such as '' Necator americanus''. This hookworm infection is a type of helminthiasis (infection) which is a type of neglected tropical disease. Signs and symptoms When adult wo ...

), which are caused by ''

'' and ''

'' * 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 infections that are common in low-income populations in developing regions of Africa, Asia, and the Americas. They are caused by a variety of pathogens, such as viruses, bac ...

projected to be controlled/eradicated by 2020 through the '' London Declaration on Neglected Tropical Diseases''.

Strongyloidiasis

This is caused by ''

''. 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

pet A pet, or companion animal, is an animal kept primarily for a person's company or entertainment rather than as a working animal, livestock, or a laboratory animal. Popular pets are often considered to have attractive appearances, intelligence ...

s. But there is an emerging hyperinfection syndrome caused by ''S. stercoralis'', which exhibits a high

mortality rate Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of d ...

(15% to 87%).

General impact

Geohelminth infection is a major health problem particularly in

rural areas In general, a rural area or a countryside is a geographic area that is located outside towns and cities. Typical rural areas have a low population density and small settlements. Agricultural areas and areas with forestry typically are describ ...

developing countries A developing country is a sovereign state with a lesser developed Industrial sector, industrial base and a lower Human Development Index (HDI) relative to other countries. However, this definition is not universally agreed upon. There is al ...

like Subsaharan Africa,

India India, officially the Republic of India ( Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the ...

and other

Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...

n countries. It is an important cause of

morbidity A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that ...

in school age children who harbour the highest intensity of worm infestation. Some of the significant morbidity attributed to intestinal

helminthiasis Helminthiasis, also known as worm infection, is any macroparasite, macroparasitic disease of humans and other animals in which a part of the body is infected with parasitism, parasitic worms, known as helminths. There are numerous species of the ...

are

malnutrition Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is "a deficiency, excess, or imbalance of energy, protein and other nutrients" which adversely affects the body's tissues ...

, growth retardation,

anaemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, t ...

vitamin A deficiency Vitamin A deficiency (VAD) or hypovitaminosis A is a lack of vitamin A in blood and tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. Nyctalopia (n ...

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 Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

Tropical diseases Intestinal infectious diseases Parasitic diseases Helminthiases Foodborne illnesses Conditions diagnosed by stool test