Soil-transmitted helminthiasis is a type of worm infection (helminthiasis) caused by different species of roundworms. It is caused specifically by worms transmitted through soil contaminated with fecal matter and are known as soil-transmitted helminths. Three types of soil-transmitted helminthiasis can be distinguished: ascariasis, hookworm infection and whipworm infection. These three types of infection are caused by the large roundworm A. lumbricoides, the hookworms Necator americanus or Ancylostoma duodenale and by the whipworm Trichuris trichiura. It has become the most common parasitic disease of humans worldwide. Approximately two billion people (about a fourth of global population) are infected as of the latest estimate, and four billion at risk, surpassing even the all-time most prevalent parasitic disease, malaria. The largest numbers of cases occur in impoverished rural areas of Sub-Saharan Africa, Latin America, Southeast Asia, and China. Its main cause, like for many types of helminth infections, is lack of sanitation, such as the practice of open defecation, lack of hygiene such as hand washing and walking barefoot on contaminated soil. It is regarded as one of the world's most important causes of intellectual and physical disability. The helminthic disease is so named because the infection is transmitted through ingestion of the nematode eggs in the soil, which is contaminated through feces. Therefore, the disease is most prevalent in warm and moist climates where sanitation and hygiene are poor and waters are unsafe, including the temperate zones during warmer months. STH is categorised among neglected tropical diseases because it inflicts tremendous disability and suffering, which can be clinically treated and relatively easily prevented (primarily through improved sanitation), yet negligible attention has been given for many years. It is now among the target diseases of London Declaration on Neglected Tropical Diseases (launched on 30 January 2012) to be controlled/eradicated by 2020. Simple prevention and control strategies are access to improved sanitation, public awareness of personal hygiene, and health education.
Types Soil-transmitted helminths are intestinal parasites and their eggs which are liberated into soil along with the feces of infected persons. Ascaris and hookworm eggs become infective as they develop into larvae in soil. Infection occurs when vegetables and fruits contaminated with egg-infested soil are consumed, or when contaminated hands or fingers are placed in the mouth. Hookworm eggs are not directly infective. They hatch in soil, releasing mobile larvae that can penetrate the skin. Thus infection is acquired through accidental contact with contaminated soil.
Ascariasis
Ascariasis of STH is caused by the large roundworm A. lumbricoides. It is estimated to be the most widespread STH, affecting approximately 1 billion people. The victims constitute about half of the populations in tropical and subtropical areas. Most conditions are mild and often show little or no symptoms. Heavy infections however are debilitating, causing severe intestinal blockage and impairing growth in children. Children, compounded with malnutrition, are most infected, with the most common age group being 3- to 8-year-olds, with an annual death of about 20,000. Children are more susceptible due to their frequent exposure to contaminated environments such as during playing, eating raw vegetables and fruits, and drinking wastewater.
Hookworm disease
Hookworm infection of STH is caused by N. americanus and A. duodenale. Mild infections produce diarrhoea and abdominal pain. More severe infections can create serious health problems for newborns, children, pregnant women, and malnourished adults. It is the leading cause of anaemia and protein deficiency in developing nations, affecting an estimated 740 million people. N. americanus is the more common hookworm, while A. duodenale is more geographically restricted. Unlike other STHs, in which school-age children are most affected, high-intensity hookworm infections are more frequent in adults, specifically women. Roughly 44 million pregnant women are estimated to be infected. The disease causes severe adverse effects in both the mother and infant, such as low birth weight, impaired milk production, and increased risk of mortality.
Trichuriasis
Whipworm (Trichuris trichiura) is the third most common STH-causing nematode in humans. According to current estimates, nearly 800 million people are infected, the majority of them children. Heavy infections could lead to acute symptoms such as diarrhoea and anaemia, and chronic symptoms such as growth retardation and impaired cognitive development. Medical conditions are more often serious since coinfection with protozoan parasites such as Giardia and Entamoeba histolytica, and with other nematodes is common. Predominantly a tropical disease of developing countries, trichuriasis is quite common in the United States.
Signs and symptoms
Symptoms become evident only when the intensity of infection is relatively high. Thus the degree of negative outcomes is directly related to worm burden; more worms means greater severity of disease.
General Most conditions of STH have a light worm burden and usually have no discernible symptoms. Heavy infections however cause a range of health problems, including abdominal pain, diarrhoea, blood and protein loss, rectal prolapse, and physical and intellectual disability. Severe ascariasis is typically a pneumonia, as the larvae invade lungs, producing fever, cough and dyspnoea during early stage of infection. Hookworm infections insinuate a skin reaction (dermatitis), increased white blood cells (eosinophils), a pulmonary reaction (pneumonitis), and skin rash (urticarial). Iron deficiency anaemia due to blood loss is a common symptom.
Malnutrition STH is often associated with malnutrition in children as it worsens their nutritional status in multiple ways. The worms can induce intestinal bleeding, competition for nutrients (malabsorption of nutrients), frequent anaemia and diarrhoea. Soil-transmitted helminths can also cause loss of appetite. These nutritional "knock on" effects of STH can have a significant impact on the mental and physical development of children. In endemic countries, communities remain suppressed due to malnourishment, intellectual disability, and physical weaknesses as a result of heavy infections.
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