Soil-transmitted helminthiasis (STH) refers to a group of parasitic diseases in humans caused by intestinal roundworms such as hookworms (Ancylostoma duodenale and Necator americanus), ascaris (Ascaris lumbricoides), and whipworm (Trichuris trichiura), collectively called soil-transmitted helminths (STHs), which are transmitted through contaminated soil. It has become the most common parasitic infection of humans worldwide. Approximately two billion people (about a third 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 Subsaharan Africa, Latin America, Southeast Asia, and China. It is regarded as one of the world's most important causes of intellectual and physical retardation.
The helminthic disease is so named because the infection is transmitted through ingestion of the nematode eggs in the soil, which is contaminated through excrements. 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, 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.
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 impair 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 environment such as during playing, eating raw vegetables and fruits, and drinking wastewater.
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. In fact it is the leading cause of anaemia and protein deficiency in developing nations, afflicting 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.
Whipworm (T. trichiura) is the third most common STH-causing nematode in humans. According to current estimate, nearly 800 million people are infected, and majority of the victims are 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.
STHs are essentially intestinal parasites and their eggs are liberated along the faeces of infected persons into the soil. Ascaris and hookworm eggs become infective as they develop into larvae in soil. Infection occurs when vegetables and fruits, contaminated with soil infested eggs, are consumed; or when hands or fingers have been contaminated with dirt carrying the eggs are put in the mouth. On the other hand, 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.
The global burden of STHs can be summarised as follows:
- approximately 807-1,121 million with ascaris
- approximately 576-740 million with hookworm
- approximately 604-795 million with whipworm
Clinical symptoms and impact
Even with a global estimate of over a million deaths a year, the greatest significance of STH is the relentless chronic effects on socio-economic conditions. In endemic countries, communities remain suppressed due to malnourishment, cognitive disability and physical weaknesses as a result of heavy infections. Most conditions of STH usually have no discernible symptoms. Morbidity becomes evident only when the intensity of infection is relatively high. Thus morbidity is directly related to worm burden; more worms means greater severity of disease. Heavy infections then cause a range of health problems, including abdominal pain, diarrhoea, blood and protein loss, rectal prolapse, and physical and mental retardation. STH is most often associated with malnutrition, to the point of life threatening, because the worms negatively affect the nutritional status as they induce intestinal bleeding, competition for nutrients, frequent anaemia and diarrhoea.
Sever ascariasis is typically a pneumonia, as the larvae invades 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.
For basic diagnosis, specific helminths can be generally identified from the faeces, and their eggs microscopically examined and enumerated using fecal egg count method. However, there are certain limitations such as the inability to identify mixed infections, and on clinical practice, the technique is inaccurate and unreliable. A novel effective method for egg analysis is the Kato-Katz technique. It is a highly accurate and rapid method for A. lumbricoides and T. trichiura; however not so much for hookworm, which could be due to fast degeneration of the rather delicate hookworm eggs.
Interventions and chemotherapy
Effective prevention and control measures include: 1) availability of clean water for personal and domestic uses; 2) sanitation and health education to promote use of latrines; 3) education on personal hygiene such as hand washing and washing of food; 4) avoiding the use of uncomposted human faeces as fertilizer. When massive intestinal infections are diagnosed, surgical removal is usually the only effective method. The principal 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. These anthelminthics are administered in a single dose are safe, relatively inexpensive, and effective for several months. Mebendazole can be given with a single dose of 300 to 500 mg, or with 100 mg twice a day for three consecutive days. Albendazole is given at a single dose of 400 mg. In cases of coinfection, combination therapy with ivermectin and diethylcarbamazine is advocated. WHO recommends annual treatment in endemic regions where the rate of prevalence is between 20 and 50%, and a bi-annual treatment if it is over 50%; and in low risk situation (i.e. less than 20% prevalence) case-by-case treatment. In addition to these, pyrantel pamoate is also equally effective on ascaris. However, it has been reported that albendazole, mebendazole, and pyrantel pamoate are not entirely effective against T. trichiura with single oral doses in population-based control. A novel drug tribendimidine, which was approved by Chinese authorities for human use in 2004, has been subjected to clinical trials showing that they are highly effective against major human flukes, ascaris (>90% cure rate) and hookworm (>82%); however with low cure rate for whipworm (<37%). Indeed, with the ever rampant coinfection with malaria and HIV, especially among African women, there is an alarming situation that the current regimes for controlling STHs are inadequate. It is more pressing for trichuriasis that the recommended drugs fail to provide positive results.
- STH is also used to abbreviate soil-transmitted helminths.
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- Centers for Disease Control and Prevention
- WHO information page
- USAID's NTD Program
- OneWorld Health
- Management Record at US NIH
- London Declaration Uniting to Combat NTDs
- WHO Western Pacific Region
- Information at Right Diagnosis
- USAID's Neglected Tropical Diseases Programme
- Global Network Neglected Tropical Diseases
- GlaxoSmithKline campaign brochure
- NTDs at Helen Keller International
- GlaxoSmithKline Developing Countries Unit
- Death to Onchocerciasis and Lymphatic Filariasis
- Disease Control Priority Project
- Types of soil-transmitted helminths at Shinpoong
- The Beehive of Nigeria
- Rural Education Action Program at Stanford
- Global Atlas of Helminth Infections
- Deworming programmes at Schools&Health
- Comprehensive strategy at ChildrenWithoutWorms
- World Intellectual Property Organization
- STH at Malaria World
- Global Health Hub
- Cure Byte
- NTDs at Global Network
- BIO Ventures for Global Health
- Pan American Health Organization
- The End Fund
- End Neglected Tropical Diseases in Africa
- Resources at TropIKA