Soil-transmitted helminths are intestinal worms that grow in contaminated wet soil and eventually infect people. Representative helminths include Ascaris lumbricoides (sometimes just called Ascaris or round worm), whipworm (Trichuris trichiura), and hookworm (Anclostoma duodenale and Necator americanus). They enter human’s body through food contaminated with helminth eggs or larvae. Although symptoms vary by the type of helminth, they include anemia, malnutrition, and bowel obstruction.
Infection risks are high in tropical/subtropical areas in sub-Saharan Africa, the Americas, China and East Asia. According to the data released by the World Health Organization (WHO) in 2016, more than 1.5 billion people, or 24% of the world’s population, are infected with soil-transmitted helminth infections worldwide.
Causes of Infection
Soil-transmitted helminths cause infectious diseases through parasitic worm eggs, or larvae, that are transmitted to human intestines through contaminated soil. These parasitic worms include roundworm, whipworm, and hookworm. Soil-transmitted helminths inhabit the intestines of their host, with the eggs of these worms being released in the feces of the patient. If an infected patient passes stool outdoors, or if the stool of an infected patient is utilized as plant fertilizer, the eggs are deposited in the soil, where they eventually hatch and, as the worms mature, continue the cycle of infection. In humans, it is roundworm and whipworm that cause infection when their eggs or larvae are ingested; the ingestion of hookworm eggs does not lead to infection. In the cause of the latter, the eggs hatch while in soil to releasing larvae that matures into a form that can then penetrate human skin, and is primarily transmitted when the would-be host is traveling without shoes on contaminated soil. There is, however, another type of hookworm that has been confirmed to be transmitted through the ingestion of its larvae.
Disease Agent：Soil-transmitted helminths, including roundworm, whipworm, and hookworm
Host：Human
Symptoms
There are normally no observable symptoms for patients who have contracted milder soil-transmitted helminth infections. Long-term or heavy infections—from either one of the parasitic worms—can cause a range of health problems, including abdominal pain, diarrhea, blood and protein loss, rectal prolapse, and physical and cognitive slow growth. Children with heavy infections especially will suffer from a growth disorder caused by iron and protein deficiencies or malnutrition.
Symptoms distinctive to each parasitic worm include the following:
Ascariasis: Roundworm infection
Heavy infections can cause intestinal blockage and impaired growth in children. When worms migrate to the lungs, this causes coughs and other symptoms.
Hookworm infection
Itching and a localized rash are often the first signs of hookworm infection. These symptoms occur when the larvae penetrate the skin. Some also cause gastrointestinal symptoms, especially for people who are infected for the first time. Meanwhile, anemia and protein deficiency due to bleeding at the site in the body where the adult worms have attached to intestines are considered to be the most serious symptoms of hookworm infection For pediatric patients, repeated infection through multiple worms can also lead to iron and protein deficiencies, causing retarded physical and/or mental development.
Whipworm infection
Mild infections will not normally manifest as observable signs or symptoms. However, patients with more serious infections may suffer from frequent and painful passage of stool. In the case of the latter, the stool will comprise a mixture of mucus, water and blood and in instances of diarrhea, typically be accompanied by a sharp, acrid odor. Long-term or repeated infections with whipworm can cause serious health problems including malnutrition, anemia, and slow physical growth, particularly in children.
Diagnosis and Treatment
Soil-transmitted helminth infections are treatable with the proper medication. Regardless of the type of parasitic worm, the standard treatment is to administer anthelmintics such as albendazole and mebendazole.
Diagnostic Methods
The standard way to diagnose roundworm, hookworm, and whipworm infection is to identify their eggs in a stool sample using a microscope.
Treatment
According to CDC data released in 2013, a same prescription applies to both adults and children: albendazole at mealtimes. The table below shows which drug should be administered based on the parasitic worm:
Drugs Administered by Type of Parasitic Worm
| Drug Administered / Type of Disease | Ascaris | Hookworm | Whipworm |
|---|---|---|---|
| Albendazole | ○ | ○ | ○ |
| Mebendazole | ○ | ○ | ○ |
| Ivermectin | ○ | × | ○ |
| Pyrantel pamoate | × | ○ | × |
Prevention
Preventive measures include:
- Avoiding any and all contact with earth contaminated with human waste, especially feces;
- Thoroughly washing hands with soap before handling food;
- Communicating to children the importance of washing hands as a means to prevent infection;
- Washing, peeling, and/or cooking all vegetables and fruits before consumption, especially in cases where the vegetable or fruit was grown in soil fertilized with manure; and
- Not defecating outdoors; using toilets connected to effective sewage disposal systems.
Furthermore, where possible, it is recommended that people in at-risk communities refrain from travelling barefoot in areas where hookworm is prevalent and by all means avoid ingestion of dirt or soil.
Regions at High Risk of Infection
Infections are widely distributed among poorer countries with warm and humid weather and poor hygiene practices in tropical and subtropical areas, with the greatest numbers in sub-Saharan Africa, the Americas, China,and Asia.
Estimated Number of Infected People
According to CDC data released in 2013, approximately 800 million to 1 billion are infected with roundworm, approximately 600 to 800 million with whipworm, and approximately 600 to 700 million with hookworm.
Estimated Number of the Deaths
Soil-transmitted helminths impair the nutrition of those infected in many ways, sometimes causing death by negatively impacting nutrition (causing intestinal bleeding, loss of appetite, diarrhea or dysentery, and reducing the absorption of micronutrients). It can also lead to complications, including ileus, that require surgical intervention. For roundworm infection, by far the most common of the soil-transmitted helminth infections, more than 60,000 die from the disease annually, according to data from CDC in 2011.
References
WHO- Neglected Tropical Diseases, accessed March 19, 2014,
http://www.who.int/neglected_diseases/mediacentre/factsheet/en/
CDC- Neglected Tropical Diseases, accessed March 19, 2014,
http://www.cdc.gov/globalhealth/ntd/diseases/