WHAT IS STH?

Soil-transmitted helminthiasis (STH) is an infection caused by the intestinal parasites roundworms (Ascaris), whipworms (Trichuris), and hookworms (Necator americanus and Ancylostoma duodenale). 
These parasites damage the health of children and adults, specifically women of reproductive age. While rarely fatal, repeated infections are common and adversely affect productivity. 
Typically, STH is most prevalent in the poorest communities in the world where water, sanitation, and hygiene are often inadequate. 
The parasites that cause STH compete with their host for nutrients from food, and some use their host for blood meals, which can impact health, development, and cognitive functioning.  
STH is preventable and treatable
 

HOW IS STH TRANSMITTED?

STH transmission occurs via two possible routes: 
Infective eggs are ingested by a human host; or 
Infective larvae in soil burrow through human skin. 
 
After undergoing their respective lifecycles in the human host, adult parasites live in the gut, where they excrete eggs that are passed into the environment via human feces. In areas where sanitation is inadequate, these infected feces contaminate soil. Thus, infection typically occurs from environmental exposures, e.g.  contaminated food, utensils, or hands, contact between bare skin and hookworm larvae in soil.
 

WHERE IS STH?

STH is among the most common diseases in the world. More than 100 countries are considered to have substantial transmission of STH. Transmission even occurs in highly developed countries like the United States. The World Health Organization (WHO) considers STH a public health problem in areas where >1% of the at-risk population has moderate-to-high intensity infection – as measured by number of eggs per gram of stool and diagnosed by stool examination. 

How do we control STH?

Although STH causes substantial illness in infected individuals, available interventions are safe and effective, and mass treatment of target populations is an effective means to reaching infected persons. 

STH Control requires the delivery of one or more interventions. Key interventions include drug treatment, sanitation provision, and any personal hygiene promotion. Children Without Worms supports an integrated approach to STH control, which includes mass treatment with effective drugs or preventive chemotherapy of target populations, and access to and use of clean water, as well as good hygiene and sanitation practices. Interventions should be followed by monitoring of impact through epidemiologic, risk population-based surveys following successful implementation of interventions over 5 or more years per WHO guidance.

MEET THE INTESTINAL WORMS

Ascaris

Name - Ascaris lumbricoides (sometimes called, “Ascaris” or “roundworm”)

Global Prevalence:  807 - 1,221 million people [1]

Mode of transmission: Ingestion of helminth eggs present on food, utensils, or dirty hands that have been exposed to feces-contaminated soil.

Geographic Location: The parasites are mainly found in warm and moist climates. Transmission is most common in poorer communities where water, sanitation, and hygiene are inadequate. In 2015, 102 countries had ≥20% prevalence of any STH species. [2]

At-risk population: The parasite primarily affects children from ages 1-14. Globally, there are 807 million to 1.2 billion people infected with Ascaris. [3]

Health effects: Mild infection: Minor abdominal discomfort. Severe infection: Blockage in the intestines.

Malnutrition can develop among those infected with intestinal worms. Children with these infections may also have mental and physical growth impairments. In cases of severe infection, the disease can lead to death – mainly due to bowl obstruction and rupture.

There were an estimated 1.31 million (0.71 – 2.35 million) disability adjusted life years resulting from Ascaris infections in 2010. [4]

Secondary Health Implications: Malnutrition can develop among those infected with intestinal worms. Children with these infections may also have mental and physical growth impairments. In cases of severe infection, the disease can lead to death – mainly due to bowl obstruction and rupture.

Whipworm

Name: Trichuris trichiura or “whipworm”

Global Prevalence: 604 - 795 infected million people [5] 

Mode of transmission: Ingestion of helminth eggs present on food, utensils, or dirty hands that have been exposed to feces-contaminated soil.

Geographic Location: The parasites are mainly found in warm and moist climates. Transmission is most common in poorer communities where water, sanitation and hygiene are inadequate. In 2015, 102 countries had ≥20% prevalence of any STH species. [6]

At-risk population: The parasite primarily infects and affects children (1-14 years old). Globally, there are 604 million to 795 million people infected with Whipworm.

Health effects: Mild infection: Usually no obvious symptoms. Severe infection: Frequent or painful stool mixed with mucus, water, and blood. Rectal prolapse may also occur.

Malnutrition can develop among those infected with intestinal worms. Children with these infections may also have mental and physical growth impairments. In cases of severe infection, the disease can lead to death – mainly due to bowl obstruction and rupture. Globally, whipworm resulted in 640,000 disability adjusted life years in 2010. [7]

Secondary Health Implications: Malnutrition can develop among those infected with intestinal worms. Children with these infections may also have mental and physical growth impairments. In cases of severe infection, the disease can lead to death – mainly due to bowl obstruction and rupture.

Hookworm

Name: Ancylostoma duodenale (“Old World” hookworm) and Necator americanus (“New World” hookworm)

Global Prevalence: 576 - 740 million people

Mode of transmission: Most often acquired by walking barefoot on feces-contaminated soil. Some hookworm infections can occur in the lining of the mouth.

Geographic Location: The parasites are mainly found in warm and moist climates. Transmission is most common in poorer communities where water, sanitation, and hygiene are inadequate. In 2015, 102 countries had ≥20% prevalence of any STH species.

At-risk population: The parasite primarily infects and affects adults (>15 years old). Women of childbearing age are particularly affected (but not necessarily more infected as compared to male adults) by hookworm. 576- 740 million people are infected with hookworm. [8]

Health effects: Hookworm causes the majority of STH-associated morbidity. Mild infection: Usually asymptomatic, but even light infections are associated with lower hemoglobin concentration. [9]
Severe: Blood loss, anemia, and protein loss. [10]

Malnutrition can develop among those infected with intestinal worms. Children with these infections may also have mental and physical growth impairments. In cases of severe infection, the disease can lead to death – mainly due to bowl obstruction and rupture.

In 2010, hookworm caused an estimated 3.2 million disability-adjusted life years (DALY). Hookworm is responsible by far for the most DALYs caused by a STH species. [11]

Secondary Health Implications: Malnutrition can develop among those infected with intestinal worms. Children with these infections may also have mental and physical growth impairments. In cases of severe infection, the disease can lead to death – mainly due to bowl obstruction and rupture.

 TRANSMISSION AND LIFECYCLE

Transmission (Worm, Environment, Host)
Ascaris Lifecycle
Adult worms  live in the small intestine. A female may produce approximately 200,000 eggs per day, which are passed via feces . Fertile eggs become infective after 18 days to several weeks , depending on the environmental conditions (optimum: moist, warm, shaded soil). After infective eggs are swallowed , the larvae hatch , invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs . The larvae mature further in the lungs (10 to 14 days), ascend to the throat, and are swallowed . Upon reaching the small intestine, they develop into adult worms . Between 2 and 3 months are required for female works to start producing infective eggs. Adult worms can live 1 to 2 years. Life cycle image and information courtesy of DPDx.
Whipworm or Trichuris Lifecycle
Eggs are passed with the stool . In the soil, the passed eggs develop into a 2-cell stage , an advanced cleavage stage , and then they embryonate ; eggs become infective in 15 to 30 days. After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine and release larvae  that mature and establish themselves as adults in the colon . The adult worms live in the cecum and ascending colon. The adult worms are fixed in that location. The females begin to produce eggs 60 to 70 days after infection. Female worms in the cecum shed between 3,000 and 20,000 eggs per day. The life span of the adults is about 1 year. Life cycle image and information courtesy of DPDx.
Hookworm Lifecycle
Eggs are passed in the stool , and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days. The released larvae grow in the feces and/or the soil , and after 5 to 10 days, they become infective larvae . These larvae can survive 3 to 4 weeks in favorable environmental conditions. On contact with the human host, the larvae penetrate the skin and are carried to the heart and then lungs. They ascend to the pharynx and are swallowed . The larvae reach the small intestine, where they mature into adults. Adult worms live in the small intestine, where they cause blood loss . Most adult worms die in 1 to 2 years, but may live several years.
Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle). In addition, infection by A. duodenale may probably also occur by the oral and transmammary route. N. americanus, however, requires a transpulmonary migration phase. Life cycle image and information courtesy of DPDx.
 

FIND OUT MORE!

WHO STH Progress Report and Strategic Plan 2012
http://apps.who.int/iris/bitstream/10665/44804/1/9789241503129_eng.pdf

Centers for Disease Control and Prevention - Parasites
http://www.cdc.gov/parasites/hookworm/
http://www.cdc.gov/parasites/ascariasis/
http://www.cdc.gov/parasites/whipworm/

 


 

1. CDC - Ascariasis. US Centers for Disease Control & Prevention. 2017. Available at: https://www.cdc.gov/parasites/ascariasis/index.html. Accessed June 19, 2017.

2. WHO. Schistosomiasis And Soil-Transmitted Helminthiases: Number Of People Treated In 2015. HTTP://www.who.int/wer; 2016.

3. CDC - Ascariasis. US Centers for Disease Control & Prevention. 2017. Available at: https://www.cdc.gov/parasites/ascariasis/index.html. Accessed June 19, 2017.

4. CDC - Ascariasis. US Centers for Disease Control & Prevention. 2017. Available at: https://www.cdc.gov/parasites/ascariasis/index.html. Accessed June 19, 2017.

5. CDC - Trichuriasis. US Centers for Disease Control & Prevention. 2017. Available at: https://www.cdc.gov/parasites/whipworm/index.html. Accessed June 19, 2017.

6. WHO. Schistosomiasis And Soil-Transmitted Helminthiases: Number Of People Treated In 2015. HTTP://www.who.int/wer; 2016.

7. Pullan R, Smith J, Jasrasaria R, Brooker S. Global numbers of infection and disease burden of soil transmitted helminth infections in 2010. Parasites & Vectors. 2014;7(1):37. doi:10.1186/1756-3305-7-37.

8. CDC - Hookworm. Cdcgov. 2017. Available at: https://www.cdc.gov/parasites/hookworm/index.html. Accessed June 19, 2017.

9. Pullan R, Smith J, Jasrasaria R, Brooker S. Global numbers of infection and disease burden of soil transmitted helminth infections in 2010. Parasites & Vectors. 2014;7(1):37. doi:10.1186/1756-3305-7-37.

10. Gyorkos, T. W., Gilbert, N. L., Larocque, R. and Casapía, M. (2011), Trichuris and hookworm infections associated with anaemia during pregnancy. Tropical Medicine & International Health, 16: 531–537. doi:10.1111/j.1365-3156.2011.02727.x

11. Pullan R, Smith J, Jasrasaria R, Brooker S. Global numbers of infection and disease burden of soil transmitted helminth infections in 2010. Parasites & Vectors. 2014;7(1):37. doi:10.1186/1756-3305-7-37.