Overview
Soil-transmitted helminth (STH) infections are 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 infections are most prevalent in the poorest communities in the world where water, sanitation, and hygiene are often inadequate. The parasites that cause STH infections compete with their host for nutrients from food, and some use their host for blood meals. This can impact overall health, development, and cognitive functioning. Intestinal worm infections are preventable and treatable.
There are three main species that infect humans:
Where is STH?
STH is among the most common diseases in the world. The World Health Organization considers STH a public health problem in areas where >2% 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 of 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 of target populations with effective drugs or preventive chemotherapy and access to clean water and improved sanitation facilities alongside good hygiene practices. Regular program monitoring is necessary to assess proper coverage of interventions. Following successful implementation of interventions for five or more years, impact assessments should be carried out to evaluate the prevalence and intensity of STH among at-risk populations (as per World Health Organization guidance). These evaluations should be representative of all populations at risk at the level of program implementation, following proper epidemiological guidelines for statistical power and representation. Examples of appropriate methods for program monitoring and impact assessments are included in the Global Monitoring and Evaluation Framework, and in our guidance materials for Children Without Worm’s Integrated Community-based Survey for Program Monitoring.
Global efforts to control intestinal worm infections
Over the last 20 years, various partners (including many STH Coalition members) have successfully increased the level of attention given to STH control. Since declaring STH as a public health problem in 2001, the World Health Organization has developed a strategic plan outlining targets and milestones that serve as a foundation for a global STH control strategy. Each year the World Health Organization publishes a report on the number of people treated within STH control programs.
World Health Organization 2030 Roadmap and STH Control
In 2020, the World Health Organization shared 2030 targets for STH control programs.
The targets include:
- Achieve and maintain elimination of STH morbidity in pre-school age children and school age children by 2030
- Reduce the number of tablets needed in preventive chemotherapy for STH
- Increase domestic financial support to preventive chemotherapy for STH
- Establish an efficient STH control programme in adolescent, pregnant, and lactating women of reproductive age
- Establish an efficient strongyloidiasis control programme in school age children
- Ensure universal access to at least basic sanitation and hygiene by 2030 in STH endemic areas
Diagnostics
STH is typically diagnosed through examination of patient stool samples using a microscope.
A variety of examination tests are available to diagnose STH, including Kato-Katz, formol-ether concentration, McMaster, FLOTAC, and Mini-FLOTAC. Of these, Kato-Katz and FLOTAC/Mini FLOTAC are most commonly used. New diagnostic methods using qPCR and PCR have also been developed, which allow laboratory technicians to efficiently test for multiple parasitic infections and, in the case of qPCR, quantify the intensity of infection with one sample. PCR diagnostics are especially important in that they are much more sensitive, thus improving detection of low-intensity infections.
For additional STH diagnostics information, please refer to the World Health Organization’s Bench Aids for the diagnosis of intestinal parasites, Second edition (2019).
Kato-Katz
FLOTAC/Mini-FLOTAC
Molecular Diagnostics
Interventions
Preventive Chemotherapy
The World Health Organization recommends preventive chemotherapy (i.e., population-level treatment with safe, effective drugs) as the primary intervention to control morbidity from and transmission of STH. Mass drug administration of effective drugs to targeted at-risk populations, irrespective of current infection status has proven to effectively reduce STH prevalence and intensity of infection in targeted communities.
Albendazole and mebendazole are the most common deworming medications. They have excellent safety profiles, are inexpensive, and are easily administered by non-medical personnel. Effective mass drug administration campaigns with these drugs, either alone or in combination, reduce the lifespan of helminths in the host, the number of infected individuals, and the duration that they can excrete eggs into the soil. Therefore, effective preventive chemotherapy campaigns also decrease the total number of infectious eggs (and the larvae resulting from eggs) in the soil, reducing the likelihood of continued transmission.
WASH – Water, Sanitation, and Hygiene
STH can be treated and sometimes cured with medication. However, infections are often cumulative, leading to more worms in a person and greater risk of serious health consequences. Preventing infection is ideal, but it is especially difficult in areas where sanitation conditions are poor. Improved and consistent Water, Sanitation, and Hygiene (WASH) facilities and practices, therefore, play a key role in preventing transmission and reducing the intensity of infections. An integrated program approach to STH includes the provision of preventive chemotherapy along with specific activities to improve access to and use of sanitation and proper hygiene practices.
WASH interventions lower the risk of infection and burden from multiple diseases, including the long-term control and even elimination of STH. Prevention of open defecation and provision of adequate sanitation facilities prevent helminth eggs passed in the feces of infected individuals from being ingested or coming into contact with humans. Hand washing with soap and thoroughly cleaning uncooked foods (e.g. fruit, vegetables) help prevent humans from ingesting eggs. Recognizing the importance of delivering WASH interventions alongside other neglected tropical disease interventions, the World Health Organization developed a toolkit that provides guidance to neglected tropical disease programs and partners on how to engage with the WASH sector to improve the delivery of WASH services to communities affected by STH and neglected tropical diseases.
At-Risk Populations
Children
STH can seriously impact the health of children aged 1 to 14 years old and women of reproductive age. Administering preventive chemotherapy to these populations is critical to minimize morbidity associated with STH.
Children harbor the highest worm burdens for roundworm and whipworm, with these infections generally having the greatest impact on children. If left untreated, STH can lead to anemia, malnutrition, and stunted growth. The World Health Organization document Helminth control in school-age children: a guide for managers of control programmes — Second edition (2011) provides additional information on how to effectively plan, implement, and monitor school-based deworming to improve child health outcomes.
Women of Reproductive Age
Hookworm infections pose an increase risk to women of reproductive age because they are strongly associated with iron-deficiency anemia during pregnancy. The World Health Organization document Deworming adolescent girls and women of reproductive age. Policy brief (2022) provides additional information on how strategic deworming can improve the health of at-risk women.