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. 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 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 WHO 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 CWW’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 (WHO) has developed a strategic plan outlining targets and milestones that serve as a foundation for a global STH control strategy. Each year WHO publishes a report on the number of people treated within STH control programs.
WHO 2030 Roadmap and STH Control
Representatives from STH endemic countries and global partners supporting STH control activities, including representatives from CWW, met in Basel, Switzerland, in October 2018 to develop new targets to guide STH control activities in 2020-2030.
The targets include:
- Achieve and maintain elimination of STH morbidity in pre-school age children (pre-SAC) and school age children (SAC) by 2030
- Reduce the number of tablets needed in preventive chemotherapy (PC) for STH
- Increase domestic financial support to PC for STH
- Establish an efficient STH control programme in adolescent, pregnant, and lactating women of reproductive age (WRA)
- Establish an efficient strongyloidiasis control programme in SAC
- Ensure universal access to at least basic sanitation and hygiene by 2030 in STH endemic areas
Soil-transmitted helminthiasis (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.
Preventative Chemotherapy (PC)
The World Health Organization (WHO) recommends PC (i.e., population-level treatment with safe, effective drugs) as the primary intervention to control morbidity from and transmission of soil-transmitted helminthiasis (STH). Mass drug administration (MDA) 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 MDA 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 PC 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 PC 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 NTD interventions, the World Health Organization (WHO) developed a toolkit that provides guidance to NTD programs and partners on how to engage with the WASH sector to improve the delivery of WASH services to communities affected by STH and NTDs.
Interventions for At-Risk Populations
STH can seriously damage the health of children 1 to 14 years old and women of childbearing age. PC coverage of these populations with suitable platforms is critical to minimize morbidity associated with STH infections.
Children harbor the highest worm burdens for Ascaris and whipworm and are the main source of environmental contamination. Additionally, Ascaris and whipworm infections generally have the greatest impact on children, as STH infection occurs concurrently with physical and intellectual development. Infection can cause anemia, malnutrition, and stunted growth.
Approximately 269 million preschool-age children (aged 1 to 4 years) and 571 million school-age children (aged 5 to 14 years) require PC for STH.
Preschool-age Children PC Platforms
- Co-administration with vitamin A supplementation
- Co-administration with existing immunization programs, e.g., polio vaccinations
- Maternal/child health days, including antenatal checkups
School-age Children PC Platforms
- In-school treatment programs
Woman of Reproductive Age
Women of reproductive age are particularly vulnerable to hookworm and the anemia caused by the parasite. Hookworm infection is strongly associated with iron-deficiency anemia during pregnancy. Approximately 250 million women of childbearing age are at risk of morbidity due to STH. PC during pregnancy (with iron supplementation) has been associated with reduced maternal anemia, increased weight of the newborn, and reduced infant mortality. 
Women of Reproductive Age PC Platforms
- Maternal/child health days, including antenatal checkups
- Community-based distribution via lymphatic filariasis elimination programs
 12. Ndibazza, J. et al. Effects of Deworming during Pregnancy on Maternal and Perinatal Outcomes in Entebbe, Uganda: A Randomized Controlled Trial. Clinical infectious Diseases. 50.4 (2010): 531–540. PMC.
Repeated PC with safe, affordable drugs at a recommended frequency helps lower both STH prevalence and intensity of infection levels below those associated with morbidity. According to the WHO, the treatment frequency for populations at risk of STH is determined by a baseline prevalence estimate among school-age children. Experts continue to debate the appropriateness of relying on prevalence estimates of school-age children when trying to understand hookworm infection levels (since that parasite primarily infects adults). Repeated treatments help reduce and maintain levels of infection below those associated with morbidity. Current recommended treatment frequencies are as follows:
Baseline STH Prevalence: <20%
Baseline STH Prevalence: ≥20% and ≤50%
Baseline STH Prevalence: >50%
Treatment: Twice annually
Certified preventive chemotherapy (PC) medications, donated by Johnson & Johnson and GlaxoSmithKline, are available free-of-charge to national ministries of health through the WHO. Get more information about these donations here.