Written by Emma To, Peer Reviewed by Nikhil Chakravarty, Edited by Courtney Coleman
Introduction
Soil-transmitted helminths (STHs) are a type of parasitic worm that live and lay eggs inside the intestines of its host (1). The term, STH, accounts for three major groups: “Ascaris lumbricoides (sometimes called just ‘Ascaris’), whipworm (Trichuris trichiura), and hookworm (Ancylostoma duodenale and Necator americanus)” (1). Figure 1 visualizes the life cycles of these common STHs (2). Generally, transmission occurs when a host ingests the eggs of these worms that have matured and become infectious in soil (3).
Figure 1: Life Cycles of Different Species of STHs. A: Ascaris lumbricoides; B: Trichuris trichiura; C: Necator americanus/Ancylostoma duodenale; D: Strongyloides stercoralis. (Source: Jourdan et al., Lancet, 2018)
Ingestion often occurs as a result of consuming contaminated produce that has not been cleaned or cooked properly, through drinking contaminated water, and when soil-contaminated hands are put in mouths (common amongst children) (3). Hookworm can also be transmitted when larvae penetrate through the skin and travel to the intestines via the lungs and larynx (2). The ingested eggs or larvae travel to the intestines (or sometimes by “larval migration through pulmonary tissue”) to hatch and lay more eggs which will be excreted via host defecation (2). The STHs, however, persist in the intestines for months to years, consuming and sustaining themselves on the tissues of their host, leading to host blood, protein, and iron loss (1,2,3) . They can also affect the nutrient absorption by the intestines or result in a general decrease in appetite (3). More mild cases of STH infection typically do not progress to severe disease; however, when the infection becomes more acute, serious symptoms, such as impairment of “physical and cognitive growth[,]” is possible (1).
Statistics
STHs are common in highly impoverished areas with limited access to clean water and necessities. In the United States, STHs are uncommon but do exist in some rural communities in the South, as well as in the Appalachia area, and may go undiagnosed due to the symptoms having significant overlap with other common diseases affecting these populations (4). According to the NIH, STHs have contributed to cycles of poverty, as these communities have limited access to adequate healthcare, as well as poor sanitation, which lead to more infections that can cause “cognitive impairments,” affecting children’s success in school and causing these children to largely remain impoverished (4).
Detailed surveillance of STHs slowed following the 1980s as sanitation standards increased and agricultural industries decreased; however, out of households with incomplete plumbing, 16.6% are Black and 16.7% are Hispanic, and this data, though not directly tied to STH infections, shows inequitable access to adequate sanitation facilities and provides important context to assess the potential for STH infections in the US (1,4,5).
The WHO estimates that approximately 24% of the global population, 1.5 billion people, are infected with STHs, with the majority of cases occurring in “sub-Saharan Africa, China, South America and Asia” (3). When looking at each common group of STHs, about 807 million to 1.1 million people are infected with Ascaris, “approximately 604-795 million with whipworm,” and “approximately 576-740 million with hookworm” (1).
Preventative Measures/Treatments
STHs are both treatable and preventable. Mebendazole and Albendazole are affordable medications that can be administered efficiently and are effective treatments for most STH infections (3). Strongyloides stercoralis, however, has to be addressed with ivermectin, another affordable and effective drug (3). Before infection can even occur, STHs can be prevented with improved sanitation practices and facilities, stopping infections at the source by reducing the likelihood of contamination with eggs or larvae in soil or water (2). Educating communities about STHs, proper food preparation, and hygiene practices will also work to eliminate STH infections as people know what to do to avoid them and protect their communities. The relatively simple treatment and prevention measures against STHs compared to their severe consequences makes education and awareness more important, especially for communities where STHs are common but information is limited.
This post is not a substitute for professional advice. If you believe that you may be experiencing a medical emergency, please contact your primary care physician, or go to the nearest Emergency Room. Results from ongoing research is constantly evolving. This post contains information that was last updated on April 10, 2023.
Works Cited
1. “Parasites - Soil-Transmitted Helminths.” CDC, 2 February 2022,
www.cdc.gov/parasites/sth/index.html. Accessed 10 February 2023.
2. Jourdan, Peter Mark, et al. "Soil-transmitted helminth infections." The Lancet 391.10117 (2018): 252-265.
3. “Soil-Transmitted Helminth Infections.” World Health Organization, 18 January 2023, www.who.int/news-room/fact-sheets/detail/soil-transmitted-helminth-infections. Accessed 10 February 2023
4. Lynn, Mary K et al. “Soil-Transmitted Helminths in the USA: a Review of Five Common Parasites and Future Directions for Avenues of Enhanced Epidemiologic Inquiry.” Current tropical medicine reports vol. 8,1 (2021): 32-42.
doi:10.1007/s40475-020-00221-2
5. Deitz, Shiloh, and Katie Meehan. "Plumbing poverty: mapping hot spots of racial and geographic inequality in US household water insecurity." Annals of the American Association of Geographers 109.4 (2019): 1092-1109.
6. “Neglected Tropical Diseases.” CDC, 7 March 2022, www.cdc.gov/globalhealth/ntd/diseases/index.html. Accessed 10 February 2023 7. Starr, Michelle C, and Susan P Montgomery. “Soil-transmitted Helminthiasis in the United States: a systematic review--1940-2010.” The American journal of tropical medicine and hygiene vol. 85,4 (2011): 680-4. doi:10.4269/ajtmh.2011.11-0214
Emma To is currently a Microbial Biology major undergraduate at UC Berkeley.
Nikhil Chakravarty is currently a second-year MPH student specializing in Epidemiology at UCLA.
Courtney Coleman is a master's degree candidate in biology at Harvard and Co-President of Students vs Pandemics.