Common Soil-Transmitted Helminths Beyond the "Big Three"
The primary soil-transmitted helminths (STHs) are limited to hookworm (Ancylostoma duodenale and Necator americanus), roundworm (Ascaris lumbricoides), and whipworm (Trichuris trichiura)—there are no other common STHs beyond these three species. 1, 2
The Complete STH Picture
The term "soil-transmitted helminths" specifically refers to these three main parasites that share a common transmission route through contaminated soil 1:
- Hookworms: Ancylostoma duodenale and Necator americanus 3
- Roundworm: Ascaris lumbricoides 3, 4
- Whipworm: Trichuris trichiura 4, 5
These are the only parasites classified as STHs because they require a period of development in warm, moist soil before becoming infectious to humans 1.
Geographic Distribution Patterns
The three STHs have distinct but overlapping geographic distributions:
- Ascaris lumbricoides and Trichuris trichiura have closely related distributions and frequently co-occur in the same communities 2
- Hookworm infection is largely independent of the other two species and has a different geographic pattern 2
- All three are found worldwide in tropical and subtropical regions with warm climates and poor sanitation 3, 1
Other Intestinal Helminths (Not STHs)
While not classified as soil-transmitted helminths, other intestinal parasites may be encountered in similar populations 3:
- Strongyloides stercoralis (strongyloidiasis): Transmitted through soil but has a unique autoinfection cycle that distinguishes it from classic STHs 3
- Enterobius vermicularis (pinworm): Transmitted via fecal-oral route, not through soil 3, 4
- Hymenolepis nana (dwarf tapeworm): Transmitted through poor hygiene, not soil 3
- Taenia species (tapeworms): Transmitted through undercooked meat, not soil 3
Clinical Implications
When evaluating patients from endemic areas, focus diagnostic efforts on the three classic STHs, as they represent the overwhelming majority of soil-transmitted infections and are the targets of mass drug administration programs. 6, 1 The World Health Organization's preventive chemotherapy strategy specifically targets these three parasites with benzimidazoles (albendazole or mebendazole) 6.