Causes of Strongyloides (Threadworm) Infection
Transmission Mechanism
Strongyloides stercoralis infection occurs when infective filariform larvae penetrate intact skin after contact with soil contaminated by human feces containing the parasite. 1, 2, 3
Primary Routes of Infection
- Percutaneous transmission is the predominant route, occurring when bare skin contacts contaminated soil containing infective filariform larvae 2, 4
- After skin penetration, larvae migrate through the venous system to the lungs, ascend the bronchi to the trachea, are coughed up and swallowed, then establish residence in the small intestine 2
- Autoinfection represents a unique feature of this parasite, where filariform larvae can develop in the gut lumen and re-penetrate the intestinal mucosa or perianal skin without leaving the host, allowing infection to persist for decades 3, 4, 5
Geographic and Environmental Factors
- The parasite is widespread throughout tropical and subtropical regions with poor sanitation, though small endemic foci exist in temperate regions including the rural southeastern United States 6, 2, 4
- Globally, 300-600 million people are infected through contact with fecally contaminated soil 3
- In the United States, strongyloidiasis is most commonly reported among refugees and immigrants from endemic areas 4
Less Common Transmission Routes
- Intimate interhuman contact can rarely transmit infection 5
- Solid organ transplantation from an infected donor represents another uncommon but documented route 5
Critical Autoinfection Cycle
The autoinfective component of the life cycle distinguishes Strongyloides from other helminths, as invasive filariform larvae can develop anew in the gut lumen, perpetuating cycles of penetration, tissue migration, and reproduction entirely within the host 3, 5. This mechanism explains why chronic infection can persist asymptomatically for decades after leaving endemic areas, and why immunosuppression can trigger catastrophic hyperinfection syndrome when large numbers of larvae pierce the bowel wall and disseminate throughout organs 3, 5, 7.