Strongyloides

Strongyloides stercoralis, often called threadworm, is an intestinal nematode (roundworm) found in tropical and subtropical regions and parts of the southeastern United States. Infection begins when filariform larvae in fecally contaminated soil penetrate intact skin — classically bare feet — then travel through the bloodstream to the lungs, ascend the airway, are swallowed, and mature in the small intestine.

What sets Strongyloides apart from other worms is autoinfection: some larvae mature back into the infectious filariform stage while still inside the host and re-penetrate the intestinal wall or perianal skin. This internal cycle lets the infection sustain itself for decades after the original exposure, long after a patient has left an endemic area.

Many chronic infections cause only mild symptoms — abdominal pain, diarrhea, and larva currens (a rapidly migrating, itchy serpiginous rash) — often with eosinophilia. The danger comes with immunosuppression, especially corticosteroid therapy or HTLV-1 infection: unchecked autoinfection can escalate into hyperinfection syndrome and disseminated disease, with massive larval migration, gram-negative sepsis, and high mortality. Diagnosis rests on finding rhabditiform larvae in stool (rather than eggs, unlike most nematodes) or positive serology; treatment is ivermectin, and screening is essential before starting immunosuppressive therapy in patients with possible exposure.

USMLE Step 1 tests Strongyloides in parasitology. High-yield points: skin penetration by larvae, the autoinfection cycle, larvae (not eggs) in stool, hyperinfection risk with corticosteroids, and ivermectin as the drug of choice.