You’re cruising through a parasite question, you recognize “eosinophilia,” maybe you even think “worms,” and then the answer choices all start blending together: hookworm, Ascaris, filariasis, schisto—wait, why is Strongyloides so testable again? The trick is that Strongyloides stercoralis isn’t just “another nematode.” It has a unique life cycle, autoinfection, and a hyperinfection syndrome that can become rapidly fatal—especially after steroids.
The Vignette (Q-Bank Style)
A 62-year-old man presents with abdominal pain, intermittent watery diarrhea, and a pruritic rash. He recently started high-dose prednisone for a COPD exacerbation. He immigrated from rural Southeast Asia decades ago. Vitals: febrile, tachycardic. Labs show leukocytosis; eosinophils are low-normal. Blood cultures later grow gram-negative rods. Stool ova and parasite exam is negative on the first sample.
Which organism is the most likely cause?
A. Ancylostoma duodenale
B. Ascaris lumbricoides
C. Enterobius vermicularis
D. Strongyloides stercoralis
E. Schistosoma mansoni
Correct Answer: D. Strongyloides stercoralis
Why this is Strongyloides
This stem is screaming hyperinfection syndrome:
- Steroids (prednisone) are the classic trigger (also HTLV-1, transplant, hematologic malignancy).
- Decades after exposure: Strongyloides can persist for years due to autoinfection.
- Gram-negative bacteremia: migrating larvae can drag gut flora across the intestinal wall → sepsis/meningitis.
- Eosinophilia can disappear in hyperinfection (so don’t let “normal eosinophils” trick you).
High-yield life cycle points (Step gold)
- Infective form: filariform larvae in soil penetrate skin.
- Travel: skin → bloodstream → lungs → trachea → swallowed → small intestine.
- In the intestine: adults produce eggs that hatch into rhabditiform larvae.
- Unique feature: autoinfection
Rhabditiform larvae can mature into filariform larvae inside the host, penetrate intestinal mucosa/perianal skin, and restart the cycle.
Classic clinical findings
- Larva currens: rapidly moving serpiginous urticarial rash, often perianal/trunk.
- GI: abdominal pain, diarrhea, malabsorption (chronic).
- Pulmonary: cough, wheeze, transient infiltrates (can mimic asthma/COPD flare).
- Hyperinfection: severe GI + pulmonary symptoms + dissemination (CNS) + gram-negative bacteremia.
Diagnosis (how they like to test it)
- Stool O&P may be negative (especially a single sample).
- Look for larvae (not eggs) in stool. Sensitivity improves with multiple samples or concentration/agar plate culture.
- In hyperinfection: larvae may be seen in sputum or bronchoalveolar lavage.
- Serology can help in chronic infection (varies by resource setting).
Treatment
- Ivermectin is first-line (especially for hyperinfection/disseminated disease).
- Albendazole is an alternative but generally less effective.
USMLE pearl: If a patient from an endemic area is about to start steroids, think about screening/treating Strongyloides first—because hyperinfection can be catastrophic.
Why Each Distractor Is Wrong (and What It Would Look Like)
A. Ancylostoma duodenale (Hookworm)
Why it’s tempting: skin penetration + eosinophilia + GI complaints can overlap.
Why it’s wrong here:
- Hookworm classically causes iron deficiency anemia from blood loss (microcytic anemia, low ferritin).
- The big “steroid → sepsis via autoinfection” story is Strongyloides, not hookworm.
- Stool findings: eggs (hookworm), not larvae.
High-yield hookworm tells
- “Ground itch” at entry site
- Iron deficiency anemia, fatigue, pica
- Treatment: albendazole/mebendazole + iron
B. Ascaris lumbricoides
Why it’s tempting: pulmonary migration (Löffler syndrome) + GI symptoms.
Why it’s wrong here:
- Ascaris does not do autoinfection → less likely to persist silently for decades and then explode with steroids.
- Typical presentations are:
- Intestinal obstruction (especially children)
- Biliary tree obstruction/pancreatitis
- Stool: eggs are typically seen.
High-yield Ascaris tells
- Large roundworm; “spaghetti-like” worms
- Obstruction symptoms, RUQ pain if biliary involvement
C. Enterobius vermicularis (Pinworm)
Why it’s tempting: pruritus clue… but pinworm pruritus is different.
Why it’s wrong here:
- Pinworm causes no hyperinfection, no bacteremia, no lung migration syndrome.
- Classic symptom is perianal pruritus at night in children.
- Diagnosis is tape test (eggs on perianal folds), not stool O&P.
High-yield pinworm tells
- Treat patient + close contacts: albendazole/mebendazole/pyrantel pamoate
E. Schistosoma mansoni
Why it’s tempting: endemic exposure + eosinophilia + GI symptoms.
Why it’s wrong here:
- Schistosomiasis is a trematode, not a nematode, and its hallmark complications differ:
- S. mansoni → portal hypertension (periportal “pipestem” fibrosis), hepatosplenomegaly
- Can cause bloody diarrhea and polyps, but not the steroid-triggered autoinfection/hyperinfection picture
- Exposure history is usually freshwater (snail intermediate host).
- Diagnosis: eggs with lateral spine in stool (for S. mansoni).
High-yield schisto tells
- Cercariae penetrate skin (“swimmer’s itch”)
- Katayama fever (acute serum sickness-like illness)
- Treatment: praziquantel
Rapid Comparison Table (What USMLE Wants You to Separate)
| Organism | Key Exposure | Hallmark Feature | Stool Finding | Big Complication |
|---|---|---|---|---|
| Strongyloides stercoralis | Soil; skin penetration | Autoinfection, larva currens | Larvae (often missed) | Hyperinfection + gram-negative sepsis (steroids/HTLV-1) |
| Hookworm (Ancylostoma/Necator) | Soil; skin penetration | Chronic blood loss | Eggs | Iron deficiency anemia |
| Ascaris lumbricoides | Fecal-oral | Large worm burden | Eggs | Bowel/biliary obstruction |
| Enterobius vermicularis | Fecal-oral; fomites | Nocturnal perianal itching | Tape test (eggs) | Mild; reinfection common |
| Schistosoma mansoni | Freshwater/snails | Eggs → granulomas | Eggs (lateral spine) | Portal HTN, periportal fibrosis |
Take-Home USMLE Triggers for Strongyloides
- Steroids + immigrant/traveler + GI/pulm symptoms = presume risk.
- Gram-negative bacteremia in this context is a huge clue.
- Autoinfection explains persistence for decades and sudden deterioration.
- Normal eosinophils do not rule it out in hyperinfection.
- Ivermectin is the go-to.