Step 1 topics → Microbiology → Nematodes (Roundworms)

Nematodes (Roundworms)

USMLE Step 1 trap: Confuses cutaneous larva migrans (dog hookworm) with larva currens (Strongyloides). Cutaneous larva migrans is caused by dog/cat hookworm (Ancylostoma braziliense), while Strongyloides causes larva currens, a faster-moving serpiginous rash near the anus.

Nematodes (roundworms) are a high-yield parasite group on USMLE Step 1, but the sheer number of organisms makes students blur them together. The key to mastering this topic is organizing by transmission route and signature presentation — not memorizing organisms in isolation. Intestinal nematodes (Ascaris, hookworm, pinworm, Strongyloides, Trichinella) each have a distinct exposure history and clinical picture. Filarial nematodes (Wuchereria, Brugia, Loa loa, Onchocerca) are tested by matching vector to tissue-specific disease.

Step 1 tests nematodes from multiple angles. Pure recall questions ask you to match organism to presentation. More commonly, you get a clinical vignette — a patient with periorbital edema and eosinophilia after eating wild boar, or a child with perianal pruritus worse at night — and you have to work backward to the organism and treatment. Passage-based questions may give you epidemiology (geographic region, occupation, barefoot exposure) and ask you to identify the organism or explain the mechanism of tissue damage. The treatment angle is especially tricky because several drugs (ivermectin, DEC, albendazole) have overlapping but not interchangeable indications.

The most common errors on USMLE Step 1 come from conflating similar-sounding syndromes: larva currens vs. cutaneous larva migrans, or assuming all filarial worms share the same vector. Students also underestimate Trichinella, incorrectly linking it to fecal-oral soil transmission rather than undercooked meat. Getting the mental model right — transmission → migration pathway → target tissue → presentation — is what separates a 'medium' topic from free points.

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Common misconceptions

Common mistake
Wrong: Cutaneous larva migrans (creeping eruption) is caused by Strongyloides.
Right: Cutaneous larva migrans is caused by dog/cat hookworm (Ancylostoma braziliense), while Strongyloides causes larva currens, a faster-moving serpiginous rash near the anus.
Cutaneous larva migrans (the classic 'creeping eruption') is caused by Ancylostoma braziliense, the dog or cat hookworm — a dead-end host migration in humans that moves slowly through the skin. Strongyloides causes larva currens, which is faster-moving, serpiginous, and characteristically appears near the perianal region or buttocks because filariform larvae re-enter the skin during the autoinfection cycle. The distinction matters clinically: larva currens signals Strongyloides, which can cause fatal hyperinfection in immunocompromised patients, so treatment urgency is very different.
Common mistake
Wrong: All filarial nematodes are transmitted by mosquitoes.
Right: Wuchereria bancrofti and Brugia malayi are transmitted by mosquitoes, but Loa loa is transmitted by the deer fly (Chrysops) and Onchocerca volvulus by the blackfly (Simulium).
Mosquitoes transmit Wuchereria bancrofti and Brugia malayi, but two major filarial pathogens use completely different vectors. Onchocerca volvulus is transmitted by the Simulium blackfly — remember 'black fly → black blindness (river blindness).' Loa loa is transmitted by the Chrysops deer fly. These vector associations are a favorite Step 1 test point because they require memorization beyond the default assumption that 'filarial = mosquito.' Tie each worm to its vector and the geographic/ecologic setting it implies.
Common mistake
Wrong: Ivermectin is safe to use empirically in all filarial infections including Loa loa.
Right: Ivermectin is contraindicated in high Loa loa microfilaremia because rapid microfilarial killing can cause encephalopathy; diethylcarbamazine (DEC) is used for Loa loa.
Ivermectin works by rapidly killing microfilariae, which is exactly the problem in high-burden Loa loa: massive microfilarial death releases inflammatory mediators that cause encephalopathy, a potentially fatal complication. This is why empiric ivermectin — often used in mass drug administration programs for onchocerciasis — is contraindicated in regions co-endemic for Loa loa unless microfilarial counts are confirmed to be low. For Loa loa with high microfilaremia, diethylcarbamazine (DEC) is used, with careful monitoring. Knowing this exception is what the exam is actually testing when it brings up ivermectin safety.
Common mistake
Wrong: Trichinella infection comes from ingesting soil-contaminated food.
Right: Trichinella spiralis is acquired by eating undercooked pork or wild game containing encysted larvae, not from soil exposure.
Trichinella spiralis has nothing to do with soil contamination or fecal-oral spread — it is acquired exclusively by eating undercooked meat containing encysted larvae, classically pork or wild game (bear, boar). The larvae encyst in striated muscle, which is why myalgia and periorbital edema (from larvae migrating into facial/orbital muscle) are the hallmarks. Conflating it with other nematodes that use a soil/fecal-oral route (like Ascaris or hookworm) leads to wrong exposure histories and wrong treatment reasoning. When you see 'camping trip + undercooked meat + periorbital edema + eosinophilia,' that is Trichinella until proven otherwise.
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What the exam tests

  1. Given a clinical scenario describing exposure history and symptoms, identify the specific intestinal nematode responsible — know the signature presentation for Ascaris (Löffler syndrome, intestinal obstruction), hookworm (iron-deficiency anemia, cutaneous entry), pinworm (nocturnal perianal pruritus, Scotch tape test), Strongyloides (larva currens, immunocompromised hyperinfection), and Trichinella (periorbital edema, myalgia, eosinophilia after eating undercooked meat).
  2. Match each filarial nematode to its specific arthropod vector and the tissue it targets: Wuchereria bancrofti and Brugia malayi (mosquito → lymphatic obstruction/elephantiasis), Loa loa (Chrysops deer fly → subcutaneous migration, Calabar swellings), and Onchocerca volvulus (Simulium blackfly → river blindness).
  3. Select the correct antinematode drug for a given organism or clinical situation, including why ivermectin cannot be used empirically for all filarial infections — specifically the encephalopathy risk in high-burden Loa loa — and when to use DEC, albendazole, or mebendazole instead.

Can you avoid these mistakes?

A 35-year-old immunosuppressed patient (on high-dose steroids) presents with severe abdominal pain, sepsis, and gram-negative bacteremia. He recently traveled to Southeast Asia and has a history of a migratory perianal rash. What organism is responsible, and why is this patient at particular risk?
A Peace Corps volunteer returns from sub-Saharan Africa with episodic swelling of the forearm and a worm visibly crossing the conjunctiva of his eye. Which organism is this, what vector transmitted it, and which antinematode drug should you avoid — and why?
A 6-year-old child has severe nighttime perianal itching. The mother reports seeing small white worms near the anus. What is the diagnostic test of choice, and what drug do you use to treat the entire household?
You are reviewing two patients: one with a slowly migrating serpiginous rash on the foot after walking barefoot on a beach, and one with a rapidly migrating serpiginous rash near the buttocks. Which organism causes each, and what distinguishes their clinical significance?

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