A 34-year-old man comes to the physician because of progressive swelling of the left lower leg for 4 months. One year ago, he had an episode of intermittent fever and tender lymphadenopathy that occurred shortly after he returned from a trip to India and resolved spontaneously. Physical examination shows 4+ nonpitting edema of the left lower leg. His leukocyte count is 8,000/mm3 with 25% eosinophils. A blood smear obtained at night confirms the diagnosis. Treatment with diethylcarbamazine is initiated. Which of the following is the most likely route of transmission of the causal pathogen?

Deposition of thread-like larvae into the skin by a female mosquito

Penetration of the skin by hookworms in feces

Penetration of the skin by cercariae from contaminated fresh water

Deposition of larvae into the skin by a female black fly

Ingestion of encysted larvae in undercooked pork

Nematodes (roundworms) for USMLE Step 1: High-Yield Microbiology Lessons

Intestinal Nematodes (Ingested) - The Oral Route

General Features: Round, unsegmented worms with a complete digestive tract.
Transmission: Acquired by ingestion of eggs or larvae.
📌 Mnemonic: Get sick if you EATTT these! (Enterobius, Ascaris, Trichinella, Trichuris).

Organism	Transmission	Key Clinical Finding(s)
Enterobius vermicularis (Pinworm)	Fecal-oral (eggs)	Perianal pruritus, especially at night. Diagnosed by tape test.
Ascaris lumbricoides (Giant roundworm)	Fecal-oral (eggs)	Loeffler's syndrome (eosinophilic pneumonitis). Intestinal obstruction.
Trichuris trichiura (Whipworm)	Fecal-oral (eggs)	Often asymptomatic. Heavy infection can lead to rectal prolapse.
Trichinella spiralis	Undercooked meat (larvae)	Myalgias, periorbital edema, eosinophilia.

⭐ Ascaris lumbricoides, the largest intestinal nematode, can cause intestinal obstruction and its larval migration phase can cause Loeffler's syndrome (eosinophilic pneumonitis).

Intestinal Nematodes (Cutaneous) - Skin Invaders

Nematodes that enter the body by penetrating the skin, often the feet. They migrate to the lungs, are coughed up, swallowed, and then mature in the small intestine.

Organism	Transmission	Key Clinical Findings
Strongyloides stercoralis	Filariform larvae penetrate skin	Autoinfection cycle; hyperinfection in immunocompromised; eosinophilia.
Ancylostoma duodenale	Filariform larvae penetrate skin	Microcytic anemia (more severe blood loss); ground itch.
Necator americanus	Filariform larvae penetrate skin	Microcytic anemia (less severe); ground itch.

Hookworm infection: skin, histology, adult, and egg

⭐ Hyperinfection Syndrome: In Strongyloides, corticosteroid use can trigger a fatal autoinfective cycle, leading to disseminated larval invasion of multiple organs and septic shock from gut bacteria translocation.

Tissue & Filarial Nematodes - Blood & Body Dwellers

These nematodes inhabit blood and subcutaneous tissues, transmitted by insects.

Organism	Vector	Clinical Features
*Wuchereria bancrofti*	Mosquito	Elephantiasis, lymphatic filariasis, lymphedema
*Brugia malayi*	Mosquito	Similar to Wuchereria, causes elephantiasis
*Onchocerca volvulus*	Female blackfly	River blindness, hyperpigmented skin, nodules
*Loa loa*	Deer fly, horse fly	African eye worm, transient Calabar swellings
*Dracunculus medinensis*	Copepod	Guinea worm disease, skin ulcer, worm emerges

Diagnosis: Blood smear (Giemsa stain) to find microfilariae (timing matters: Loa loa during day, Wuchereria at night).
Treatment: Diethylcarbamazine (DEC) for most, but Ivermectin for Onchocerciasis.

⭐ River Blindness: Onchocerca volvulus microfilariae in the eye cause irreversible blindness. Treatment with Ivermectin is key, but it does not kill adult worms.

Nematode Pharmacology - The Worm Killers

Drug	Mechanism (brief)	Key Indications (Nematodes)
Bendazoles (Albendazole)	Inhibits parasitic microtubule polymerization.	Broad-spectrum: Ascaris, Hookworm, Pinworm, Whipworm.
Pyrantel Pamoate	Depolarizing neuromuscular blocker → spastic paralysis.	Pinworm (Enterobius), Roundworm (Ascaris).
Ivermectin	Activates glutamate-gated Cl- channels → flaccid paralysis.	Strongyloides, Onchocerca volvulus (River Blindness).
Diethylcarbamazine (DEC)	Sensitizes microfilariae to phagocytosis.	Loa loa, Wuchereria bancrofti (Elephantiasis).

High‑Yield Points - ⚡ Biggest Takeaways

Enterobius vermicularis (pinworm) is diagnosed via the Scotch tape test and causes intense perianal itching.

Ascaris lumbricoides, the giant roundworm, can cause intestinal obstruction and Loeffler's syndrome.

Trichinella spiralis is contracted from undercooked pork and leads to myalgias and periorbital edema.

Strongyloides stercoralis is unique for its autoinfection capability, dangerous in the immunocompromised.

Hookworms (Ancylostoma, Necator) cause iron deficiency anemia by feeding on blood.

Onchocerca volvulus, transmitted by the blackfly, results in river blindness.

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