Enterobius vermicularis
Also known as: pinworm
Enterobius vermicularis, commonly called pinworm, is an intestinal nematode that causes nocturnal perianal itching, most often in children. It is the most common helminth infection in the United States and is diagnosed with the tape test.
Enterobius vermicularis is a small white roundworm (nematode) that infects the human gastrointestinal tract. Known as pinworm, it is the most common helminth infection in the United States, and it overwhelmingly affects school-age children and their household contacts.
Transmission is fecal-oral. A person ingests microscopic eggs from contaminated hands, bedding, or surfaces; larvae hatch in the intestine and adult worms settle in the cecum and colon. At night, gravid female worms migrate out to the perianal skin to lay thousands of eggs, producing the classic symptom: intense perianal itching that is worst at night. Scratching transfers eggs back to the fingers, perpetuating reinfection and household spread. Because the worms remain in the gut lumen and do not invade tissue, infection typically does not cause the eosinophilia seen with invasive helminths.
Diagnosis uses the tape test (scotch tape test): clear adhesive tape pressed against the perianal skin in the early morning picks up eggs, which appear football-shaped and flattened on one side under the microscope. Stool ova-and-parasite exams are usually negative, since eggs are laid on the skin rather than passed in stool. Treatment is a benzimidazole (albendazole or mebendazole) or pyrantel pamoate, with a repeat dose about two weeks later and simultaneous treatment of household contacts to break the reinfection cycle.
The USMLE Step 1 tests Enterobius in parasitology. Recognize the vignette of a child with nocturnal perianal pruritus, name the tape test as the diagnostic step, and know the drugs of choice — plus the detail that eosinophilia is characteristically absent.
Key takeaways
- Enterobius vermicularis (pinworm) is the most common helminth infection in the United States, primarily affecting children.
- Transmission is fecal-oral, and the hallmark symptom is perianal itching that is worst at night.
- Diagnosis is by the tape test; stool studies are usually negative and eosinophilia is typically absent.
- Treatment is albendazole, mebendazole, or pyrantel pamoate, repeated in about two weeks and given to household contacts.
- USMLE Step 1 vignettes pair a child with nocturnal perianal pruritus and ask for the organism, test, or treatment.