Most common helminth infection in the US, especially in children — nocturnal perianal pruritus diagnosed by tape test, treated with single-dose albendazole or pyrantel pamoate.
Also known as: pinworm, Enterobius vermicularis, enterobiasis, seatworm
Overview
Intestinal infection by Enterobius vermicularis, a small white nematode living in the colon. Female worms migrate to the perianal area at night to deposit eggs, causing the characteristic pruritus.
Epidemiology
Most common helminthic infection in temperate climates — ~40 million US cases. Peak in school-age children (5-10 years); spreads readily within households, daycare, schools, and institutional settings. Re-infection common from fingernails and bedding.
Try two board-style Pinworm questions
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Question 1Infectious DiseaseEasy
A 6-year-old boy is brought to clinic for perianal itching that wakes him at night. His mother reports she has seen small white threads on his anus in the morning. Two of his siblings have similar symptoms. Exam reveals perianal excoriations. Cellophane tape test demonstrates ovoid, asymmetric eggs flattened on one side. Which of the following is the first-line pharmacologic treatment?
APraziquantel
BIvermectin
CMetronidazole
DAlbendazole
Reveal answer & full explanation
Correct answer: D — Albendazole
APraziquantel
BIvermectin
CMetronidazole
DAlbendazole✓
Why Albendazole is correct
This is classic enterobiasis (pinworm, Enterobius vermicularis), confirmed by the characteristic asymmetric, ovoid eggs flattened on one side on the cellophane tape test, nocturnal perianal pruritus, and visible white threads.
First-line therapy is a single dose of albendazole (or mebendazole or pyrantel pamoate), repeated in 2 weeks to kill newly hatched worms.
All household contacts should be treated simultaneously, and bedding should be laundered.
Albendazole inhibits microtubule polymerization in the parasite, depleting glycogen stores and killing the worm.
Why the others are wrong
C) Metronidazole — treats protozoal infections such as giardiasis, trichomoniasis, and amebiasis; not active against nematodes.
A) Praziquantel — used for cestodes (tapeworms) and trematodes (schistosomes); acts by increasing tegument permeability to calcium; not used for pinworm.
B) Ivermectin — reserved for strongyloidiasis, onchocerciasis, and scabies; has activity against some nematodes but is not first-line for enterobiasis.
Question 2Infectious DiseaseMedium
A 5-year-old girl presents with two weeks of nocturnal anal pruritus and irritability. She attends preschool and her younger brother has similar complaints. Exam shows perianal excoriations without erythema. A cellophane tape applied to the perianal region in the early morning is examined under microscopy, revealing oval eggs flattened on one side. Which of the following is the recommended first-line therapy?
ANitazoxanide oral suspension
BTrimethoprim-sulfamethoxazole course
CDoxycycline oral course
DPyrantel pamoate single dose
Reveal answer & full explanation
Correct answer: D — Pyrantel pamoate single dose
ANitazoxanide oral suspension
BTrimethoprim-sulfamethoxazole course
CDoxycycline oral course
DPyrantel pamoate single dose✓
Why Pyrantel pamoate single dose is correct
The cellophane tape test showing oval eggs flattened on one side confirms Enterobius vermicularis (pinworm)
First-line management: a single oral dose of pyrantel pamoate, mebendazole, or albendazole, with a second dose 2 weeks later to eradicate worms hatched after the first dose
Household contacts should be treated simultaneously and bedding/clothing laundered in hot water
Pyrantel pamoate mechanism: causes neuromuscular blockade in the worm, leading to paralysis and expulsion
Reinfection is common because eggs persist in the environment, making the repeat dose and hygiene measures essential
Why the others are wrong
A) Nitazoxanide oral suspension — nitazoxanide is approved for Giardia and Cryptosporidium, both protozoa; it has no role in helminth (pinworm) therapy
B) Trimethoprim-sulfamethoxazole course — TMP-SMX targets bacteria and Pneumocystis, not Enterobius
C) Doxycycline oral course — doxycycline is an antibacterial used for rickettsial disease, Lyme disease, and atypical pneumonias; it has no role in helminth therapy
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Adult worms (8-13 mm females, 2-5 mm males) live in the cecum. Gravid females migrate at night to the perianal skin, releasing thousands of eggs that become infective within hours. Eggs are deposited on bedding, fingers, clothing, and household surfaces; ingestion (often hand-to-mouth from scratching) restarts the cycle. Eggs survive on surfaces for ~2 weeks.
Clinical presentation
Symptoms
Nocturnal perianal pruritus — hallmark symptom
Restless sleep, irritability
Vulvovaginitis in young girls (worm migration into the vagina)
Visible adult worms (small white threads ~1 cm) on perianal skin at night or in stool
Generally well-appearing child
Classic findings
Young child with new nocturnal perianal itching and restless sleep, family members also affected — pinworm. Demonstration of adult worms by parents at night is sometimes the first clue.
Differential diagnosis
Atopic or contact dermatitis (perianal) — Chronic eczematous changes, broader distribution; treat with topical steroids and emollient
Candidal intertrigo — Beefy red erythema with satellite lesions; KOH positive
Streptococcal perianal dermatitis — Sharply demarcated bright erythema in young children; strep culture positive
Hemorrhoids — Pruritus with rectal bleeding, palpable veins; less common in children
Scabies — Generalized pruritus, burrows in webspaces, family members affected
Vulvovaginitis — Young girl with vaginal pruritus/discharge — pinworm can migrate; consider as part of differential
Diagnostic workup
Diagnostic criteria
Identification of characteristic eggs on tape test or adult worms by direct observation in a patient with consistent symptoms.
Labs
Scotch tape test ('cellophane tape test'): apply transparent tape to perianal skin in the morning before bathing or defecating; examine under microscope for eggs (asymmetric ovoid 'D-shaped' eggs)
Sensitivity increases with multiple specimens on consecutive mornings
Stool ova and parasite exam — poor sensitivity for pinworm (eggs not in stool typically)
Visual identification of adult worms at night sufficient for diagnosis
Imaging
Not required
Diagnostic algorithm
flowchart TD
A[School-age child<br/>nocturnal perianal itching] --> B[Scotch tape test<br/>early AM x several days]
B --> C{Eggs or worms<br/>identified?}
C -->|Yes| D[Albendazole 400 mg PO x 1<br/>OR pyrantel pamoate 11 mg/kg x 1]
D --> E[Repeat dose in 2 weeks]
D --> F[Treat all household contacts]
D --> G[Hygiene: short nails,<br/>AM bath, wash bedding hot]
C -->|No, but high suspicion| H[Empiric treatment reasonable]
C -->|Negative + atypical Sx| I[Consider alternative dx]
Pinworm diagnosis and management algorithm.
Treatment
First-line
Albendazole 400 mg PO × 1; repeat in 2 weeks (to eliminate worms from re-ingested eggs)
Pyrantel pamoate 11 mg/kg PO (max 1 g) × 1; repeat in 2 weeks (available OTC)
Mebendazole 100 mg PO × 1; repeat in 2 weeks (limited availability in US)
Treat all household members simultaneously to prevent re-infection
Hygiene measures (critical):
• Morning bathing/showering to remove eggs
• Daily change of underwear and pajamas
• Wash bedding and towels in hot water
• Keep fingernails short; discourage nail biting and thumb-sucking
• Frequent handwashing, especially before meals and after toileting
Second-line / adjunct
Re-treatment if symptoms recur (re-infection or treatment failure)
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