Pinworm (Enterobius vermicularis)

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

• Metronidazole — treats protozoal infections such as giardiasis, trichomoniasis, and amebiasis; not active against nematodes.
• Praziquantel — used for cestodes (tapeworms) and trematodes (schistosomes); acts by increasing tegument permeability to calcium; not used for pinworm.
• Ivermectin — reserved for strongyloidiasis, onchocerciasis, and scabies; has activity against some nematodes but is not first-line for enterobiasis.
• Permethrin — first-line topical therapy for scabies, which also causes nighttime itching that spreads through a household; a topical scabicide does not eradicate intestinal worms, and the tape-test eggs confirm pinworm rather than mites.

Why Single dose now and again at 2 weeks for everyone living in the home is correct

• Itching returned about a month after one dose, and a second household member now has symptoms: this is ongoing transmission within the family (reinfection), not drug failure, so the same drug is used again.
• Pyrantel pamoate, albendazole, and mebendazole kill worms in the gut but cannot kill eggs; eggs swallowed around the time of a dose hatch afterward and grow into egg-laying adults in about a month.
• The CDC regimen is therefore a single dose followed by a second dose of the same drug 2 weeks later, which kills the worms that were still eggs at the first dose before they can lay eggs of their own; her first course stopped after dose one.
• CDC advises treating everyone in the household, and the caregivers of the diagnosed child, at the same time; the AAP Red Book likewise supports treating the household together when several members are infected or infections recur - both apply here.
• Pinworm is often asymptomatic, so the parents and older brother may carry worms and reseed the children; they are treated without first needing their own tape tests.
• Pyrantel pamoate is sold over the counter and dosed at 11 mg/kg (maximum 1 g) for each person, adults included, at both time points.

Why the others are wrong

• Single dose now and again at 2 weeks for the two symptomatic children - the schedule is right, but pinworm carriers are often itch-free; with two infected members, untreated asymptomatic parents or the 10-year-old keep seeding eggs onto fingers, bedding, and shared surfaces, and the children are reinfected (the trap is treating only the people who are itching).
• Once daily for 3 consecutive days for everyone living in the home - the right people, but all three doses land within the same few days; they kill the worms present now and miss the worms that hatch from eggs swallowed around that time, which mature after the last dose and restart the household cycle about a month later; a 3-day pyrantel course is a CDC regimen for hookworm, not pinworm (the trap is assuming more consecutive doses make a more thorough course).
• Once daily for 3 consecutive days for the two symptomatic children - this combines both errors: the three doses fall too close together to reach the next generation of worms, and the asymptomatic parents and 10-year-old are left untreated to reinfect the children.
• Single dose now for everyone living in the home, repeated if itching recurs - the right drug for the right people, but the 2-week dose is scheduled, not held for symptoms: worms that hatch from eggs swallowed around the first dose mature and lay eggs before itching returns, so by the time a repeat is triggered the household cycle has restarted, exactly as it did after her single dose (the trap is treating the second dose as optional).

Additional high-yield points

• The 2-week dose is scheduled, not held until symptoms return: by the time itching recurs, new females are already laying eggs around the anus and the household cycle has restarted - the pattern this girl shows after her single dose.
• Recurrence after treatment usually reflects reinfection, so retreat with the same agent. Albendazole (400 mg) and mebendazole (100 mg) are equally acceptable first-line choices, each given as one dose repeated at 2 weeks.
• Mechanism: pyrantel is a depolarizing neuromuscular blocker that causes spastic paralysis of the worm, which is then passed in stool; albendazole and mebendazole bind parasite beta-tubulin and block microtubule assembly.
• A pregnant household contact is not treated automatically with the rest of the family: CDC advises considering treatment in pregnancy only when the infection is compromising the pregnancy (for example, weight loss or sleeplessness), and even then withholding it until the third trimester, when any risk to the fetus is likely lower. For a child younger than 2 years, CDC advises weighing risks and benefits before treating.
• Hygiene: bathe in the morning to remove eggs laid overnight, change underwear daily, keep nails short, wash hands with soap and warm water and scrub under the nails, discourage scratching and nail biting, and launder bedding and clothing in hot water; eggs can survive on objects for 2 to 3 weeks.
• Daycare or institutional outbreaks follow the same rule: mass, simultaneous treatment, repeated 2 weeks later.
• Diagnosis is the tape test done in the morning before bathing or defecation; one test detects about 50% of infections and tests on 3 mornings about 90%. Stool ova and parasites testing is insensitive, and eosinophilia is usually absent because the worm does not invade tissue.