NEMATODES – ENTEROBIUS VERMICULARIS

ENTEROBIUS VERMICULARIS

**definition → Enterobius vermicularis is the pinworm, a small white nematode that lives in the human large intestine and causes enterobiasis

cause → ingestion of embryonated eggs from contaminated hands, clothes, bedding, food or water (fecal‑oral route)

type → only one species infects humans (Enterobius vermicularis); no other clinically important types

morphology

• adult male ≈ 2 cm long, 0.3 mm wide, curved tail, single spicule, copulatory bursa absent
• adult female ≈ 8–13 mm long, 0.3 mm wide, long pointed tail, larger than male, can lay up to 20 000 eggs/day
• egg → oval, flattened on one side, 50‑60 µm × 25‑30 µm, contains a single embryo, sticky outer surface

life cycle (step‑wise)

1. ingestion of embryonated eggs →
2. eggs hatch in duodenum, releasing rhabditiform larvae →
3. larvae mature to adult worms in the caecum and appendix →
4. adult males and females mate in the lumen →
5. gravid females migrate at night to the perianal skin →
6. each female deposits ≈ 10 000 sticky eggs on the perianal skin →
7. eggs become infective in 2‑4 h →
8. eggs are spread by scratching, hand‑to‑mouth contact, clothing, bedding → restart cycle

pathogenesis (step‑wise)

• eggs → irritate perianal skin → itching (especially at night) → scratching → secondary bacterial infection
• adult worms → mild inflammation of caecum/appendix → possible appendicitis or colicky pain

clinical features

• intense perianal itching, worse at night
• restlessness, difficulty sleeping
• irritability, especially in children
• abdominal discomfort, occasional nausea/vomiting
• loss of appetite, mild weight loss
• secondary bacterial infection of scratched skin (impetigo, cellulitis)

complications

• impetigo or cellulitis from scratching
• rare appendicitis or intussusception due to worm burden

laboratory diagnosis

• Scotch‑tape (cellophane) test: press adhesive tape on perianal skin in early morning, mount on slide, look for characteristic eggs under microscope
• direct perianal swab: similar to tape test, may be used if tape unavailable
• stool examination: usually negative because eggs are not passed in feces, but occasional adult worms may be seen

management (treatment & prevention)

• single dose of albendazole 400 mg or mebendazole 100 mg; repeat after 2 weeks to kill newly hatched worms
• pyrantel pamoate 11 mg/kg as alternative single dose, repeat after 2 weeks
• treat all household members simultaneously to prevent reinfection
• strict personal hygiene: daily morning bathing, nail trimming, washing night‑clothes and bedding in hot water, frequent hand washing, keep fingernails short

memory trick for life cycle → “I H L A F E S”
I = Ingest eggs, H = Hatch in duodenum, L = Larvae become adults, A = Adults mate, F = Female migrates to perianal skin, E = Eggs laid, S = Spread by scratching

memory rhyme for clinical picture → “Itchy Night, Restless Fight, Belly Ache, Weight’s Light”

(All points are concise, point‑wise, use arrows for flow, and follow the BHMS NCH/MUHS exam style.)