What parasite infestation is most commonly associated with itching in the perianal area and autoinfection?

Pinworm (Enterobius vermicularis) Infestation

Enterobius vermicularis (pinworm) is the parasite that causes perianal itching and autoinfection. 1, 2, 3

Mechanism of Autoinfection

• Autoinfection occurs through the fecal-oral route when eggs deposited in the perianal area are transferred to the mouth via contaminated fingers, particularly after scratching the itchy perianal region 3, 4, 5
• Female pinworms migrate from the cecum and ascending colon to the perianal area at night to deposit eggs, triggering intense nocturnal pruritus 6, 7
• Scratching causes pinworm eggs to stick to fingers, and reinfection results when fingers are placed in the mouth 4
• Eggs can contaminate bedclothes, underwear, hands, and food; even airborne eggs can be swallowed and cause infection 4

Clinical Presentation

• Nocturnal perianal itching is the cardinal and most common symptom, often leading to sleep disturbances and irritability, especially in children 2, 3, 5
• Additional symptoms include weight loss, irritability, diarrhea, and abdominal pain 1, 2, 3
• In females, pinworms may colonize the genital tract causing vaginal discharge 1, 2, 3
• Eosinophilia may be present, particularly with heavy infections or eosinophilic colitis (a rare complication) 2, 3

Diagnosis

• The cellophane tape test (sellotape test) is the preferred diagnostic method, performed by applying adhesive tape to the perianal skin in the morning before bathing or defecation to collect eggs 2, 3, 5
• A single test has approximately 50% sensitivity, but performing the test on three consecutive mornings increases sensitivity to approximately 90% 5
• Stool examination is not recommended as pinworms and eggs are not usually passed in stool 5

Treatment

First-line treatment options include: 2

• Albendazole 400 mg as a single oral dose 2
• Mebendazole 100 mg as a single oral dose 2, 4, 5
• Both medications must be repeated after 2 weeks to kill newly hatched worms and prevent reinfection 2, 5

Treatment of all household members should be considered, especially with multiple or repeated symptomatic infections, as reinfection is extremely common even with effective medication 5

Prevention Measures to Break Autoinfection Cycle

• Wash hands and fingernails with soap frequently during the day, especially before eating and after using the toilet 4, 5
• Wear tight underpants both day and night and change them daily 4
• Clip fingernails short and avoid finger-sucking, nail-biting, and scratching in the anogenital area 5
• For several days after treatment, clean bedroom floors by vacuuming or damp mopping (avoid dry sweeping that stirs up dust) 4
• After treatment, wash bed linens and night clothes without shaking them 4

Common Pitfall

Despite effective treatment, recurrences are common and are typically due to repeated cycles of autoinfection rather than treatment failure, given the short life span of adult pinworms 5. This necessitates strict adherence to hygiene measures and consideration of treating all household contacts.