What is the most likely diagnosis for a female patient presenting with nocturnal anal itching, excoriations around the anus, and no changes in bowel habits, following a month-long stay in a summer camp cabin with other girls?

Pinworms (Enterobius vermicularis)

The most likely diagnosis is pinworms (Enterobius vermicularis), given the classic presentation of nocturnal anal itching with perianal excoriations following close contact in a communal living setting, without changes in bowel habits or other systemic symptoms. 1

Clinical Reasoning

The clinical presentation strongly points to pinworm infection based on several key features:

• Nocturnal pruritus ani is the cardinal symptom of pinworm infestation, occurring because female worms migrate to the perianal area at night to deposit eggs 1, 2, 3
• The excoriations around the anus result from scratching in response to the intense itching 1
• The 2-4 week prepatent period aligns with symptom onset a few days after returning from camp, where transmission occurred during the month-long stay 1
• Pinworms are highly contagious and spread via the fecal-oral route, making transmission in close quarters like summer camp cabins extremely common 1, 4, 2
• The infection affects approximately 30% of children worldwide, with the highest prevalence in children aged 5-14 years, and is facilitated by overcrowding 2, 5

Why Other Diagnoses Are Less Likely

Giardiasis presents with diarrhea and abdominal cramping, not isolated perianal itching, and would not explain the physical findings 1

Internal hemorrhoids can cause perianal itching through mucus discharge, but this patient has no palpable hemorrhoid on examination and lacks the typical bleeding or prolapse symptoms 1, 6

Lyme disease does not cause isolated perianal itching or excoriations and would present with systemic symptoms, rash (erythema migrans), or arthritis [@general medical knowledge@]

Scabies causes generalized pruritus that worsens at night but typically involves the hands, wrists, and web spaces between fingers, not isolated perianal involvement 1

Diagnostic Approach

• The cellophane tape test (sellotape test) is the diagnostic method of choice, performed by placing the sticky side of tape on the perianal skin in the morning before bathing, then examining it under microscopy for ova 1
• A single test has approximately 50% sensitivity, but three tests performed on consecutive mornings increase sensitivity to approximately 90% 1, 2
• Stool examination is not recommended because pinworms and eggs are not usually passed in stool 1, 2
• Direct visualization of adult worms (tiny white threads, 6-7 mm in length) in the perianal area provides definitive diagnosis 1, 4, 7

Treatment Recommendations

First-line treatment is mebendazole 100 mg as a single oral dose, repeated in two weeks 1, 4, 2

Alternative regimens include:

• Albendazole 400 mg as a single dose, repeated in two weeks 1, 2
• Pyrantel pamoate 11 mg/kg (maximum 1 g) as a single dose, repeated in two weeks 2

Mebendazole and albendazole are both adulticidal and ovicidal, making them superior to pyrantel pamoate, which is only adulticidal 2

Treatment of all household members should be considered, especially with multiple or repeated symptomatic infections, because reinfection is extremely common 4, 2, 5

Prevention of Reinfection

Critical hygiene measures to prevent reinfection include:

• Wash hands and fingernails with soap frequently during the day, especially before eating and after using the toilet 1, 4
• Wear tight underpants both day and night and change them daily 4
• Clean bedroom floors by vacuuming or damp mopping for several days after treatment; avoid dry sweeping that stirs up dust 4
• Wash bed linens and night clothes after treatment without shaking them 4
• Clip fingernails short and avoid finger-sucking, nail-biting, and scratching in the anogenital area 2

Common Pitfalls

Recurrences are common even with effective medication due to repeated cycles of autoinfection, given the short life span of adult pinworms and the ease of reinfection from contaminated surfaces or other household members 2, 5

In females, ectopic migration can cause vaginal colonization, which may serve as a reservoir for recurrent infection even in the absence of gastrointestinal symptoms 7, 5

Eggs floating in the air can be swallowed and cause infection, so environmental decontamination is essential 4