Dermatology · Infectious Dermatoses
The facts most likely to be tested
Scabies is caused by the mite Sarcoptes scabiei and is transmitted through prolonged skin-to-skin contact.
The hallmark clinical presentation is intense, nocturnal pruritus that worsens at night.
Physical examination reveals linear burrows in the interdigital web spaces, wrists, and axillary folds.
Diagnosis is confirmed via skin scraping and microscopic visualization of the mite, eggs, or scybala (fecal pellets).
First-line treatment for classic scabies is topical permethrin 5% cream applied from the neck down and washed off after 8–14 hours.
Oral ivermectin is the treatment of choice for crusted (Norwegian) scabies or in patients who fail topical therapy.
All close contacts must be treated simultaneously, regardless of whether they are currently symptomatic, to prevent reinfestation.
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A 24-year-old medical student presents to the clinic complaining of a persistent, intense pruritic rash that has kept him awake for the past two weeks. He notes that his roommate has developed similar symptoms. Physical examination reveals multiple erythematous papules and thin, linear burrows located in the interdigital web spaces of both hands and the flexor surfaces of the wrists. There is no evidence of secondary bacterial infection.
What is the most appropriate initial management for this patient and his household contacts?
Topical permethrin 5% cream
The patient's presentation of nocturnal pruritus and pathognomonic linear burrows in interdigital spaces is diagnostic for scabies, which requires treatment with topical permethrin for the patient and all close contacts.
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Etiology / Epidemiology
Caused by Sarcoptes scabiei; highly contagious via prolonged skin-to-skin contact or fomites.
Clinical Manifestations
Intense nocturnal pruritus and burrows in interdigital spaces or flexor surfaces.
Diagnosis
Microscopic skin scraping of a burrow is the gold standard for identifying mites, eggs, or scybala.
Treatment
Permethrin 5% cream applied neck-to-toe; do not use in infants <2 months.
Prognosis
Pruritus may persist for 2-4 weeks post-treatment; treat all household contacts simultaneously.
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Epidemiology & Etiology
Transmission occurs primarily through prolonged direct contact in crowded settings like nursing homes or schools. The mite Sarcoptes scabiei var. hominis burrows into the stratum corneum. Fomite transmission is possible but less common than direct contact.
Pertinent Anatomy
Mites prefer thin, warm skin. Classic sites include interdigital web spaces, wrists, axillae, and the periumbilical region. In infants, involvement often extends to the palms and soles.
Pathophysiology
The female mite burrows into the epidermis to deposit eggs and feces (scybala). A Type IV delayed hypersensitivity reaction to mite proteins causes the intense pruritus. The incubation period for primary infestation is 4-6 weeks.
Clinical Manifestations
Patients present with nocturnal pruritus that is disproportionate to physical findings. Pathognomonic burrows appear as thin, gray-brown, wavy lines. Crusted (Norwegian) scabies is a severe, hyperkeratotic variant seen in immunocompromised patients, presenting with thick scales and high mite burden.
Diagnosis
The microscopic skin scraping of an intact burrow is the diagnostic gold standard. Mineral oil is applied to the site to facilitate collection. If burrows are not visible, a dermoscopy can reveal the delta-wing sign of the mite head.
Treatment
Permethrin 5% cream is the first-line therapy, applied from the neck down and washed off after 8-14 hours. Oral ivermectin is indicated for crusted scabies or treatment failure. All household members must be treated simultaneously to prevent reinfestation.
Prognosis
Post-scabietic pruritus is common and does not indicate treatment failure; it is managed with topical steroids or antihistamines. Failure to resolve after 4 weeks suggests treatment resistance or improper application technique.
Differential Diagnosis
Atopic dermatitis: lacks burrows and nocturnal exacerbation
Pediculosis: involves hair-bearing areas rather than interdigital spaces
Contact dermatitis: localized to exposure site, lacks burrows
Drug eruption: generalized, usually follows medication initiation
Bullous pemphigoid: presents with tense bullae rather than papules