Skabies

01Pathophysiology

Scabies is caused by the obligate human ectoparasite Sarcoptes scabiei var. hominis, a microscopic mite that completes its entire life cycle within the human epidermis. The female mite burrows into the stratum corneum, creating a tunnel where she deposits 2 to 3 eggs per day over her 4 to 6 week lifespan. These burrows are the hallmark lesion of scabies and correlate directly with the linear, thread-like tracks seen on physical examination.

The intense pruritus that defines scabies is not caused by the mite itself but rather by a type IV (delayed-type) hypersensitivity reaction to the mite's body, eggs, and fecal material (scybala). This is a critical concept: in a primary (first-time) infestation, there is a sensitization period of 4 to 6 weeks before symptoms develop. During this asymptomatic window, the patient is already infested and contagious but has no itching. In contrast, re-infestation triggers symptoms within 1 to 3 days because the immune system is already primed. This delayed hypersensitivity mechanism explains why the degree of itching is disproportionate to the actual mite burden, which in classic scabies is typically only 10 to 15 mites on the entire body.

Crusted (Norwegian) scabies represents the opposite end of the spectrum and occurs in patients with impaired cell-mediated immunity. Because the host cannot mount an effective immune response, mite numbers proliferate unchecked, reaching millions of mites on the skin surface. This produces thick, crusted, hyperkeratotic plaques rather than the typical papules and burrows. Paradoxically, pruritus may be minimal or absent because the hypersensitivity response is blunted. Crusted scabies is extremely contagious due to the enormous mite burden and can be transmitted even through brief contact or fomites (bedding, clothing).

The predilection for certain body sites is explained by the mite's preference for thin skin with few hair follicles. The interdigital web spaces, flexor surfaces of the wrists, and genital skin provide optimal conditions for burrowing. In infants, the skin barrier is thinner overall, which is why involvement of the palms, soles, face, and scalp is common in this age group but unusual in adults.

02Classification and Clinical Manifestation

03Diagnostic Workup

The workup for scabies is primarily clinical. In a typical exam vignette, the diagnosis is established by the combination of intense nocturnal pruritus, characteristic distribution of papules and burrows, and the presence of similarly symptomatic household members or close contacts. When the clinical picture is classic, no further testing is necessary to make the diagnosis.

When confirmation is needed, the best initial test is a skin scraping with mineral oil preparation. The technique involves placing a drop of mineral oil over an intact, unexcoriated burrow (the interdigital web spaces are the best site), scraping superficially with a #15 scalpel blade, transferring the material to a glass slide, and examining under low-power light microscopy. A positive result showing mites, eggs, or scybala is diagnostic. However, the sensitivity is limited because the mite burden in classic scabies is so low; a negative scraping does not exclude the diagnosis, and clinical judgment should prevail.

Dermoscopy has emerged as a valuable non-invasive adjunct. The pathognomonic finding is the "delta-wing jet sign", which is a triangular dark structure at the leading end of a burrow corresponding to the anterior portion of the mite. This finding has higher sensitivity than visual inspection alone and can guide the clinician to the optimal site for scraping.

Skin biopsy is reserved for diagnostically challenging situations: bullous scabies mimicking bullous pemphigoid, crusted scabies mimicking psoriasis or seborrheic dermatitis, or nodular scabies mimicking lymphoma. Histology typically shows a spongiotic dermatitis with eosinophils, and cross-sections of the mite or its products may be visible within the stratum corneum.

An important clinical pearl for the exam: the presence of pruritic symptoms in multiple household members or close contacts is a powerful diagnostic clue that should immediately raise suspicion for scabies, even before any testing is performed.

04Management and Treatment

Acute management centers on eradication of the mite. Permethrin 5% cream is the treatment of choice for classic scabies in nearly all patient populations. The cream must be applied meticulously to every square centimeter of skin from the jawline to the soles of the feet, with particular attention to the interdigital web spaces, wrists, axillae, umbilicus, buttocks, and genitalia. Under the fingernails should also be treated, as mites accumulate there from scratching. The cream is left on for 8 to 14 hours (typically applied at bedtime and washed off the following morning) and then repeated once after 1 to 2 weeks to kill any newly hatched larvae that survived the initial treatment.

Oral ivermectin at a dose of 200 mcg/kg is an alternative for patients who cannot tolerate topical therapy, for cases that fail permethrin, or as part of combination therapy for crusted scabies. For classic scabies, a single dose repeated in 1 to 2 weeks is usually sufficient. For crusted scabies, combination therapy with both oral ivermectin (multiple doses on days 1, 2, 8, 9, and 15) and daily permethrin application is required because of the extraordinary mite burden. Crusted scabies also necessitates strict contact isolation and thorough environmental decontamination of clothing, bedding, and furniture.

A point frequently tested: ivermectin is contraindicated in pregnant or lactating women and in children weighing less than 15 kg. In these populations, permethrin remains the agent of choice.

Simultaneous treatment of all household members and close contacts is mandatory, regardless of whether they have symptoms. This is because of the 4 to 6 week asymptomatic sensitization period; contacts may be infested and contagious without knowing it. Failure to treat contacts simultaneously is the most common cause of treatment failure and re-infestation.

Environmental measures include washing all clothing, bedding, and towels in hot water (at least 60 degrees Celsius or 140 degrees Fahrenheit) and drying on high heat, or sealing items in a plastic bag for at least 72 hours (mites cannot survive off the human host for more than 48 to 72 hours).

Post-treatment pruritus is expected and can persist for 2 to 4 weeks after successful eradication. This residual itching is caused by the ongoing immune response to dead mite antigens in the skin and should be managed with oral antihistamines and low-potency topical corticosteroids. Patients should be counseled that continued itching does not necessarily indicate treatment failure. True treatment failure or re-infestation should be suspected only if new burrows appear or if itching worsens beyond 4 weeks.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap on scabies questions is post-treatment pruritus being mistaken for treatment failure. The vignette will describe a patient who was appropriately treated with permethrin, completed the regimen, and returns 1 to 2 weeks later still itching. The temptation is to select "re-treat with permethrin" or "switch to ivermectin," but the correct answer is symptomatic management with antihistamines and reassurance, provided no new burrows are present. Itching can persist for up to 4 weeks after all mites have been killed because of the lingering hypersensitivity response. Students who do not understand the immunologic basis of scabies pruritus will fall for this distractor every time.

A second common testing angle involves recognition of crusted scabies in immunocompromised patients. The vignette will present an HIV-positive or transplant patient with a widespread, thick, scaly eruption and minimal itching. The intended wrong answers are psoriasis or seborrheic dermatitis. The key discriminator is either the mention of an institutional outbreak among contacts (because crusted scabies is extraordinarily contagious) or the finding of mites on scraping. Remember that crusted scabies requires combination oral and topical therapy, not permethrin alone.

A third pearl: exam writers love to test the concept that all household contacts must be treated simultaneously, even if asymptomatic. If the vignette describes a child with classic scabies and asks for the next best step, the correct management includes treating the entire household. A common wrong answer is to treat only the symptomatic individual.

Finally, watch for age-based distribution differences. In adults, scabies spares the head and neck. In infants, the face, scalp, palms, and soles are commonly involved. If the vignette describes a vesiculopustular eruption on an infant's palms and soles with an irritable, poorly sleeping baby and a mother who is also scratching, the diagnosis is scabies until proven otherwise.

The core competency being tested across scabies questions is the ability to integrate epidemiologic clues (close contacts, institutional setting, immunocompromised host), recognize pattern-based distribution, and correctly sequence management decisions, particularly the distinction between persistent immune-mediated symptoms and true treatment failure.