Scabies: Sarcoptes Mite Infestation, Burrow Recognition, and Systemic Scabicide Therapy

Clinical Overview

Scabies is a highly contagious parasitic infestation caused by the human itch mite Sarcoptes scabiei var. hominis. This microscopic arachnid burrows into stratum corneum, triggering delayed-type hypersensitivity reaction manifesting as intense pruritus, papules, and characteristic burrows. Scabies affects 300+ million people annually worldwide, particularly in resource-limited settings and institutional environments (nursing homes, prisons, military barracks) where overcrowding facilitates transmission.

Epidemiology

Scabies prevalence varies 0.2-12% depending on socioeconomic status and geographic region. Transmission: direct prolonged skin-to-skin contact (15-30 minutes in endemic areas, longer in non-endemic). Fomite transmission rare despite persistence on fomites (mites survive 24-36 hours). Sexual transmission common. Attack rate household contacts 50% within 1 month if untreated. Crusted scabies (Norwegian scabies): severe variant with poor hygiene, immunosuppression, or chronic neurologic disease; mite burden 10,000-100,000 vs. typical 10-15 mites; highly contagious despite minimal pruritus. Institutional scabies outbreaks common if unrecognized; rapid spread through contact.

Pathophysiology

Female mites tunnel into stratum corneum at 0.5-1mm per day, depositing eggs and feces. Immune response to mite antigens and fecal matter produces Th2-mediated reaction with IL-4, IL-5 production (eosinophilia development) and Th1-mediated response with IFN-gamma (skin infiltration of T cells, macrophages). Pruritus driven by histamine release from mast cells and substance P elevation. Pruritus often worsens at night (nocturnal predominance) and paradoxically improves with successful treatment despite ongoing mite presence initially. Secondary bacterial infection (impetigo) develops in 10-25% of cases due to scratching. Crusted scabies: massive mite proliferation (1000-fold higher) due to absent/impaired immune response; minimal pruritus despite heavy infestation, permitting unrecognized spread in institutional settings.

Clinical Presentation

Typical scabies: intense pruritus (90% of patients), worse at night. Papules 2-3mm erythematous, distributed on fingers/hands (70%), genitals (50%), axillae (40%), waist/belt line (35%), buttocks (30%). Characteristic burrows: linear or S-shaped, 1-10mm long, subtle white or skin-colored lines visible on palm/sole/webbing. Crusted areas from scratching. Face/scalp spared in adults (except infants, elderly). Post-scabetic pruritus: persistent pruritus 2-4 weeks post-treatment despite mite eradication due to persistent immune response.

Crusted scabies (institutional variant): hyperkeratotic plaques on palms/soles, minimal pruritus, possible fever/malaise. Minimal burrows visible despite massive mite burden. Contagious despite absence of prominent symptoms.

Diagnosis

Diagnosis primarily clinical based on characteristic pruritus with burrows on hands/genitals. Dermoscopy identifies burrows (seen as dark linear structures). Microscopy: scrapings from burrow (mineral oil mount) visualized 10x-40x, showing mite body, eggs, or fecal pellets (sensitivity 40-60%). Negative scraping does NOT exclude scabies; multiple attempts increase sensitivity to 70%. Dermatoscopy (10x magnification) shows delta-shaped mites within burrow. Histopathology (rarely needed): shows mite within keratin, inflammatory infiltrate.

Dermoscopy identifies burrows (seen as dark linear structures with mite bodies). Microscopic examination (scrapings from burrow using mineral oil mount or KOH prep) visualized at 10x-40x magnification showing mite body, eggs, or fecal pellets is gold standard, with sensitivity 40-60% on initial attempt. Multiple scraping attempts increase sensitivity to 70-75%. Negative scraping does NOT exclude scabies; clinical presentation sufficient for diagnosis in suspected cases with compatible presentation. Dermatoscopy (10x magnification) demonstrates delta-shaped mites within burrow structures. Histopathology (rarely needed for diagnosis): shows mite within keratin, inflammatory infiltrate characteristic. Immunohistochemical staining rarely employed. Culture negative (scabies not cultivable on standard media). Serologic testing (IgG against Sarcoptes antigens) used in some research settings but not clinically standard.

Treatment Algorithm

Typical Scabies - First-Line Topical: Permethrin 5% cream applied from neck down (full body including genitals, inframammary folds, toe webs, soles) at bedtime. Leave on 8-14 hours, then shower. Repeat second application 1 week later. 95% efficacy. Crotamiton 10% cream alternative (similar efficacy, fewer systemic effects) applied similarly. Sulfur 5-10% ointment for infants <2 months, pregnant women (permethrin safety unclear in pregnancy).

Typical Scabies - Systemic (Oral): Ivermectin 200mcg/kg oral dose, repeated 1-2 weeks later. Efficacy 90-95%, requires 2 doses for typical scabies. Preferred if unable to apply topicals (infants, bedridden), institutional outbreaks (easier implementation). Less effective than permethrin for crusted scabies (requires 3-5 doses weekly).

Crusted Scabies: Aggressive therapy required. Combination approach: topical permethrin 5% for 24 hours (then wash), repeated every 2-3 days for 1-2 weeks (heavy mite burden). Oral ivermectin 200mcg/kg weekly x 5 doses. Keratolytic agents (salicylic acid 10% ointment) applied to hyperkeratotic areas preceding scabicide. Hospitalization often required for compliance/decontamination.

Household/Environmental Decontamination: Wash all clothing/bedding in hot water (>50°C), dry in hot dryer. Items unable to wash placed in sealed plastic bags x 1 week (mites die without host). Furniture: vacuum thoroughly, spray with permethrin spray if severe. Treat all household contacts simultaneously regardless of symptoms (mites contagious during incubation period).

Prognosis

Scabies: 95% cure with single permethrin application (repeat 1 week later). Ivermectin: 90% cure (requires 2 doses). Post-scabetic pruritus persists 2-4 weeks despite successful mite eradication; topical corticosteroids (triamcinolone 0.1% cream) and antihistamines control during this period. Relapse: 5-10% if inadequate household contact treatment or re-exposure. Crusted scabies: requires aggressive therapy; relapses common (30%) if treatment inadequate.

When to See a Dermatologist

Dermatologists recognize scabies, confirm diagnosis, initiate treatment, manage institutional outbreaks, and treat crusted scabies variants. Coordinate with infection control if institutional outbreak.

Frequently Asked Questions

Q: How contagious is scabies?
A: Highly contagious. Requires 15-30 minutes skin-to-skin contact for mite transmission. 50% of household contacts infected within 1 month if untreated. All close contacts require simultaneous treatment regardless of symptoms.

Q: Do I need to treat my whole house?
A: Hot wash clothing/bedding and seal other items x 1 week. Mites die without human host within 3-5 days on fomites. Extensive environmental treatment (fumigation) unnecessary. Good hygiene sufficient.

Q: Why does itching continue after treatment?
A: Post-scabetic pruritus (continued itching despite mite death) results from persistent immune sensitization. Resolves 2-4 weeks. Topical steroids and antihistamines provide relief while awaiting resolution. This is NOT treatment failure; continue observation.

Q: Can scabies be transmitted sexually?
A: Yes. Sexual transmission common, particularly if genital involvement in partner. Both partners require treatment simultaneously. Barrier protection (condoms) reduces but does not eliminate transmission risk.

References