Scabies

Etiology And Pathophysiology

Caused by burrowing and toxic antigen release by female Sarcoptes scabiei var hominis mite.
Transmitted predominantly via direct physical and sexual contact.
Fomite transmission remains uncommon since mites die within 72 hours off human host.
Adult female measures 0.4 mm, possesses four sets of legs, and features hemispheric body with transverse corrugations and dorsal spines.
Gravid female exudes keratolytic enzymes, excavating shallow stratum corneum wells within 30 minutes.
Tract extends 0.5 to 5.0 mm daily strictly along stratum granulosum boundary.
Mite deposits 10 to 25 oval eggs alongside brown fecal pellets termed scybala.
Eggs hatch within 3 to 5 days, releasing larvae that migrate rapidly to skin surface.
Larvae molt into nymphs, achieving complete reproductive maturity within 2 to 3 weeks.
Primary initial infestation exhibits 1-month latent asymptomatic phase while antigen sensitization occurs.
Subsequent reinfestation triggers intense immunologic hypersensitivity reaction within mere hours.

Primary pathognomonic lesion represents grey, threadlike, serpentine burrow terminating in minute papule.
Characterized universally by intense, generalized pruritus worsening significantly at night.
Typical older child distribution involves interdigital spaces, wrist flexors, anterior axillary folds, buttocks, umbilicus, and genitalia.
Head, neck, palms, and soles remain characteristically spared in older immunocompetent patients.

Clinical Variant	Diagnostic Morphology	Typical Anatomical Distribution	Target Pediatric Population
Infantile Scabies	Bullae, pustules, wheals, and diffuse eczematous eruption masking classical burrows.	Face, scalp, neck, palms, and soles consistently affected.	Infants and toddlers.
Nodular Scabies	Pruritic, red-brown or violaceous firm nodules representing pronounced hypersensitivity reaction.	Covered areas including axillae, groin, and male genitalia.	Children and sexually active adolescents.
Crusted (Norwegian)	Massive hyperinfestation featuring psoriasiform hyperkeratotic plaques, thick crusts, and dystrophic nails.	Scalp, face, neck, hands, feet, with dense subungual mite populations.	Immunocompromised, Down syndrome, institutionalized patients.
Canine Scabies	Tiny papules, vesicles, wheals lacking classic burrows due to host incompatibility.	Arms, chest, abdomen representing direct contact points with infested animal.	Pet owners exposed to Sarcoptes scabiei var canis.

Severe excoriations precipitate secondary pyodermas including impetigo, ecthyma, folliculitis, and cellulitis.
Streptococcal superinfection potentially triggers acute poststreptococcal glomerulonephritis.
Massive infestation in crusted variants causes significant peripheral eosinophilia and generalized lymphadenopathy.
Histopathology of nodular scabies mimics malignant lymphoid neoplasms, demonstrating deep perivascular infiltrate containing atypical mononuclear cells, histiocytes, and plasma cells.
Diagnostic confirmation requires microscopic visualization of mites, ova, or scybala from targeted epidermal scrapings.

Treat all household contacts and caretakers simultaneously regardless of clinical symptom presence.
Launder all clothing, towels, and bed linens utilizing hot water exceeding 130°F followed by high-heat drying.
Isolate non-washable fomites in sealed plastic bags for minimum 72 hours.
Post-scabietic pruritus frequently persists for weeks secondary to lingering retained mite antigens.
Manage residual pruritus aggressively utilizing topical corticosteroids and systemic antihistamines.
Canine variant remains self-limiting in humans; mandates prompt veterinary treatment of infested animal.

Medication Agent	Administration Protocol	Clinical Notes And Contraindications
Permethrin 5% Cream	Apply entire body neck down. Wash off after 8 to 14 hours. Repeat application in 1 week.	First-line gold standard therapy. Infant protocol mandates scalp and facial application. Safe for infants >2 months.
Oral Ivermectin	Single oral dose of 200 micrograms/kg. Repeat identical dose in 14 days.	Indicated for severe, crusted, or endemic institutional outbreaks. Contraindicated in children <15 kg or <5 years.
Crotamiton 10% Lotion	Apply topically twice daily for 14 consecutive days.	Recommended alternative specifically for neonates and infants <2 months.
Benzyl Benzoate 10-25%	Administer two to three sequential applications at 12 to 24-hour intervals.	Highly irritant preparation. Exacerbates pruritus and preexisting eczematous dermatitis.
Lindane 1% Lotion	Apply neck down, wash off strictly after 8 to 12 hours.	Carries significant risk of systemic neurotoxicity. Strictly contraindicated in infants <2 months, pregnancy, and breastfeeding.