• Clinical science

Scabies

Abstract

Scabies is a parasitic skin infestation caused by the Sarcoptes scabiei var. hominis (S. scabiei) mite, which is primarily transmitted via direct human-to-human contact. The female scabies mite burrows into the superficial skin layer, causing severe pruritus, particularly at night. Primary lesions commonly include erythematous papules, vesicles, or burrows. Treatment involves topical medical therapy (e.g., permethrin) and decontamination of all clothing and textiles.

Etiology

  • Pathogen: Sarcoptes scabiei var. hominis
  • Transmission
    • Highly contagious
    • Typically via direct physical contact
    • Indirect transmission is rare (e.g., sharing textiles such as bedding, towels, or clothes); more common in crusted scabies S. scabiei
  • Risk factors: crowded living conditions (e.g., institutions such as nursing homes, child care facilities, and prisons)

References:[1][2][3][4]

Pathophysiology

  • The fertilized, female mite tunnels into the superficial skin layer (stratum corneum), forming burrows in which she lays her eggs and deposits feces (scybala).
  • After laying up to 240 eggs over the course of 2 months, the female parasite dies on site.
  • Following a period of 3 weeks, the larvae mature into adult mites, maintaining the infestation cycle.
  • The excretions of the mites and their decomposing bodies contain antigens which cause an immunological response (see type IV hypersensitivity reaction), presenting as severe pruritus and excoriations.

References:[5][6]

Clinical features

  • Incubation period: approximately 3–6 weeks following infestation.
  • Intense pruritus at night
  • Burning sensation
  • Skin lesions
    • Elongated, erythematous papules
    • Burrows of 2–10 mm in length
    • Scattered vesicles filled with clear or cloudy fluid
    • Excoriations, pustules, and secondary infection
    • Bullous or nodular formation (especially in children)
    • Formation of crusts
    • Post-inflammatory hyperpigmentation
  • Predilection sites
    • Wrists (flexor surface)
    • Medial aspect of fingers
    • Interdigital folds (hands and feet)
    • Male genitalia (e.g., scrotum, penis)
    • All other intertriginous areas of the skin (anterior axillary fold, buttocks)
    • Areas surrounding the nipple = mamillary region
    • Periumbilical area or waist
    • Knees (flexor surface)
    • Elbows
    • Feet (dorsal and lateral aspect)
    • Additionally in children, elderly persons, and immunosuppressed patients: scalp, face, neck, under the nail, palms of hands, and soles of feet

References:[3][5][7][8]

Subtypes and variants

  • Crusted scabies (Norwegian scabies)
    • Definition: a rare, severe, and highly contagious form of scabies that presents with a large number of scabies mites and eggs on the skin
    • Epidemiology: typically occurs in immunosuppressed (e.g., HIV), debilitated, or elderly patients
    • Clinical features
      • Slightly pronounced or absent pruritus
      • Lesions
        • Thick crusts or scales on an erythematous base with irregular borders
        • May have a wart-like appearance and fissures
        • Nail changes (i.e., dystrophic, thick)
      • Location
        • Typical areas include the scalp, hands, and feet
        • May involve the whole integument (especially if left untreated)
    • Treatment: rapid and aggressive medical therapy with a scabicidal agent to prevent an outbreak

References:[5]

Diagnostics

  • Typical history and skin lesions on clinical examination (see “Symptoms/clinical findings” above)
  • Environmental diagnosis (direct contact with infected persons)
  • Detection of mites, larvae, ova, or mite feces
    • Revealed in dermoscopy
    • Microscopic examination of the skin
    • Skin scraping and histology

Scabies may be mistaken for eczema, especially as the topical use of glucocorticoids initially alleviates symptoms!

References:[5]

Treatment

  • Medical therapy: topical application of a scabicidal agent
    • Drug of choice: permethrin 5% lotion
      • Mechanism of action: inhibition of voltage-gated sodium channels in the mite → delayed repolarization of neurons → paralysis and death of the mite
    • Alternatives
      • Lindane 1% lotion: in the case of treatment failure or side effects
        • Mechanism of action: blocks GABA channels → neurotoxicity in the mite
      • Oral ivermectin: especially indicated in large outbreaks or severe forms of scabies
      • Others: crotamiton 10% cream or lotion, sulfur ointment
    • Symptomatic treatment of pruritus
  • Possible antibiotics (for secondary or complicated infections; drug of choice depends on culture and sensitivity of pus swab)
  • General measures
    • Wash all textiles (e.g., clothing and bedding) (day 1 and day 8 post-treatment)
    • Vacuum all carpets and furniture; the bags should be disposed of immediately.
    • All contacts within the household should be treated for scabies infestation even if asymptomatic.

References:[9]

Complications

  • Bacterial superinfection
  • Generalized involvement
    • Eczema
    • Pyoderma
    • Lymphadenitis

References:[3]

We list the most important complications. The selection is not exhaustive.

Prevention

  • The local health care department should be notified of a suspected threat of community outbreak of scabies and the following measures implemented for:
    • Single non-crusted case
      • Following direct contact with the skin or a patient with scabies, hands should be thoroughly washed (also underneath the nails), then disinfected with an agent that is effective against scabies mites.
      • Appropriate identification and treatment of scabies in the affected individual, with adequate follow-up. All contacts (including in hospital) should receive prophylactic treatment.
      • Avoid skin-to-skin contact for 8 hours after initial treatment.
      • Increased surveillance for new cases
    • Multiple non-crusted cases
      • In addition to the above, an institution-based awareness and education program
      • Adequate recording of epidemiological data
    • Crusted scabies case: same as above, but more rapid and aggressive approach

References:[10]