Scabies

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 use of scabicidal agents (e.g., permethrin) and prevention of transmission and reinfection (e.g., decontamination of clothing and bedding, treatment of close contacts).

• Pathogen: Sarcoptes scabiei var. hominis
• Transmission
  • Highly contagious
  • Typically via direct prolonged physical (skin-to-skin or sexual) contact
  • Rarely indirect transmission (e.g., sharing textiles such as bedding, towels, or clothes)
  • Commonly affects children ; and individuals living closely with other people (e.g., in nursing homes or jails)
• Risk factors: : crowded living conditions (e.g., institutions such as nursing homes, hostels, child care facilities, and prisons)

Scabies mites cannot survive more than 2–3 days away from human skin. ^[1]

• 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 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:^[3]

• Incubation period: approximately 3–6 weeks following infestation.
• Intense pruritus that increases 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

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

Crusted scabies is highly contagious and should be treated aggressively in affected individuals and their close contacts. Appropriate isolation precautions should also be taken to prevent transmission. ^[1]

• Diagnosis is typically clinical.
• Detection of mites, larvae, ova, or mite feces on any of the following: ^[2][4]
  • Dermoscopy
  • Microscopic examination of the skin (e.g., skin scraping): low sensitivity

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

• Other arthropod bites, e.g., bedbugs, ticks, mosquitoes
• Delusional parasitosis
• Urticaria
• Atopic dermatitis
• Allergic contact dermatitis
• Seborrheic dermatitis
• Lichen sclerosus
• Genital lichen planus
• Folliculitis
• Impetigo
• Tinea

The differential diagnoses listed here are not exhaustive.

General principles ^[2][4]

• Start treatment with a scabicidal agent.
• Treat pruritus with oral antihistamines and/or topical corticosteroids as needed. ^[5]
• Treat secondary bacterial infection of the lesions with topical or systemic antibiotics; see “Antibiotic therapy for skin and soft tissue infections” for details. ^[5]
• Advise all patients
  • On measures to prevent transmission and reinfection
  • To trim fingernails to reduce the risk of excoriation and superinfection
  • To stay home from school or work until treatment is completed ^[1]
• For suspected treatment failure, use of a different scabicidal agent is recommended. ^[4]

Itching may persist for up to 2 weeks after treatment. If symptoms last longer, consider treatment failure, reinfection, or alternative diagnoses. ^[2][4]

Scabicidal agents ^[2][4][6]

First-line (for adults and children)

• Permethrin 5% cream ^[2][4]
• Dose may be repeated in one week. ^[2]

A single application of permethrin 5% cream is usually curative, but symptoms of pruritus may persist for up to 2 weeks. ^[2]

Mechanism of action of permethrin: inhibition of voltage-gated sodium channels in the mite → delayed repolarization of neurons → paralysis and death of the mite

Alternatives

• For adults only
  • Oral ivermectin (off-label) ^[4]
  • Crotamiton 10% cream ^[5][6]
  • Lindane 1% lotion : Consider only in nonpregnant nonlactating adults who do not respond to or cannot tolerate other agents. ^[4]
• For adults and children: sulfur 5–10% ointment ^[5][6]

Oral ivermectin has limited ovicidal action. If used to treat scabies, a second dose is required after 14 days. ^[4]

Mechanism of action of lindane: blocks GABA channels → neurotoxicity in the mite

Prevention of transmission and reinfection ^[1]

• Wash all clothing and bedding in hot water ≥ 50°C (≥ 122°F). ^[1][2]
• Treat all of the following individuals simultaneously with the patient:
  • Household members within the last month
  • Sexual partners within the last two months ^[1][2]
• Advise household members to avoid direct, prolonged contact with the patient's skin, bedding, and clothing until treatment completion.
• In the event of a suspected outbreak:
  • Notify the local health department.
  • Increase surveillance to identify new cases.

• Bacterial superinfection : can lead to post-streptococcal glomerulonephritis
• Generalized involvement
  • Eczema
  • Pyoderma
  • Lymphadenitis

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