Scabies

Definition and Aetiology

• Scabies is a parasitic infestation of the skin caused by the mite Sarcoptes scabiei var. hominis .

• Transmission occurs primarily through close physical contact (including sexual contact) and, to a lesser extent, through fomites such as bedding or towels.

• The mites burrow into the epidermis to lay eggs, triggering a hypersensitivity reaction that gives rise to pruritus, rash, and, in some cases, burrows.

Epidemiology and Risk Factors

• Prevalent worldwide, but higher rates in settings of crowding or close living arrangements, such as residential aged care facilities, institutions, and remote Aboriginal and Torres Strait Islander communities.

• High-risk groups include children, immunocompromised individuals, and older adults in residential care.

• Outbreaks can occur quickly in schools, long-term care facilities, and other close-contact communities.

Pathophysiology

• Female mites burrow into the stratum corneum to lay eggs, and larvae hatch and mature within the skin.

• Intense pruritus is predominantly due to a type IV hypersensitivity reaction to mite antigens.

• The pruritus often becomes more severe at night, and patients may present with excoriations and secondary infection.

Clinical Features

1. Classic (“Typical”) Scabies

   • Intense pruritus (worse at night).

   • Burrows: short, wavy, greyish lines often seen on the web spaces of the fingers, wrists, waist, buttocks, and genital area.

   • Polymorphic rash: papules, vesicles, nodules, or excoriations may be present.

2. Crusted (Norwegian) Scabies

   • Highly contagious variant seen in immunocompromised patients or those with limited ability to scratch.

   • Presents with thick, hyperkeratotic crusts containing very large mite loads.

   • May lack the severe pruritus typical of classic scabies, yet it is far more infectious.

   • Considered a public health concern; notifiable in some Australian jurisdictions.

3. Scabies in Infants and Younger Children

   • Can involve the head, neck, palms, and soles, in addition to typical sites.

   • May present with widespread vesiculopustular lesions or eczematous changes.

   • Nodular scabies can appear in some children, presenting with persistent nodules, usually in the groin or axillae.

Diagnosis

• Clinical diagnosis is usually made based on a typical history (night-time pruritus, contacts with similar symptoms) and examination findings (burrows, papules, distribution).

• Definitive diagnosis can be achieved by microscopy of skin scrapings, though in practice, many GPs rely on clinical suspicion and response to treatment.

• Delayed diagnosis is common because early signs can be subtle or atypical, particularly in immunocompromised or older patients.

Differential Diagnoses

• Eczema (atopic dermatitis)

• Papular urticaria (insect bite hypersensitivity)

• Contact dermatitis

• Tinea (dermatophyte infection)

• Psoriasis (less commonly confused, but possible with mild disease)

Complications

1. Secondary Bacterial Infection

   • Excoriations can lead to impetigo and deeper skin/soft tissue infections, frequently with Staphylococcus aureus or Streptococcus pyogenes .

   • In remote settings, scabies-related skin infection can predispose to serious complications such as acute rheumatic fever or post-streptococcal glomerulonephritis.

2. Crusted Scabies

   • If undiagnosed, the huge mite burden poses a significant outbreak risk.

   • Often complicated by bacteraemia, ulceration, and significant morbidity.

Management

General Measures

• Examine close contacts (family members, sexual partners, etc.) and treat empirically if clinical suspicion is high.

• Wash clothes, towels, and bedding used in the last few days in hot water (≥60°C), or subject them to heat via ironing or a hot clothes dryer.

• Non-washable items can be sealed in plastic bags for 8 days (mites rarely survive longer than 7 days away from a host).

• Vacuum carpets and furniture; for items that cannot be washed or placed in the sun, avoid body contact for at least 3 days.

• In institutional outbreaks (e.g., aged care):

  • Treat all residents and staff who have had significant contact.

  • Identify the potential index case (often crusted scabies).

  • Consider quarantine of affected sections if feasible.

Pharmacological Treatment (Adults and Children ≥6 months)

1. Permethrin 5% Cream (Topical)

   • Apply to clean, dry, cool skin from the neck down (include scalp and face in infants, older adults, and patients in northern Australia).

   • Leave on for 8–12 hours (usually overnight); reapply to hands if washed.

   • Repeat in 7 days to eradicate newly hatched mites.

   • Safe in pregnancy and breastfeeding; first-line for these groups.

2. Ivermectin (Oral)

   • Dose: 200 micrograms/kg, given with fatty food to enhance absorption.

   • Usually repeated in 7 days.

   • Particularly useful in large-scale outbreaks or in patients who struggle with topical application.

   • Not recommended for children <15 kg or pregnant/breastfeeding women.

3. Benzyl Benzoate 25% Emulsion (Topical)

   • Effective but can be irritating to the skin, potentially affecting adherence.

   • Often reserved for patients who cannot tolerate permethrin or ivermectin.

   • For children: dilute according to age (e.g., 1:1 for ages 2–12, 1:3 for ages 6 months–2 years) and apply similarly for 24 hours, then wash off.

   • Repeat in 7 days.

4. Crotamiton 10% Cream or Sulfur Ointment

   • Possible second-line or salvage therapies in specific situations (e.g., infants younger than 6 months if permethrin is not tolerated).

Resolving Post-Scabies Itch

• Itch can persist for up to 3 weeks or more after successful treatment. A moderately potent topical corticosteroidcan be used to reduce residual inflammatory changes.

• Persistent nodules may require an intralesional corticosteroid injection if they are particularly symptomatic.

Special Considerations for Infants <6 months

• Although permethrin 5% cream is not officially registered for infants <6 months, it is recommended in Australia due to the serious morbidity of untreated scabies .

• Apply to the whole body, including head and neck (avoiding eyes, mucous membranes).

• Cover the infant’s hands to prevent oral ingestion of cream.

• Repeat in 7 days.

Crusted (Norwegian) Scabies

• Highly infectious variant; can present in immunocompromised or physically incapacitated people.

• Management requires expert consultation (Infectious Diseases/Dermatology).

• Combined therapy: multiple oral ivermectin doses plus topical scabicide (often permethrin) applied more frequently (e.g., every second day initially), plus a keratolytic agent (e.g., 5–10% salicylic acid in cream).

• Secondary bacterial infection is common; consider empirical antibiotic coverage.

• Isolation or at least strict contact precautions in institutions until effectively treated.

Follow-Up and Prevention of Recurrence

• All household and close contacts should be treated or closely monitored to break the cycle of transmission.

• If there is persistent pruritus beyond 3–4 weeks, consider inadequate application, re-infestation, or an alternative diagnosis.

• For recalcitrant cases or repeated treatment failures, ensure correct usage, contact tracing, and consider switching between oral and topical regimens or seeking specialist referral.