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Scabies

Image : “ScabiesD08” This image shows multiple excoriations and small, erythematous papules all of which cause itchiness.
By Cixia. License: Public Domain


Definition

Scabies is a contagious, superficial skin infection caused by the mite Sarcoptes scabiei.

It is a transmissible, ectoparasitic infection characterized by superficial burrows, intense pruritus, and secondary infections.

Epidemiology

The disease has a prevalence of 4–10% in epidemic areas.

Over 300 million cases have been reported worldwide, with nodular scabies being more common among children.

Risk factors

Scabies outbreaks are more common in institutions that are over 30 years old, containing more than 120 beds and with a bed to worker ratio greater than 10:1.

Pathophysiology of Scabies

The parasite is transmitted via 3 main mechanisms:

  1. Human to human transmission via direct body contact: For instance, children playing in close contact, sharing of beds, and intercourse are considered the main exposure risks.
  2. Environmental transmission occurs through contact with fomites and mites from dust. The parasite survives for about 2–5 days outside the human body at room temperature and pressure.
  3. Zoophilic transmission is rare.

After mating, the 0.3–0.4 mm large female mites dig into the horny layer of the epidermis, where they form tunnel-like ducts for the deposition of their eggs. A few days later, they die there. Three weeks after egg deposition, the mite grubs emerge from the eggs and then drill back to the skin surface — the life cycle repeats.

The infection takes the form of:

  • Initial infection: Sensitization occurs in several weeks.
  • Reinfestation: The patient is already sensitized, and pruritus occurs within 24 hours. In immunocompromised patients, there is an imminent risk of developing hyperkeratotic/crusted scabies.

Symptoms and Diagnosis of Scabies

Acarodermatitis Hand

Image: “Acarodermatitis” by Sven Teschke. License: CC BY-SA 3.0

Severe nocturnal itching is typical for scabies. This itching is caused by the mite antigens that are released during the decay of the mite bodies. As a clinical correlation of the mite ducts, convoluted and partially palpable skin efflorescences can be found. Later, papules and vesicles develop. Mainly, warm body regions with little horny skin are affected. These regions include the mammilla region, the interdigital folds, the male genital area, the axillary folds, and the wrists. Due to scratching, secondary efflorescences like excoriations, crusts, and eczema develop. Bacterial superinfections of the skin can also occur as a complication.

Note: Predilection sites of scabies should be remembered for exams!

Characteristic primary and secondary efflorescences at scabies

Primary efflorescences have a comma or dot shape. Straight efflorescences are mostly scratching marks!

Primary Efflorescences Secondary Efflorescences
Comma-like or irregularly convoluted mite ducts Crusts
Papules Scratching
Vesicles Impetigo
Rash

Source: Robert Koch Institute

The latency between infection and the onset of the symptoms amounts to roughly 3 weeks (see above: grub hatching). Thus, one should ask for possibly previous exposures within this period while obtaining medical history (e.g., vacation). The attack of several persons within a family or a social facility (e.g., kindergarten, hospital wards) is especially suspicious.

For the detection of scabies, dermatoscopy (detection of the mite ducts) and microscopy are appropriate measures. Skin samples for microscopic examination are obtained via tape pull-off or abrasion.

Norwegian Scabies

The massive mite infestation of immunosuppressed patients (e.g., patients with AIDSleukemia, or undergoing long-term glucocorticoid/cytostatic therapy) is referred to as Norwegian Scabies. Due to the decreased immune reactions in these people, they may experience almost no itching. Skin is ichthyosiform with dirty-brown papules, bark formation, and desquamation.

Differential Diagnosis

Especially in older patients with nocturnal pruritus, delusional parasitosis should be excluded. Also, in the context of organic psychoses (e.g., alcohol deprivation delirium or exsiccosis), chronic tactile hallucinosis often occurs where the patient is convinced that animals or insects live under his skin.

Therapy

The 3 main pillars of successful treatment of scabies are hygienic measures, medicamentous therapy, and, most of all, the compliance of the affected person. The first-line treatment is the antiscabietic agent permethrin. The permethrin cream is applied to the whole integument, including fingernails and the tips of the toes. Further treatment options are benzyl benzoate, ivermectin, and allethrin. To avoid auto-reinfection, clothes, bedding, and towels have to be changed and washed daily during therapy.

Important Final Fact about Scabies

Outside the human body, the mites survive no more than 2–4 days at room temperature.

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