Scabies

Pathophysiology & Transmission Dynamics

The Cause and the Mite's Life Cycle

• Cause: The female Sarcoptes scabiei mite is tiny (0.3–0.4 mm) and hard to see clearly without magnification.
• Burrowing: The female mite secretes protein-digesting enzymes to tunnel winding burrows into the outermost layer of the skin (stratum corneum). Inside these burrows she lays 2–3 eggs a day and leaves her waste (fecal pellets).
• Life cycle: Eggs hatch into larvae in 3–4 days and pass through nymph stages to become adult mites within 10–14 days. Females live for 1–2 months on the human body.

The Allergic Hypersensitivity Reaction

• Itching and red bumps do not start as soon as the mites arrive. They are caused by the host's delayed (type IV) hypersensitivity reaction to mite proteins, eggs and feces.
• In a first infestation, the incubation period is 2–6 weeks. With reinfestation, however, the allergic reaction flares within just 1–4 days because T-cell immune memory is already in place.

Clinical Forms of Scabies

Classic Scabies

• Burrows: The most characteristic sign: a slightly raised, wavy, thread-like gray or grayish-white line 2–15 mm long, often ending in a tiny blister that contains the female mite.
• Itchy bumps and blisters: Scattered red papules concentrated at typical sites where the skin is thin: finger webs, inner wrists, armpits, around the navel, buttocks, the buttock crease and the genitals (scabies nodules on the glans or scrotum in men).
• Night-time itch: The itch becomes much worse at night because the warmth of the bed makes the female mites move about and become more active.

Crusted (Norwegian) Scabies

• Who is at risk: People with weakened immunity (HIV, long-term corticosteroid use), people with psychiatric conditions, and frail, bedridden older adults.
• Features: Because the immune system can no longer keep the mites in check, their numbers climb into the millions (compared with 10–15 mites in classic scabies).
• Lesions: Thick, hard, grayish-green cracked scale that builds up like tree bark across the body (including the scalp, face, palms and soles). There is little or no itch, but the condition is highly contagious.

Scabies in Infants and Young Children

• Unlike in adults, lesions in young children are spread across the whole body, including the face, scalp, neck, palms and soles. Pustules, blisters and impetigo develop easily because a child's skin is so sensitive.

Differential Diagnosis of Itchy Rashes in Skin Folds or Across the Body

How Scabies Is Diagnosed at Haute-Derma

Contact and Medical History: We ask whether others are itchy too (roommates, family members, nursery or dormitory contacts).

Clinical Examination and Dermoscopy:

• The finger webs and wrists are searched for burrows.
• The "delta-wing jet" sign on dermoscopy: the mite's head and front legs form a dark, pigmented triangle at the end of the burrow.

Microscopy (Skin Scraping Test):

• A drop of mineral oil or 10% KOH is placed on an intact burrow or blister, which is then gently scraped with a scalpel blade (or a fine needle).
• The sample is examined under a light microscope at ×10/×40 for adult mites, larvae, eggs or fecal pellets (scybala).

Treatment, Outbreak Control and Recovery at Haute-Derma

Topical Treatment (Permethrin, the Gold Standard)

• Permethrin 5% cream:
  • How to apply: Cover all skin from the neck down to the soles of the feet, paying attention to under the nails, between the fingers and toes, the buttock crease and the genitals. Children under 2 and older adults should also have the scalp, face and ears treated.
  • Contact time: Leave the cream on for 8–14 hours (usually applied at bedtime), then wash it off with warm water.
  • Frequency: Two applications exactly 7 days apart (the second kills mites newly hatched from any surviving eggs).
• Alternatives: 6–10% sulfur ointment (safe for infants under 2 months and in pregnancy) or 25% benzyl benzoate.

Oral Treatment (Systemic Ivermectin)

• Oral ivermectin (200 µg/kg):
  • Recommended for widespread scabies, failure of topical treatment, institutional outbreaks or crusted (Norwegian) scabies.
  • Frequency: A single dose taken with food, repeated after 7–14 days.
  • Crusted scabies: Oral ivermectin (days 1, 2, 8, 9 and 15) is combined with daily permethrin 5% and a keratolytic (salicylic acid) to break down the thick crusts.

Essential Infection Control and Environmental Measures

• Treat everyone at once: All household members and close skin-to-skin contacts must be treated on the same day, whether or not they itch.
• Personal items: Wash all clothing, bedding and towels in hot water (>60 °C) and dry them on a high heat setting.
• Items that cannot be washed: Seal them in a tightly tied plastic bag for at least 72 hours to 7 days (mites away from a human host die of dehydration within 2–3 days).

Managing Post-Scabietic Itch

• Be aware: Itching can continue for 2–4 weeks after all mites have been killed, because dead mites and their waste remain in the skin and keep the allergic reaction going.
• What to do: Take an oral second-generation H1 antihistamine, use a gentle moisturizer and apply hydrocortisone 1% cream for a short time. Do not keep reapplying scabicide on your own, as this can cause irritant contact dermatitis.