Summary

Scabies is a common parasitic infestation of the skin caused by the mite Sarcoptes scabiei var. hominis. The female mite burrows into the stratum corneum to lay eggs. The hallmark is Intense Pruritus, particularly at night, caused by a Type IV hypersensitivity reaction to the mite and its faeces ("scybala"). It is transmitted by prolonged skin-to-skin contact. [1,2]

Clinical Pearls

Burrows: The pathognomonic sign. Silvery, linear or serpiginous tracks (5-10mm long), typically in finger web spaces, wrists, and feet.

The Itch Lag: In a primary infestation, itching takes 4-6 weeks to develop (time for sensitization). In re-infestation, itching starts within 24 hours.

Crusted Scabies (Norwegian Scabies): Occurs in immunocompromised or elderly patients who cannot scratch (loss of mechanical removal). They develop hyperkeratotic crusts containing Millions of mites (vs 10-15 in classic scabies). It is highly contagious (can spread via bedding/dust), but paradoxically NOT ITCHY (due to immune anergy).

The "Post-Scabetic Itch": Patients often return 2 weeks after treatment complaining they are still itching. This is normal! The allergic reaction continues until the dead mite antigens are shed with the skin turnover (~4 weeks). Don't retreat unless new burrows appear.

Scabies is frequently misdiagnosed as Eczema, Impetigo, or Drug Reaction.

• The Clue: "The Itch that wakes the dead". Sleep disturbance is near universal.
• The Family: "Is anyone else scratching?" (A positive answer is diagnostic gold).

Mechanism of Itch (The Delay)

The itch is NOT caused by the mite biting. It is a Type IV (Delayed) Hypersensitivity Reaction to mite proteins (feces, eggs, saliva).

• Primary Infestation: The immune system takes 4-6 weeks to recognize the antigen. During this "Incubation Period", the patient is asymptomatic but infectious.
• Re-infestation: The immune system is primed. Itching starts within 24 hours.
• Implication: You must treat contacts even if they have NO symptoms, because they might be in the 4-week incubation phase.

The life of Sarcoptes scabiei is entirely on the human host. Away from the skin, they die within 36-72 hours.

The Burrowing Cycle

1. Insemination: Occurs in a "mating pocket" primarily on the hands/wrists.
2. The Male: Dies shortly after mating.
3. The Female:
   • She is 0.3mm long (just visible to the naked eye as a speck).
   • She tunnels into the stratum corneum (not the dermis).
   • She secretes enzymes to digest keratin.
   • Speed: 2-3 mm per day.
   • Lifespan: 4-6 weeks (lays ~3 eggs/day).
4. The Eggs:
   • Laid behind the female in the burrow.
   • Hatch in 3-4 days.
   • Larvae crawl to the skin surface to moult.

Why is it nocturnal?

The itch is worse at night because:

1. Temperature: The warmth of the bed increases mite activity.
2. Distraction: Lack of daytime stimuli makes the itch perception dominant.
3. Circadian: Cortisol (anti-inflammatory) dips at night.

1. Classic Scabies

• Burden: Low. Only 10-15 mites on the entire body.
• Transmission: Requires prolonged contact (15-20 mins).
• Host: Immunocompetent.

2. Crusted (Norwegian) Scabies

• Burden: Massive. Millions of mites.
• Appearance: Thick, psoriasiform, white/grey crusts. Nail dystrophy.
• Host: Immunocompressed (HIV, Transplant), Elderly, Down Syndrome (Genetic predisposition?), Sensory Neuropathy (Leprosy).
• Transmission: HIGHLY infectious. Can spread via aerosolised skin flakes (dust).
• Itch: Absent or Minimal. (Anergy).

3. Nodular Scabies

• Appearance: Red-brown, indurated nodules (5-10mm) on penis, scrotum, axillae.
• Mechanism: A persistent hypersensitivity reaction to dead mite parts.
• Course: Can persist for months after successful cure.
• Treatment: Intralesional steroids (not more Permethrin!).

• Bullous Pemphigoid: Can present as intense itch before blisters appear. (Elderly).
• Dermatitis Herpetiformis: Coeliac disease association. Very itchy elbows/knees.
• Uraemic Pruritus: Renal failure.
• Polycythaemia Rubra Vera: Itch after hot bath.

Case 1: The "Eczema" Misdiagnosis

History: 45M. 3 months of "eczema" on hands. Treated with Betnovate. Itch got WORSE (Steroids reduce inflammation but allow mites to breed -> "Scabies Incognito"). Clue: Wife started itching last week. Exam: Classic burrows found on wrist. Action: Permethrin for both. Steroids stopped. Learning: If eczema is not responding to steroids (and is spreading), think Scabies.

Case 2: The Nursing Home Outbreak

Scenario: 5 residents on one floor have "itchy rash". Staff member also itchy. Diagnosis: Scabies outbreak. Index Case: An elderly resident with dementia had thick white crusts on fingers (Crusted Scabies) - misdiagnosed as Psoriasis for 6 months. Management:

• Index Patient: Oral Ivermectin + Isolation.
• Mass Treatment: All 40 residents + All 60 staff treated on the SAME DAY (Blitz strategy).
• Cleaning: All bedding/curtains washed. Outcome: Outbreak controlled.

Case 3: The "Resistant" Scabies

History: 20M. Treated 3 times with Permethrin. Still itching. Dermoscopy: No burrows. No mites. Erythema only. Diagnosis: Post-Scabetic Eczema. Management: Potent steroid (Dermovate) for 1 week. Result: Itch resolved. Learning: Do not keep throwing Acaricides at a dead infestation. Treat the eczema.

1. Superinfection (Impetiginisation)

• Cause: Staph aureus or Strep pyogenes entering excoriations.
• Signs: Honey-coloured crusts, pustules, cellulitis.
• Risk: Sepsis (in elderly).

2. Post-Streptococcal Sequelae

• In tropical regions/Indigenous populations, Scabies + Strep =
  • Post-Strep Glomerulonephritis (PSGN): Haematuria, Hypertension.
  • Rheumatic Heart Disease: Repeated Strep infection.
• Global Impact: Treating scabies reduces renal and cardiac disease in these populations.

3. Bullous Scabies

• Mimic: Bullous Pemphigoid.
• Mechanism: Severe hypersensitivity causing blisters.
• Diagnosis: Biopsy shows mites.

1. The "10 Commandments" of Permethrin Application

The vast majority of "resistance" is actually application failure.

1. Everyone Treats: Patient + Household + Sexual Partners. Simultaneously.
2. No Symptoms? Don't Care: Treat them anyway (Incubation Period).
3. Cool Skin: Apply after a tepid shower. Hot skin absorbs the cream into systemic circulation (less effective on skin). Dry thoroughly first.
4. Coverage: From the Jawline Down (Adults) or Top of Head Down (Infants/Elderly).
5. The "Hiding Spots":
   • Under fingernails (use a toothbrush).
   • Between toes.
   • Cleft of buttocks.
   • Navel.
   • External Genitalia.
6. Time: Leave on for 8-12 hours (Overnight).
7. Hand Hygiene: If you wash your hands during the night (e.g., toilet), you must Re-apply to hands.
8. The Second Dose: You MUST repeat the entire process on Day 7. (Permethrin kills mites but not eggs. The second dose kills the larvae that hatched during the week).
9. Laundry: On the morning of washing off:
   • Wash sheets/clothes @ 60°C.
   • OR: Tumble dry hot.
   • OR: Seal in bin bag for 72 hours (mites starve).
10. Expect Itch: Warn the patient: "You will still itch for 2 weeks. This is the allergy fading. It does not mean it failed."

2. Pharmacotherapy Options

First Line: Topical Permethrin 5% (Lyclear)

• Action: Neurotoxin (Na+ channel blocker). Causes mite paralysis.
• Pros: Safe in Pregnancy (Category B) and Breastfeeding. Safe > 2 months age.
• Cons: Skin irritation.
• Dose: 30g tube (Adult). Need 2 tubes (Day 0 and Day 7).

Second Line: Oral Ivermectin (Stromectol)

• Indication:
  • Institutional outbreaks (easier logistics).
  • Crusted Scabies (Combined with topical).
  • Application difficulty (physical disability).
  • Refractory cases.
• Dose: 200 mcg/kg. Taken with fatty meal (increases absorption).
• Regimen: Dose at Day 0 and Day 14.
• Contraindications: Pregnancy, Children < 15kg.

Third Line: Malathion 0.5% (Derbac M)

• Action: Organophosphate (cholinesterase inhibitor).
• Use: If Permethrin allergy.
• Time: Needs 24 hours application.

Fourth Line: Sulphur Ointment (6-10%)

• Use: Infants < 2 months, Pregnant women (historical safety). Smells bad.

This is a medical urgency. Isolation: Single room. Contact Precautions (Gowns/Gloves). Treatment: Dual Therapy is Mandatory.

1. Oral Ivermectin: Days 1, 2, 8, 9, and 15. (Grade 1).
   • Severe cases (Grade 3) may need Days 1, 2, 8, 9, 15, 22, 29.
2. Topical Permethrin: Every 2-3 days.
3. Keratolytics: Salicylic acid 5% to remove the thick crusts (which shield the mites).
4. Cleaning: Daily terminal clean of room.

Scenario: Patient returns at Day 14 saying "I'm still itchy". Decision: Is it Failure or Post-Scabetic Itch?

Action:

• If Post-Scabetic: Potent Steroid (Betnovate) + Emollients. Antihistamines. Reassure.
• If Failure: Check "10 Commandments". Switch to Ivermectin?

• Cure: >95% if applied correctly. Failure is usually due to poor application or missed contacts.
• Itch: Persists for 2-4 weeks post-cure. Treat with emollients/steroids.

Key Guidelines

Landmark Evidence

1. Cochrane Review (Strong & Johnstone)

• Permethrin is superior to Lindane and Crotamiton. Oral Ivermectin is effective but slower acting than Permethrin.

1. Permethrin (Synthetic Pyrethroid)

• Mechanism: Binds to voltage-gated sodium channels in the mite nerve cell membrane -> Prolonged depolarisation -> Paralysis -> Death.
• Resistance: kdr mutations (Knockdown Resistance) have been documented in Australia and France. However, treatment failure is 90% application error, not resistance.
• Safety: Poorly absorbed through skin (<2%). Rapidly metabolised by esterases. Safe in neonates (>2 months).

2. Ivermectin (Macrocyclic Lactone)

• Mechanism: Binds to glutamate-gated chloride channels in invertebrate nerve/muscle cells -> Hyperpolarisation -> Paralysis.
• Note: Does NOT cross the human Blood-Brain Barrier (P-glycoprotein efflux pump prevents it) - thus safe in adults. In infants (<15kg), the BBB is immature, hence the contraindication.

3. Sulphur (Precipitated)

• Mechanism: Unknown (likely reacts with cysteine in mite cells -> Hydrogen sulphide).
• Role: The only safe option for infants < 2 months.

"Why am I still itching?" Imagine the mite is a "splinter" in your skin.

1. Week 0: You catch the mite.
2. Week 4: Your body becomes allergic to the "splinter". You start itching.
3. Treatment Day 1: Killing the mite doesn't remove the "splinter". The dead mite body, eggs, and poop are still trapped in your skin layers.
4. Weeks 5-8: Your skin grows out (turnover time 28 days). As the skin sheds, the splinter falls out. The itch stops. Key Message: Itch for 2-4 weeks after killing the mite is Normal. Use moisturisers and antihistamines. Do not use more Lyclear.

What is Scabies?

It is a skin infestation by tiny mites (too small to see) that dig tunnels in your skin.

Is it because I'm dirty?

No. Scabies affects everyone, regardless of hygiene. It spreads through close skin contact (like holding hands) or sharing a bed.

How do I use the cream?

You must cover every inch of skin from the chin down, even where there is no rash. Include under fingernails, genitals, and between toes. Put it on before bed, leave for 12 hours, then wash off. You MUST repeat it in 7 days to kill any eggs that hatch.

Does my family need it?

Yes. Everyone you live with must treat themselves on the same day, even if they have no itch. They might be infected but not itching yet.

OSCE Station: Explaining Treatment

Scenario: "Explain to a patient how to use Permethrin cream." Candidate Checklist:

1. Who: "You and your partner must do it on the same night."
2. How: "Apply it everywhere from the jawline down. Pay special attention to the webs of your fingers and genitals."
3. When: "Leave it on for 12 hours (overnight)."
4. Repeat: "You must do it again in 7 days."
5. Wash: "Wash bedding at 60 degrees tomorrow morning."
6. Itch: "The itch will persist for 2 weeks. This is normal."

Viva Question: The "Itch Lag"

Examiner: "Why do we treat asymptomatic contacts?" Candidate: "Because the itch is a Type IV hypersensitivity reaction. In a first infection, it takes 4-6 weeks for the itch to develop. Contacts may be infected and infectious during this 'lag phase' without realising it."

Viva Question: Neurotoxicity of Ivermectin

Examiner: "Why is Ivermectin avoided in young children (<15kg)?" Candidate: "The Blood-Brain Barrier (BBB) is not fully formed. Ivermectin acts on GABA receptors. In mammals, these are protected behind the BBB. If the BBB is permeable, Ivermectin can cause CNS depression."