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Dermatology
Infectious Disease

Scabies

High EvidenceUpdated: 2025-12-22

On This Page

Red Flags

  • Crusted (Norwegian) scabies
  • Outbreaks in care homes
  • Widespread pustules in infants
  • New onset haematuria (Post-Strep GLN)
Overview

Scabies

1. Introduction & Epidemiology

Summary

Scabies is a ubiquitous ectoparasitic infestation caused by the mite Sarcoptes scabiei var. hominis. It is characterized by intense pruritus (worse at night) and specific skin lesions (burrows). Transmission requires prolonged skin-to-skin contact (15-20 mins). It is NOT a disease of poor hygiene.

Key Learning Points

  • The Itch: Intense, nocturnal, spares the head (adults).
  • The Treatment: Two applications, one week apart.
  • The Contacts: Treat everyone simultaneously.
  • The Wash: Hot wash bedding the morning after.
  • The Wait: Itch persists for 4 weeks post-cure.

Red Flags (When to Worry)

  • Crusted Plaques: Suggests immune failure (HIV/HTLV-1). Immediate isolation.
  • Honey-Crusted Lesions: Suggests secondary Staph Impetigo -> Risk of Sepsis/GN.
  • Haematuria: Suggests renal involvement.

Epidemiology

  • Prevalence: 200 million cases globally at any time.
  • Outbreaks: Institutional settings (Nursing homes, Prisons, Nurseries) are high risk.
  • Cycle: Epidemics occur every 15-20 years as herd immunity wanes.

Global Health Impact

A Neglected Tropical Disease (WHO).

  • Ranking: The WHO designated Scabies a Neglected Tropical Disease (NTD) in 2017.
  • Burden: 0.21% of total global DALYs (Disability-Adjusted Life Years). High morbidity due to secondary infection / renal failure.
  • Hotspots: Pacific Islands (prevalence >20%), Northern Australia (Aboriginal communities), Latin America.
  • Complication: Scabies -> Impetigo (Staph/Strep) -> Glomerulonephritis -> Chronic Kidney Disease. This sequence is a major driver of CKD in developing nations.

Sexual Health Context

  • Classification: Scabies is a sexually transmissible infection (STI) in adults.
  • Screening: Check for "The Big 5" (Chlamydia, Gonorrhoea, Syphilis, HIV, Hep B).
  • Partner Notification: Current partner + Past partners (1 month).
  • Fact: You can get it from holding hands, but sex is a great way to transmit it.

Historical Context

The "Seven Year Itch".

  • Ancient Rome: Celsus described "scabies" (from scabere - to scratch).
  • 1687: Bonomo and Cestoni discovered the mite using a microscope (The first time a microorganism was proven to cause disease - predating bacteria!).
  • Napoleonic Wars: "The Itch" decimated troops.
  • WWII: Scabies was rampant in air raid shelters.

Evolution of Treatment

From Sulphur to Neurotoxins.

  • Pre-1940s: Sulphur ointment was the only option. Effectiveness: Good. Compliance: 0% (smell).
  • 1940s: Benzyl Benzoate introduced during WWII. Painful but effective.
  • 1970s: Lindane (Neurotoxic pesticide). Now banned in many countries due to seizure risk.
  • 1980s: Permethrin (Synthetic Pyrethroid). The current gold standard.
  • 1990s: Ivermectin (Nobel Prize winning drug for River Blindness) found effective for Scabies.
2. Pathophysiology

The Mite (Sarcoptes scabiei)

  • Size: Female is 0.3-0.4mm. Male is smaller.
  • Life Cycle:
    1. Burrowing: Female burrows into stratum corneum (2-3mm/day).
    2. Egg Laying: Lays 2-3 eggs/day for 4-6 weeks (Life span).
    3. Hatching: Eggs hatch into 3-legged Larvae in 3-4 days.
    4. Moulting: Larvae moult into Nymphs -> Adults on the skin surface.
    5. Mating: Occurs on the skin surface. Male dies. Female starts a new burrow.
    6. Total Cycle: Egg to Adult takes 10-14 days.
  • Mite Load:
    • Classic Scabies: 10-15 mites TOTAL per person. (Hard to find).
    • Crusted Scabies: Millions of mites. (Easy to find).

The "Itch" Mechanism

  • Delayed Hypersensitivity (Type IV): The itch is an allergic reaction to mite faeces (scybala) and proteins.
  • Incubation:
    • First Infestation: 4-6 weeks (sensitization delay).
    • Re-infestation: 24-48 hours (immediate reaction due to memory T-cells).

Anatomy of a Burrow

What lies beneath.

  • Location: Deep stratum corneum (does not reach dermis, hence no bleeding unless scratched).
  • The Tunnel: Serpiginous track filled with mite faeces (scybala).
  • The Occupant: The female mite resides at the blind end of the tunnel.
  • The Eggs: Laid in a trail behind her (like Hansel and Gretel's crumbs).
  • The Roof: Tiny holes in the stratum corneum allow oxygen in and nymphs out.

The Host Defence (Immunology)

  • Th1 Response: Interferon-gamma (IFN-y) drives the cellular response (macrophages) to kill mites.
  • Th2 Response: Antibody (IgE) response causes the itch (Histamine/Eosinophils).
  • The Balance:
    • Classic Scabies: High Th1 (Controls mites, but high symptoms).
    • Crusted Scabies: High Th2 (Low itch) but Failed Th1 (Uncontrolled mite proliferation). This is why crusted scabies is non-itchy but teeming with mites.
  • Complement: Mites secrete "Scabies Mite Inactivated Protease Paralogues" (SMIPPs) that inhibit the human complement system.
2. Clinical Presentation

Symptoms

Signs

Paediatric Considerations (The "Infant Pattern")

Babies are different.

Clinical Variants

  1. Crusted (Norwegian) Scabies:
    • Target: Immunocompromised, Elderly, Down's Syndrome.
    • Features: Thick, hyperkeratotic crusts (psoriasis-like). NON-ITCHY (often).
    • Danger: Millions of mites. Highly contagious (airborne scales).
    • Management: Critical medical emergency (Isolation).
  2. Bullous Scabies: Mimics Bullous Pemphigoid (elderly).
  3. Nodular Scabies: Persistent nodules even after mites are dead (immune reaction).

Animal Scabies (Sarcoptic Mange)

Can you catch it from Fido?

The Battle: Classic vs Crusted Scabies

FeatureClassic ScabiesCrusted (Norwegian) Scabies
Mite Load10-15 mites> Million mites
ItchSevere (Intolerable)Mild / Absent (Immune failure)
AppearanceBurrows, ExcoriationsThick, scaling plaques (Psoriasiform)
TransmissionProlonged Contact (15m)Brief Contact / Airborne Scales
HostImmunocompetentImmunosuppressed (HIV, Elderly, Down's)
IsolationAvoid contactStrict Isolation (Barrier Nursing)
Intense Pruritus
"Keep you awake" itch. The hallmark.
Nocturnal Worsening
Classic feature (possibly due to warmth).
Family History
Itching in other household members.
3. Diagnosis

Bedside Tests

  1. Ink Burrow Test: Rub non-toxic ink (or fountain pen) over the burrow. Wipe off with alcohol. Ink remains in the track.
  2. Dermatoscopy (The "Delta Wing" Sign):
    • Look for a dark triangular structure (the mite's head/legs) at the end of a burrow.
    • "Jet plane with a contrail".

Dermatoscopy Masterclass

How to spot the "Jet Plane".

  • Equipment: Dermatoscope (Polarised light helps).
  • The "Contrail": The white, scaly burrow.
  • The "Jet": A tiny, dark brown triangle at the leading edge. This is the mite's head and front legs.
  • The Target: Focus on the web spaces and wrists.
  • Sensitivity: MUCH higher than naked eye examination.
  • Pearl: If you see the triangle, you can confirm diagnosis 100%.
  1. Needle Extraction: Experienced hands can pick out the mite for microscopy.

IACS Criteria (International Alliance for the Control of Scabies)

  • Confirmed Scabies: Mite/Egg/Faeces seen on microscopy OR Dermoscopy.
  • Clinical Scabies: Diffuse itch + Male Genital lesions OR Typical Burrows.
  • Suspected Scabies: Diffuse itch + Typical lesions (but no burrows/genital nodules) + Contact history.

Differential Diagnosis: The Great Mimic

ConditionDifferentiatorClues
Atopic EczemaHx of atopy. Spares web spaces (usually).Lichenification (thickening). Personal/Family Hx of asthma/hayfever.
Prurigo NodularisChronic picking. Nodules on extensor surfaces."Butterfly sign" (spares mid-back where hands can't reach).
Insect BitesGrouped bites ("Breakfast, Lunch, Dinner").Pet history (fleas). Exposed sites only.
Bullous PemphigoidElderly. Itch precedes blisters.Immunofluorescence +ve. Tense blisters. rarely affects web spaces.
Dermatitis HerpetiformisCoeliac Disease. Elbows/Knees/Buttocks.Burning itch. Vesicles.
FolliculitisPustules centred on hair follicles.No burrows. Bacterial swab +ve.
Lichen Planus"The 6 Ps" (Purple, Polygonal, Pruritic...).Wrists (flexor), Wickham's striae (lace-like pattern in mouth).
Neurotic ExcoriationLinear gouges. No primary lesions.Psychiatric history. "Meth mites" (drug induced).
2. Visual Summary Panel

Image Integration Plan

Image TypeSourceStatus
Management AlgorithmAI-generatedPENDING

Image 1: "Dermatoscopy of Scabies" - High-mag view showing the 'Delta Wing' sign (gray burrow with dark triangular mite). Image 2: "Burrow Distribution Map" - Graphic highlighting web spaces, wrists, genitals, and nipples. Image 3: "Crusted Scabies" - Photo of thick hyperkeratotic plaques on the hands of an elderly patient.

4. Management

The "Two Tube" Rule is non-negotiable.

General Principles

  • Treat Everyone: Patient + All Household Contacts + Sexual Contacts (last month).
  • Simultaneous: everyone applies cream on the SAME night.
  • Repeat: Must verify cure with a SECOND dose 7 days later (kills new hatchlings).

Pharmacotherapy

  1. First Line: Permethrin 5% Dermal Cream
    • Method: Apply creating a "body suit" from jawline down to toes. Include navel, beneath nails, between buttocks.
    • Duration: Leave on for 8-12 hours (overnight).
    • Re-apply: If hands are washed during this time.
    • Repeat: Day 0 and Day 7.
  2. Second Line: Malathion 0.5% Liquid
    • Apply for 24 hours. Good alternative if permethrin resistance suspected.
  3. Oral: Ivermectin (Stromectol)
    • Indication: Institutional outbreaks, Crusted Scabies, or Topical Failure.
    • Dose: 200 mcg/kg (Day 0 and Day 14).
    • Contraindication: Pregnancy, Children <15kg.

The Emerging Crisis: Permethrin Resistance

It's not working like it used to.

  • Observation: Increasing reports of treatment failure despite correct application.
  • Mechanism: kdr (knockdown resistance) mutations in the mite's voltage-gated sodium channels.
  • Evidence: Studies in Australia and Europe show delayed mite killing times.
  • Action: If 2 courses of Permethrin fail -> Switch to Malathion or Ivermectin immediately. Do not keep repeating Permethrin.

Why Treatment Fails (The Top 5)

  1. Index Case Failure: Missed a patch (e.g., under nails, natal cleft).
  2. Contact Failure: Asymptomatic contacts didn't treat themselves.
  3. Fomite Failure: Didn't wash the sheets.
  4. Resistance: True biological resistance.
  5. Post-Scabies Itch: Mistaken for failure (The "Phantom Itch").

Drug Comparison Table

DrugEfficacyProsCons
Permethrin 5%Gold StandardHigh cure rate (if used correctly)Resistance rising; Messy cream
Malathion 0.5%Second LineLiquid (easier for hairy areas)Bad smell; Flammable
IvermectinExcellentOral (Easy compliance)Cost; Not for pregnancy/kids
Benzyl BenzoateConsultant OnlyVery cheapBurns the skin (Irritant)
Sulphur OintmentHistoricSafe in infants/pregnancySmells like rotten eggs; Stains

Alternative Protocols (When First-Line Fails)

  1. Benzyl Benzoate (Ascabiol):

    • Mechanism: Neurotoxin to mites.
    • Protocol: Apply 25% emulsion for 24 hours. Repeat in 7 days.
    • Warning: Intense burning sensation (dilute 50/50 with water for children).
    • Role: Cheap, effective, but painful. Used in developing world.

  2. Precipitated Sulphur (6-33%):

    • Mechanism: Keratolytic and toxic to mites/fungi.
    • Protocol: Apply nightly for 3 consecutive nights. Wash off 24 hours after last dose.
    • Role: Safest option for infants <2 months and pregnant women (Category C but used historically).
    • Disadvantage: Terrible odour, stains clothes yellow.

  3. Tea Tree Oil (Melaleuca):

    • Evidence: 5% TTO kills mites in vitro.
    • Role: Adjunct only. Do not use as monotherapy.

  4. Moxidectin (The Future?):

    • Mechanism: Similar to Ivermectin (Macrocyclic lactone) but LONGER half-life (20-40 days).
    • Advantage: Single dose might be curative (covers the whole egg hatching cycle).
    • Status: Showing promise in clinical trials (phase 2/3). Not yet routine.

Future: A Scabies Vaccine?

  • Target: Mite proteins (SMIPPs).
  • Goal: Induce IgE/IgG response to prevent infestation establishment.
  • Status: Pre-clinical (Rabbit models).
  • Need: Critical for eradication in endemic zones where drug compliance is hard.

Special Populations

  1. Pregnancy:
    • Permethrin: Category B. Safe. First line.
    • Ivermectin: Category C. Avoid unless crusted/severe benefit > risk.
    • Sulphur: Safe but unpleasant.
  2. Lactation:
    • Permethrin: Safe. Wash off nipples before feeding.
    • Ivermectin: Avoid (excreted in milk).
  3. The Elderly:
    • Presentation: Often have less itch (senescent immune system) but HIGHER mite load.
    • Risk: High risk of Crusted Scabies masking as "dry skin" or "psoriasis".
    • Action: Use a low threshold to treat "itchy dry skin" in the elderly.
  4. Immunocompromised (HIV, Transplant):
    • Risk: Develop Crusted Scabies rapidly.
    • Presentation: Thick, white/grey scales. NOT always itchy.
    • Mistake: Often misdiagnosed as Psoriasis or drug reaction.
    • Management: Aggressive early use of Oral Ivermectin.

The "Crusted Scabies" Protocol (Specialist)

Requires aggressive therapy.

  1. Isolation: Barrier nursing essential.
  2. Combination Therapy: Topical Permethrin AND Oral Ivermectin.
  3. Keratolytics: Salicylic acid ointment to dissolve thick crusts (penetration).
  4. Daily: Treatment often required daily for 7 days, then bi-weekly.

Syndromic Management (Resource-Limited Settings)

When you can't see the mite.

  • Protocol: IMCI (Integrated Management of Childhood Illness).
  • Rule: If a child has "severe itch" + "skin lesions" -> Treat for Scabies.
  • Rationale: The cost of missed scabies (Renal Failure/Heart Disease from Strep) > Cost of overtreatment.
  • Mass Treatment: In villages with >10% prevalence, the WHO recommends treating the ENTIRE village.
5. Environmental Management

Decontamination is key.

  • Timing: Mites die after 3 days without a host.
  • Laundry: Wash bedding/towels used in the last 3 days at >50°C (Hot wash).
  • Bagging: Items that can't be washed (shoes, coats) -> Sealed plastic bag for 72 hours.
  • Vacuuming: Carpets and soft furnishings.

The Biology of Decontamination

Why hot wash?

  • Survival: Mites survive 24-36 hours at 21°C and 40-80% humidity.
  • Death Point: Mites die within 10 minutes at 50°C.
  • Freezing: Mites die if frozen (-25°C for 2 hours) - option for delicate fabrics, but bagging is easier.
  • Fomites: Transmission via clothes/bedding is rare in classic scabies (low mite count) but guaranteed in crusted scabies.
  • Rule of Thumb: "If in doubt, bag it out."

Clinical Vignette: The Nursing Home Mystery

Scenario: A nursing home has 5 residents with "persistent eczema" and 3 night staff complaining of itchy wrists. The Error: Residents were treated with topical steroids for 3 months. The Consequence: Steroids reduced the itch (anti-inflammatory) but suppressed the immune response, allowing mites to proliferate (Promoted "Incognito Scabies"). The Diagnosis: One resident developed thick crusts on fingers. Scrapings revealed hundreds of mites (Crusted Scabies). The Fix: Mass administration of Oral Ivermectin + Permethrin for closest contacts + Ward Closure.

Institutional Outbreak Management

When it spreads in a care home.

  1. Declare Outbreak: Two or more cases affecting residents/staff.
  2. The "Blitz":
    • Treat ALL residents and ALL staff (symptomatic or not) on the SAME day.
    • Treat all visitors from the last month.
  3. Floor Plan: Divide the home into zones. Nurse cohorts.
  4. Laundry: Massive laundry operation (commercial heat).
  5. Surveillance: Daily skin checks for 6 weeks.
  6. Ivermectin: Often required for mass drug administration (MDA) due to logistics of cream.
6. Complications
  • Post-Scabies Itch: Pruritus persists for 2-4 weeks after CURE.
    • Cause: Dead mites/eggs remain in skin until pushed out by epidermal turnover.
    • Management: Steroids (Topical/Oral) + Antihistamines. Do NOT re-treat unless new burrows.

Algorithm: The "Still Itching" Patient

Is it failure or just the ghost of scabies?

  1. Check Compliance: Did they treat contacts? Did they wash it off too early?
    • If NO -> Re-treat.
  2. Check Dermoscopy: Are there NEW burrows with mites?
    • If YES -> Resistance (Switch agents).
  3. Check History: Is the itch different? (e.g., burning, widespread eczema).
    • If YES -> Post-Scabies Eczema (Steroids).
  4. Nodules: Are they persisting on genitals?
    • If YES -> Nodular Scabies (Intralesional Steroids).
  5. Psychology: No signs, but patient convinced bugs are crawling?
    • If YES -> Delusional Parasitosis.

The Psychological Toll

  • Stigma: "Dirty" disease label (False).
  • Anxiety: "Formication" (sensation of crawling).
  • Delusional Parasitosis: Some patients develop a fixed false belief of infestation after a cured episode. Needs psychiatric input.
  • Secondary Infection: Staph aureus (Impetigo).
  • Post-Streptococcal Glomerulonephritis (PSGN):
    • Mechanism: Scabies mites excrete proteins that inhibit complement, allowing Strep pyogenes to flourish in superinfected burrows.
    • The Sequence: Scabies -> Scratching -> Impetigo -> Nephritogenic Strep -> Immune Complexes in Kidney -> Haematuria/Oliguria.
    • Impact: It is NOT just "an itch". It causes chronic renal failure decades later.
    • Global: 97% of PSGN cases globally are linked to Scabies-endemic areas.

The "Paint It On" Protocol

Most treatment failures are application failures.

Step 1: Preparation (Day 0 - Evening)

  • Nails: Cut finger and toe nails short. Scrub under them with a brush (mites hide here to escape the cream).
  • Skin: Take a cool shower. Dry thoroughly. DO NOT apply to hot, sweaty skin (absorption risk).
  • Remove Jewellery: Rings, watches, bracelets must come off.

Step 2: Application (The Body Suit)

  • Product: Use Permethrin 5% (Lyclear / Elimite).
  • Extent: From the jawline down to the soles of the feet.
    • Adults: Neck down.
    • Elderly/Immunocompromised: Include face/scalp (avoid eyes).
    • Infants: Include face/scalp.
  • The "Missed Bits" Checklist:
    • Web spaces of fingers and toes
    • Under fingernails (use a toothpick to push cream under)
    • Navel (Umbo)
    • Nipples and Areola
    • Genitals and Perineum
    • Gluteal fold (Crack of bum)
    • Soles of feet

Step 3: Assessment (The Wait)

  • Duration: Leave on for 8-12 hours (overnight).
  • Hand Washing: If you wash your hands (toilet/water), you MUST RE-APPLY cream to hands immediately.

Step 4: Removal (Day 1 - Morning)

  • Wash: Shower off the cream.
  • Laundry: Strip the bed. Wash sheets, pillowcases, towels, and Pyjamas from the last 3 days at >50°C.
  • Coat/Shoes: Put them in a plastic bag for 72 hours.

Step 5: The Partner

  • Simultaneous: Your partner/kids must do this EXACT same process tonight, even if they don't itch.

Step 6: The Repeat (Day 7)

  • Repeat: Do steps 1-4 again in 7 days. This kills the nymphs that hatched from eggs (cream kills mites, not all eggs).

Frequently Asked Questions

  • "Did I get this from a toilet seat?": Unlikely. Mites need skin warmth.
  • "Can I get it from my dog?": No. Dog scabies (mange) causes a temporary itch but the mites die in humans.
  • "I'm still itching! Is it back?": Post-scabies itch lasts 4 weeks. Treat with steroids/moisturizers, NOT more poison.
  • "Do I need to fumigate my house?": No. Just wash the bedding. Mites die in 3 days off the body.
8. References
  1. BASHH: UK National Guidelines on the Management of Scabies (2016).
  2. IUSTI: European Guideline (2017).
  3. Cochrane: Permethrin is superior to Lindane and Crotamiton.

Evidence Base Summary

  • Ivermectin vs Permethrin: A 2018 meta-analysis showed Oral Ivermectin is EQUALLY effective as Permethrin, but easier to use (compliance). However, it is more expensive.
  • Mass Drug Administration (MDA): In high-prevalence islands (e.g., Fiji), giving Ivermectin to the WHOLE population reduced prevalence by >90%.
  • Tea Tree Oil: Shows in vitro killing activity but NO clinical evidence to support use yet.

Reviewer's Note

"Scabies is often misdiagnosed as eczema, leading to months of unnecessary misery and steroid use which fuels the fire. The key is the 'Ink Test' and the 'Two Tube Rule'. Don't just prescribe; educate on the laundry process."

— Dr. Sarah Miller, Consultant Dermatologist.

Legal & Work

  • Exclusion: Patients should be excluded from school/work until after the first treatment has been washed off (Day 1).
  • Notification: Outbreaks in care homes must be notified to Public Health. Single cases do not.

Resources

  • BAD Scabies Patient Info Leaflet
  • NHS Scabies Overview
  • CDC Parasites - Scabies

Patient Handout Summary

Take Home Message: > 1. Apply cream everywhere from jaw to toes. > 2. Leave overnight. > 3. Wash bedding hot. > 4. Everyone in the house treats tonight. > 5. Repeat everything in 7 days. > 6. Itch will last for weeks - this is normal.

The "Exit Interview" Checklist (Clinician to Patient)

  • Have you identified ALL household contacts?
  • Do you have enough cream for everyone (2 tubes per adult)?
  • Do you understand the machine washing rule (50 degrees)?
  • Do you know NOT to wash your hands after applying?
  • Do you promise to repeat it in 7 days?

Clinical Pearls (For Students)

  • The "Pen Test": If you can draw a line along the burrow with ink, it's scabies.
  • The "Nodule Rule": Itchy nodules on the penis are Scabies until proven otherwise.
  • The "Steroid Trap": If eczematous skin gets WORSE with steroids, scrape it.
  • The "Clean" Patient: Scabies loves clean people too. Don't be biased.
  • The "Scalp" Exception: Always check the scalp in infants and bed-bound elderly.

Comparison Table: The "Itchy" Differential

FeatureScabiesEczemaInsect Bites
Web SpacesYesRareNo
Night ItchSevereModerateMild
ContactsItchyClearClear
OnsetGradualChronicSudden

Copyright © 2025 MedVellum. All rights reserved. This content is for educational purposes only and does not constitute medical advice.

Last updated: 2025-12-22

At a Glance

EvidenceHigh
Last Updated2025-12-22

Red Flags

  • Crusted (Norwegian) scabies
  • Outbreaks in care homes
  • Widespread pustules in infants
  • New onset haematuria (Post-Strep GLN)

Clinical Pearls

  • Impetigo (Staph/Strep) -
  • Chronic Kidney Disease. This sequence is a major driver of CKD in developing nations.
  • Adults on the skin surface.
  • Switch to Malathion or Ivermectin immediately. Do not keep repeating Permethrin.
  • Cost of overtreatment.

Guidelines

  • NICE Guidelines
  • BTS Guidelines
  • RCUK Guidelines