Scabies

Overview

• Fitzpatrick's Dermatology, p. 3303

The Mite

• Pearl-white, translucent, oval, eyeless, with 4 pairs of stubby legs
• Adult female: 0.4 x 0.3 mm (just below naked-eye visibility); male slightly smaller
• Cannot fly or jump
• Survives off the host for up to 3 days (in a test tube) or 7 days in mineral oil - this enables fomite transmission
• A normal host harbors 3-50 mites; infants/elderly may harbor 50-250; crusted scabies patients can harbor millions

Life Cycle

Transmission

• Primary route: Close personal contact (skin-to-skin, including sexual contact and household contact)
• Fomite transmission: Much more common in crusted scabies (6,000 mites/g have been detected in bed linens, floors, curtains). Live mites have been recovered from bedroom floors, overstuffed chairs, and couches in infected households

Clinical Features

Incubation & Symptoms

• Initial infestation: symptoms appear 4-6 weeks after exposure (some patients take up to 3 months; some are never sensitized)
• Reinfestation: symptoms within 2-3 days (sensitized immune response)
• Cardinal symptom: Intense pruritus, classically worse at night

Distribution

Scabies: scattered erythematous papules on the buttocks and thighs - Rosen's Emergency Medicine

Skin Lesions

• Excoriations, eczematous dermatitis, small papules/pustules (<5 mm)
• Pathognomonic lesion: the burrow - a thin, thread-like, linear or J-shaped/S-shaped tunnel, 1-10 mm long

Scabies: thread-like burrows in web spaces of the fingers and knuckles - Fitzpatrick's Dermatology, p. 3304

Crusted (Norwegian) Scabies

• Hyperkeratotic plaques on palms and soles; nail thickening and dystrophy
• Harbors thousands to millions of mites
• Minimal or absent pruritus (paradoxically)
• Extremely contagious - anyone in the general vicinity is at risk

Diagnosis

Differential Diagnosis

• Pityriasis rosea
• Papular urticaria
• Secondary syphilis
• Folliculitis
• Contact dermatitis / atopic dermatitis
• Dermatitis herpetiformis
• Lichen planus
• Psoriasis
• Seborrheic dermatitis

Treatment

First-Line

• Goldman-Cecil Medicine, p. 1572; Rosen's Emergency Medicine, p. 2411

Alternative

• Lindane 1% lotion or cream: apply neck down, wash off after 8 hours. Contraindicated in children <10 years (neurotoxicity risk). Now largely abandoned given safety concerns.
• Permethrin 1% lotion (less effective than 5%)

For Crusted Scabies

• Combination of oral ivermectin (multiple doses) + topical permethrin is recommended
• Due to massive mite burden, more aggressive treatment is needed

Key Management Points

1. Treat all close contacts simultaneously, including asymptomatic household members - they may be carriers
2. Decontaminate clothing and bedding: machine wash at >60°C or seal in a plastic bag for 72 hours (mites die off the host within 3 days)
3. Post-treatment pruritus can persist for 2-4 weeks due to ongoing hypersensitivity reaction to dead mites/eggs - does not indicate treatment failure
4. Oral antihistamines or topical corticosteroids may be used to manage residual pruritus
5. Scabies nodules (firm, persistent nodules - especially on genitals) may persist for months after successful treatment

Special Populations

Complications

• Secondary bacterial infection (impetigo from Staphylococcus aureus or Streptococcus pyogenes) - particularly in resource-limited settings, this is a significant driver of post-streptococcal glomerulonephritis
• Post-scabetic itch (persistent pruritus after cure)
• Scabies nodules

Scabies Treatment Guidelines - India (ICMR STW) - Applicable to Karnataka

Important note: Karnataka does not have a separate state-specific scabies treatment protocol. The state follows the ICMR Standard Treatment Workflow (STW) for Scabies issued by the Department of Health Research, Ministry of Health & Family Welfare, Government of India (July 2022, ICD-10: B86). This is the authoritative guideline used at all levels of the healthcare system across India including Karnataka's NHM facilities, PHCs, CHCs, and government hospitals.

Core Principle (ICMR STW Motto)

"TREAT THE ENTIRE SKIN, NOT LESIONS ALONE; TREAT THE FAMILY/CONTACTS, NOT THE PATIENT ALONE"

First-Line Treatment

1. Permethrin 5% Cream (Topical - Preferred)

2. Oral Ivermectin (for poor compliance or inadequate response to topicals)

3. Alternative Topical: 1% Gamma Benzene Hexachloride (GBHC/Lindane)

• Apply same as permethrin (neck down, 8-12 hours, wash off)
• Avoid in infants (neurotoxicity risk)
• Now used less frequently due to safety concerns

Symptomatic / Adjunct Treatment

• Antihistamines: Prescribe as needed for itch relief (e.g., cetirizine, chlorpheniramine)
• Post-treatment itch: Reassure patients that itching can persist for several weeks after successful treatment - does not mean treatment failure; continue antihistamines

Crusted (Norwegian) Scabies - Intensive Regimen

• Nodular lesions: Potent topical steroid (Clobetasol propionate) or intralesional Triamcinolone acetonide 10 mg/mL for persistent scabies nodules

Post-Treatment Advice

General Measures (Public Health / Contact Management)

1. All family members and close contacts must be treated simultaneously, even if asymptomatic - to prevent re-infestation
2. Screen and treat sexual partners
3. In institutional settings (hostels, schools, old-age homes, prisons), consider selective or mass treatment depending on outbreak size
4. Patient education is essential on correct application technique

Prescribing at Different Levels of Care in Karnataka

Summary Quick-Reference Card

SCABIES - ICMR STW (India / Karnataka)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
FIRST LINE:  Permethrin 5% cream, neck down, 8-12h overnight
             Repeat at 7-14 days
             Adults: 30g | Children <5yr: 15g

ORAL:        Ivermectin 200 mcg/kg (max 12mg) with food
             Two doses, 1 week apart
             ✗ Avoid in: <5yr / <15kg / pregnancy

ALTERNATIVE: GBHC 1% (avoid in infants)

CRUSTED:     Ivermectin D1,2,8,9,15 + Permethrin daily x7d
             then twice weekly + salicylic acid keratolytic

ADJUNCT:     Antihistamines for itch
             Clobetasol for nodules

CONTACTS:    Treat ALL simultaneously
LINEN:       Hot wash or bag for 3 days
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Scabies vs Fungal Infection: Clinical Differentiation

Head-to-Head Comparison

The Most Discriminating Features

1. The Burrow - Pathognomonic for Scabies

• Confirm with ink test: apply black felt-tip ink, wipe with alcohol - burrow retains ink
• Fitzpatrick's Dermatology, p. 3305

2. Nocturnal Pruritus

3. Distribution Pattern

4. Household Clustering

5. Lesion Shape

• Scabies: no central clearing, no ring shape; diffuse papulovesicular eruption
• Tinea corporis: annular/circinate plaques with a well-defined scaly advancing border and central clearing - sometimes called "ringworm" pattern

The Notorious Overlap: Tinea Incognito vs Scabies

• History of prior topical steroid use
• Loss of annular pattern
• Absence of burrows
• KOH scraping from the edge of the lesion shows hyphae

Quick Bedside Differentiating Tests

Summary Mnemonic

Additional Confounders to Keep in Mind

• Candidal intertrigo (intertriginous areas, satellite pustules, moist red rash) can mimic scabies in groin/axillae - but no burrows, satellite lesions present, not nocturnal
• Pityriasis versicolor (Malassezia) - hypo/hyperpigmented macules, fine scale, no itch or mild itch, no burrows; trunk/upper back distribution
• Scabies + tinea co-infection can occur; both must be treated

Clinical Assessment of All 4 Images

Disclaimer: This is a clinical image interpretation for educational/decision-support purposes. It does not replace an in-person examination, KOH scraping, or formal dermatological diagnosis. Please correlate with history (nocturnal itch, family contacts affected, duration).

Image-by-Image Analysis

Image 1 (WA0000) - Dorsum of foot, circled lesion

• Could be a prurigo nodule (from chronic scratching), a healing insect bite reaction, or a dermatofibroma
• The location (dorsal foot) is atypical for both scabies and tinea corporis
• If this is from chronic scratch-induced trauma, it may be a secondary response to underlying scabies elsewhere

Image 2 (WA0003) - Buttock/outer thigh region

• Fine surface scale + hypopigmentation on a hair-bearing area fits tinea or pityriasis versicolor (Malassezia)
• The absence of an annular ring could mean: early tinea, or pityriasis versicolor (which does not form rings - just macular/patch hypopigmentation with fine scale)
• Could also be post-inflammatory hypopigmentation from a healed lesion
• No features of scabies here (no burrows, no papulovesicles, no characteristic interdigital/wrist distribution)

Image 3 (WA0001) - Leg/thigh

• This is the most classic image of the set
• Annular shape + raised scaly advancing border + central clearing = textbook tinea corporis
• The peripheral collarette scale on the inner margin of the border is characteristic
• Complete absence of burrows makes scabies very unlikely for this lesion
• In the Indian context with T. indotineae prevalence, this pattern is extremely common

Image 4 (WA0004) - Leg/thigh

• Thick, adherent yellowish-brown keratotic central crust/plug
• Hemorrhagic (bloody) crust on one - indicating recent vigorous scratching
• Hyperpigmented raised rim around each
• No burrows, no ring pattern, no central clearing

• These are hyperkeratotic crusted nodules - a well-recognized presentation of scabies nodules (nodular scabies), which are firm, pruritic, reddish-brown nodules caused by a hypersensitivity reaction to mite antigens
• The location (inner thigh/genital-adjacent region) is a classic site for scabies nodules - they preferentially occur on covered areas: genitals, buttocks, inner thighs, axillae
• The vigorous scratching evidenced by hemorrhagic crust strongly suggests intense pruritus - hallmark of scabies
• This is distinct from tinea, which does not produce this type of heavily crusted nodule with pruritic scratch marks

Overall Summary

Clinical Impression

• WA0001 + WA0003 → Tinea corporis / fungal infection
• WA0004 → Scabies nodules (nodular scabies), a form that persists for weeks-months post-infestation or indicates active heavy infestation

1. Is the itch worse at night? - If yes, strongly favors active scabies
2. Are other family members itching? - If yes, strongly favors scabies
3. Any involvement of finger web spaces, wrists, genitals? - Scabies distribution
4. Any prior steroid cream use? - Can mask tinea (tinea incognito) and worsen scabies

Recommended Bedside Confirmation

• KOH scraping from the scaly border of the ring lesion (WA0001) → hyphae = tinea confirmed
• Mineral oil scraping from one of the crusted nodules (WA0004) → mite/eggs = scabies confirmed
• If both confirm - treat simultaneously with permethrin 5% (scabies) + topical antifungal (tinea)