Scabies

Aetiology & Pathogen

• Adult female: 0.4 x 0.3 mm - just too small to see with the naked eye
• Pearl-like, translucent, white, oval, with 4 pairs of short stubby legs
• Can survive 3 days off a host (up to 7 days in mineral oil)
• Cannot fly or jump

1. Female excavates a burrow in the stratum corneum at 0.5-5 mm/day, reaching the stratum granulosum boundary
2. She lays 2-4 eggs/day (up to 50 over her 30-day lifespan)
3. Eggs hatch in 10-12 days; larvae mature on skin surface
4. Larvae molt into nymphs, nymphs into adults
5. Male lives on skin surface, enters burrows to copulate

• Fitzpatrick's Dermatology, Vol. 1-2, p. 3303-3304

Transmission

• Primary: close personal contact (skin-to-skin) - sexual or non-sexual
• Fomites: less common in classic scabies; very important in crusted scabies (thousands of mites/gram of debris from bedding, floors, furniture)
• Equally prevalent in children and sexually active adults - nonsexual household transmission is just as effective

Clinical Features

• Interdigital webs of hands
• Sides of fingers, volar wrists, lateral palms
• Elbows, axillae
• Scrotum, penis, labia, areolae
• Buttocks, abdomen

Thread-like burrows in the web spaces of the fingers - a classic scabies finding (Fitzpatrick's Dermatology)

Crusted (Norwegian) Scabies

• HIV/AIDS, HTLV-1 infection
• Solid organ transplant recipients
• Elderly, dementia patients
• Leprosy, paraplegia (impaired sensory function or inability to scratch)
• Systemic glucocorticoid use

Crusted scabies: hyperkeratotic plaques populated with thousands of mites (Fitzpatrick's Dermatology)

Diagnosis

1. Skin scraping: Drop mineral oil on burrow → scrape longitudinally with a #15 scalpel blade → apply to glass slide → examine under low power for mites, eggs, or fecal pellets (scybala). Do not scrape excoriated areas.
2. Dermoscopy: The classic finding is the "delta-wing jet" sign - dense scabies head parts + translucent body + eggs + S-shaped burrow
3. Skin biopsy: May reveal mite transected in the stratum corneum (histopathology)
4. Confocal microscopy: In vivo examination of the mite

Differential Diagnosis

Complications

• Secondary bacterial infection (impetiginization) with Streptococcus pyogenes or Staphylococcus aureus
• Post-streptococcal glomerulonephritis from scabies-induced pyoderma
• Lymphangitis and septicemia (especially in crusted scabies)
• Bullous pemphigoid - scabies infestation can trigger this autoimmune blistering condition
• Fitzpatrick's Dermatology, Vol. 1-2, p. 3305

Treatment

Principles

• Treat the patient and all household contacts/close contacts simultaneously, even if asymptomatic
• A second application at 1-2 weeks is required to kill nymphs that hatched after initial treatment
• Fomite control (wash clothing, bedding, towels in hot water; items that cannot be washed should be sealed in a plastic bag for 1 week)

First-line: Permethrin 5% Cream

• Apply from neck down to entire body, including under nails, umbilicus, nipples, genitals
• In infants and young children: treat face and scalp as well
• Apply at bedtime; wash off after 8-12 hours
• Repeat in 1-2 weeks

Oral Ivermectin

• Off-label in many countries but widely used
• Standard: 200 mcg/kg as a single dose, repeated at 2 weeks
• Cure rate may be slightly lower than permethrin with a single dose
• For crusted scabies or heavily infested/immunocompromised patients: multiple doses recommended (e.g., days 1, 2, 8, 9, 15 with concurrent topical therapy)
• Not recommended in pregnancy, breastfeeding, or children <5 kg

Alternative Topical Agents

Symptomatic Relief

• Topical or oral corticosteroids for post-scabetic itch (pruritus can persist 2-4 weeks after successful treatment due to ongoing hypersensitivity reaction - this does not indicate treatment failure)
• Oral antihistamines for itch
• Topical/oral antibiotics if secondary impetiginization present
• Fitzpatrick's Dermatology, Vol. 1-2, p. 3305-3306; Rosen's Emergency Medicine, p. 2411-2412

Special Populations

Environmental Control

• Wash all worn clothing, bedding, towels in hot water (>50°C) and dry on high heat
• Items that cannot be washed: seal in plastic bag for 72 hours (classic scabies) or 1 week (crusted scabies)
• Vacuum furniture and floors; more important for crusted scabies
• Spraying insecticides on surfaces is generally not recommended for classic scabies

1. Nocturnal itch + interdigital burrows = scabies until proven otherwise
2. Treat all contacts simultaneously or re-infestation is virtually guaranteed
3. Itch persisting for weeks after treatment ≠ treatment failure (it is hypersensitivity)
4. Crusted scabies: consider in any immunocompromised patient with hyperkeratotic skin changes and minimal itch
5. Post-scabetic nodules on the scrotum/areolae can persist for months - they are a granulomatous reaction and do not indicate active infestation

Pityriasis Rosea

Epidemiology

• Worldwide distribution; affects all races and ethnic groups
• Peak incidence: ages 10-35 years (range 3 months to 83 years)
• Slight female predominance (ratio ~1.39:1)
• Incidence: ~0.64 per 100 dermatologic patients; up to 3-30 per 1,000 patients by some reports
• May show seasonal variation (spring and fall), though not universally confirmed
• Case clustering has been documented, supporting an infectious aetiology
• Relapse is rare (1.8-3.7% of cases)
• Fitzpatrick's Dermatology (9th ed.), Ch. 31

Aetiology & Pathogenesis

• HHV-7 DNA and mRNA detected in lesional and non-lesional skin, peripheral blood mononuclear cells, and cell-free plasma of PR patients
• HHV-7 viremia demonstrated during active PR
• Case clustering and possible seasonal variation are consistent with an infectious trigger
• Resemblance to other known viral exanthems
• Prodromal symptoms in up to 69% of patients

• Fitzpatrick's Dermatology (9th ed.), Ch. 31; Tintinalli's Emergency Medicine (9th ed.)

Clinical Features

Two-Phase Presentation

• Present in ~80% of cases (range: 12-94%)
• A single, well-demarcated, oval to round plaque
• Color: pink, rose, erythematous, or less commonly hyperpigmented
• Size: 2-10 cm (usually >3 cm)
• Has a slightly depressed centre with a fine "collarette" of scale - characteristically with the open edge of scale pointing inward (toward the centre)
• Most common on the trunk (50%), also on extremities and neck
• In adults: remains isolated for ~2 weeks before the secondary eruption
• In children: interval is shorter, only ~4 days

Herald patch: large erythematous oval patch - the initial and most distinctive lesion of pityriasis rosea (Goldman-Cecil Medicine)

• Begins ~2 weeks after the herald patch
• Numerous smaller (1-2 cm), oval papules and plaques with the same fine collarette of scale
• Distributed on trunk and proximal extremities
• Their long axes run parallel to the lines of skin cleavage (Langer's lines) - on the back, this produces the classic "Christmas tree" distribution
• Palms, soles, and face typically spared (in adults)

Symptoms

• Pruritus: severe in 25%, mild-to-moderate in 50%, absent in 25%
• Prodromal/systemic symptoms (malaise, headache, nausea, upper respiratory, GI symptoms): present in 5-69% of patients, may precede or accompany the rash
• Lymphadenopathy: may occur with flu-like symptoms

Mucosal Involvement

• Punctate hemorrhagic lesions
• Ulcerations (most common oral form)
• Erythematous macules or plaques
• Bullous or annular lesions

Atypical Variants (~20% of cases)

Natural Course & Duration

• Self-limited: resolves in 4-16 weeks (typically 6-8 weeks in most sources)
• Resolves without scarring in immunocompetent individuals
• Recurrences are rare
• No long-term complications in healthy individuals

Pregnancy

• Occurs more frequently in pregnant women than the general population
• In one study of 38 pregnant women with PR: 13% miscarriage rate (vs ~10% baseline)
• Of 8 cases beginning before 15 weeks gestation: 62% resulted in abortion
• 27% premature delivery rate (but no birth defects reported)
• 18% of infants showed hypotonia, weak motility, or hyporeactivity
• Recommendation: Pregnant women with PR, especially in the first trimester, should be followed carefully for adverse outcomes.

Diagnosis

• Spongiosis with mild acanthosis
• Focal parakeratosis
• Lymphocytic exocytosis
• Perivascular lymphohistiocytic infiltrate
• Extravasated erythrocytes (in purpuric variant)

Differential Diagnosis

Treatment

Symptomatic / First-line

• Oral antihistamines - for pruritus
• Topical corticosteroids (e.g., triamcinolone 0.1% cream) - for pruritus and inflammation
• Emollients (petroleum jelly-based preparations)
• Topical zinc oxide or calamine lotion - particularly useful in the ED setting
• Natural sunlight or UVB phototherapy - may speed resolution of lesions

Antiviral (for severe/widespread disease)

• Oral acyclovir - has shown benefit in hastening recovery and lessening symptoms in patients with severe or widespread eruption. Several studies support its use but evidence is not definitive.
  • Typically: acyclovir 800 mg 5x/day for 1 week

What is NOT indicated

• Macrolides (erythromycin) - earlier studies suggested benefit but more recent data do not support use
• Systemic steroids: generally not indicated; may be used cautiously in very severe or vesicular cases
• No isolation or activity restrictions are needed
• Fitzpatrick's Dermatology (9th ed.), Ch. 31; Tintinalli's Emergency Medicine (9th ed.); Rosen's Emergency Medicine (10th ed.)

Key Points for Exam & Clinic

1. Herald patch precedes the secondary eruption by ~2 weeks (adults) - present in 80% of cases
2. The Christmas tree pattern on the back reflects alignment of oval plaques along skin cleavage lines
3. The collarette of scale with the free edge pointing inward is a hallmark - opposite to tinea where scale points outward
4. Always order VDRL/RPR - secondary syphilis is the #1 differential, especially if palms/soles involved
5. Self-limited - reassurance is the most important part of management
6. HHV-6/HHV-7 reactivation is the leading aetiologic hypothesis
7. Pregnancy + first trimester = higher risk of miscarriage; counsel and monitor carefully
8. Drug-induced PR should be considered in all cases - review the medication list