Differentiating Disseminated Scabies from Bullous Pemphigoid
Disseminated scabies and bullous pemphigoid can be distinguished by identifying burrows and mites on skin scraping for scabies, versus linear IgG/C3 deposits on direct immunofluorescence for bullous pemphigoid, though critically, scabies can trigger true bullous pemphigoid or mimic it completely, requiring both parasitological and immunological workup in elderly patients with bullous eruptions. 1, 2
Clinical Presentation Differences
Bullous Pemphigoid
- Classic presentation: Tense bullae on erythematous or normal-appearing skin with severe pruritus, primarily affecting elderly patients (age >70 years) 3, 4
- Non-bullous variants (20% of cases): Excoriations, prurigo-like lesions, eczematous patches, urticarial plaques, or dyshidrosiform lesions without visible blisters 3, 4
- Distribution: Generalized or localized, typically sparing the head and neck, with absence of mucosal involvement in most cases 3
- Associated conditions: Strong association with neurological disorders including dementia, Parkinson's disease, and stroke 5, 4
Disseminated (Bullous) Scabies
- Classic scabies features: Burrows in web spaces between fingers, excoriated papules, pustules, and vesicles with intense pruritus 1
- Bullous variant: Tense blisters on erythematous base that clinically mimic bullous pemphigoid, often with persistent pruritus despite topical corticosteroids 1, 2
- Distribution: Affects lower trunk, arms, legs, with characteristic burrows on hands and wrists 2
- Family/contact history: Other household members or close contacts developing similar pruritic symptoms 1
Critical Diagnostic Algorithm
Step 1: Immediate Parasitological Assessment
- Perform skin scraping with mineral oil preparation from suspected burrows (especially web spaces, wrists, borders of hands) to identify Sarcoptes scabiei mites, ova, or scybala 1, 2
- Use dermatoscopy to visualize burrows and mites directly 6
- This must be done first in any elderly patient with new-onset pruritic bullous eruption, as scabies can completely masquerade as bullous pemphigoid 1
Step 2: Immunopathological Workup
- Direct immunofluorescence (DIF) from perilesional skin is essential and mandatory for bullous pemphigoid diagnosis, showing linear IgG and/or C3 deposits along the dermoepidermal junction 3, 7
- Histopathology from intact recent bulla: Subepidermal bullae with eosinophils and/or neutrophils, dermal infiltrate of eosinophils, marginalization of eosinophils along dermoepidermal junction 3
- Serum testing: ELISA for anti-BP180 and anti-BP230 antibodies, or indirect immunofluorescence on salt-split skin showing IgG binding to epidermal side 3, 7
Step 3: Interpret Combined Results
Three possible scenarios exist:
Pure bullous scabies: Positive skin scraping + negative DIF + resolution with antiscabetic treatment alone 2, 8
Scabies-triggered bullous pemphigoid (Koebner phenomenon): Positive skin scraping + positive DIF with linear IgG/C3 + requires both antiscabetic treatment AND immunosuppressive therapy 6
Pure bullous pemphigoid: Negative skin scraping + positive DIF + drug history review (diuretics, psycholeptics over past 1-6 months) 3, 7
Diagnostic Pitfalls and How to Avoid Them
Major Pitfall #1: Assuming Negative Scabies Scraping Rules Out Scabies
- Scabies can coexist with true bullous pemphigoid, with both positive DIF findings and active mite infestation requiring dual treatment 9, 6
- Always examine family members and close contacts for scabies symptoms 1
Major Pitfall #2: Relying on Histopathology Alone
- Bullous scabies can show identical histopathology to bullous pemphigoid: subepidermal bullae with eosinophils, and even positive DIF in some cases 1, 9
- The presence of eosinophils does not exclude scabies, as long-standing scabies activates Th2 immune response causing eosinophilia 6
Major Pitfall #3: Treating with Corticosteroids Without Ruling Out Scabies
- Corticosteroid treatment can worsen scabies by suppressing immune response, leading to crusted (Norwegian) scabies 1
- If bullous pemphigoid treatment shows improvement but not complete resolution, actively search for concurrent scabies 1
Major Pitfall #4: Missing Non-Bullous Bullous Pemphigoid
- Up to 20% of bullous pemphigoid cases present without bullae, showing only refractory pruritus in elderly patients 3, 4
- DIF remains positive even in non-bullous variants and is essential for diagnosis 7
Treatment Implications Based on Diagnosis
If Pure Bullous Scabies (Negative DIF)
- Topical permethrin or lindane lotion followed by crotamiton cream results in complete resolution of bullous lesions 2, 8
- Treat all household contacts simultaneously 1
If Scabies-Triggered Bullous Pemphigoid (Positive DIF + Positive Scraping)
- Treat scabies first with permethrin, then initiate bullous pemphigoid therapy 6
- First-line for bullous pemphigoid: Superpotent topical corticosteroids (clobetasol propionate 0.05% cream) 20g/day to whole body except face, providing superior disease control with significantly lower mortality than systemic steroids 5, 4
- Monitor for bullous pemphigoid relapse even after scabies clearance 6
If Pure Bullous Pemphigoid (Negative Scraping + Positive DIF)
- Clobetasol propionate 0.05% cream as first-line: 20g/day for generalized disease, taper after 15 days once controlled, reduce to 10g weekly maintenance after 4 months 5, 4
- Avoid high-dose systemic corticosteroids (>40mg prednisolone daily) as first-line in elderly patients, as this doubles mortality risk 5
- Alternative for high-risk elderly: Doxycycline 200mg/day with nicotinamide achieves 73.8% response rate with reduced mortality 5, 4