Dermatology Workup

2. Skin Scraping (Second-line)

• Deep scrape (until bleeding) for Demodex; superficial for Sarcoptes/Cheyletiella
• Sensitivity: Sarcoptes 50-70%, Demodex 70-80%; repeat if high suspicion
• Diagnostic threshold: >5 Demodex mites/low-power field

3. Dermatophyte Test Medium (DTM)

• Culture 2-3 weeks; Microsporum canis (most common) produces red/pink color
• Sensitivity 70-80%; negative result does not exclude ringworm
• Confirmatory: Sabouraud agar or MALDI-TOF for definitive species ID

4. Skin Biopsy (Definitive)

• 6-8 mm punch from active lesion margin; special stains (PAS, GMS) for fungi
• Indicated: Refractory cases, suspected autoimmune, chronic scaling, neoplasia
• Sample multiple sites increases diagnostic yield

Pruritus Scoring

Visual Analog Scale (VAS): 0-10 owner-reported (0 = none, 10 = severe); trending more important than absolute Canine Pruritus Assessment Tool (CPAT): Scratching bouts, biting frequency, sleep disruption, lesion severity Feline: Grooming frequency, alopecia, self-trauma patterns

Common Differentials by Species

Dogs

1. Atopic Dermatitis: 10-15% prevalence; onset 1-3 years; face/ears/neck/paws; diagnosis: clinical (parasites/food/infection ruled out)
2. Flea Allergy Dermatitis (FAD): Single flea triggers reaction; caudal dorsum, tail, rear limbs; MANDATORY flea prevention trial
3. Food Allergy: 8-12 week elimination diet (novel protein); year-round (non-seasonal); concurrent GI signs common
4. Demodex canis: Localized vs. generalized; indicator of immunosuppression; requires treatment + underlying disease investigation
5. Sarcoptes scabiei: 50-70% diagnostic sensitivity; severe pruritus disproportionate to lesions; trial therapy justified

Cats

1. Atopy: Often presents as over-grooming/miliary dermatitis; self-trauma common; exclusion diagnosis
2. FAD: Miliary dermatitis pathognomonic; single flea sufficient; strict flea control mandatory
3. Food Allergy: 8-12 week elimination (novel protein); presents with pruritus or GI signs
4. Demodex cati: Rare; generalized mange uncommon; associated with FIV/FeLV/immunosuppression
5. Sarcoptes: Rare in cats; presumptive diagnosis often necessary

Malassezia Considerations

Epidemiology: Lipophilic yeast; overgrowth pathogenic in atopy, seborrhea, otitis, maceration Predisposed breeds: Basset hounds, Cocker spaniels, Poodles, Shih Tzus Diagnosis: Cytology (Diff-Quik); culture not standard (normal flora) Treatment: Address underlying cause (critical); topical miconazole/terbinafine 2-4 weeks; systemic fluconazole 5-10 mg/kg daily (dogs)

Treatment Tiers

Tier 1: Environmental control (flea prevention all household animals), novel protein trial, bathing, humidity reduction Tier 2: Topical azoles (Malassezia), antibacterial (chlorhexidine), antihistamines (cetirizine, hydroxyzine 10-30% efficacy) Tier 3: Systemic corticosteroids (0.5-1 mg/kg BID initial, taper essential); cephalexin 22-33 mg/kg BID (pyoderma, 2-4 weeks) Tier 4: Cyclosporine 5-10 mg/kg BID (dogs/cats, 4-6 week onset); oclacitinib (JAK inhibitor, 24-48 hour onset); lokivetmab (IL-31 antibody, canine-specific) Tier 5: Specialist referral for IDST, allergen immunotherapy, advanced dermatopathology

Key Species Differences

• Dogs: Corticosteroids effective but taper essential; JAK inhibitors preferred for rapid pruritus control; cyclosporine for long-term
• Cats: Diabetes risk with corticosteroids; cyclosporine/JAK inhibitors preferred; elimination diet often diagnostic for food allergy

Limitations

• Negative skin scrape does not exclude parasitic disease (50-80% sensitivity); repeat or trial therapy justified
• Negative DTM does not exclude dermatophytosis; clinical judgment essential
• Atopy/FAD/food allergy remain clinical diagnoses (exclusion-based); no gold-standard test exists
• Underlying immunosuppression must be identified before treating secondary Demodex
• Referral to board-certified dermatologist recommended for refractory, chronic, or suspected autoimmune cases