dermatology-workup

# Dermatology Workup ## Overview Evidence-based diagnostic progression from non-invasive to invasive techniques. Includes pruritus quantification (VAS, CPAT), differential diagnosis by species, Malassezia management, and multimodal treatment strategies. ## When to Use - User presents pruritic/alopecic dermatologic cases (dogs, cats, exotics) - User needs diagnostic escalation guidance or differential diagnosis - Keywords: pruritus, dermatitis, Demodex, Sarcoptes, atopy, FAD, food allergy, Malassezia, cytology, DTM, biopsy, seborrhea ## Diagnostic Ladder **1. Cytology** (First-line, <24 hours) - Acetate tape or swab, Diff-Quik staining; yields cocci, yeast, bacteria - **Malassezia**: Peanut-shaped organisms; overgrowth suggests predisposition (atopy, otitis) - Limitation: Cannot detect Demodex/Sarcoptes **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