Dermatology on the NAVLE tests pattern recognition: the right breed, distribution, and cytology finding together point to the diagnosis. The classic traps are Demodex vs Sarcoptes (contagious vs not), pemphigus foliaceus cytology, and the species differences in allergy presentation.
Canine Atopic Dermatitis (CAD)
IgE-mediated type I hypersensitivity to environmental allergens (dust mites, pollens, molds). Most common allergic skin disease in dogs. Breeds: West Highland White Terrier, Golden Retriever, Labrador, Bulldog, Boxer, German Shepherd. Age of onset: 1–3 years. Distribution: ventrum, axillae, inguinal region, paws (licking), periocular, perilabial—NOT dorsum. Primary sign is PRURITUS—skin lesions are secondary from self-trauma.
Treatment: Oclacitinib (Apoquel, JAK1 inhibitor), lokivetmab (Cytopoint, anti-IL-31 monoclonal antibody), cyclosporine, allergen-specific immunotherapy (ASIT). Short-term steroids for acute flares. Always address secondary Malassezia and Staphylococcus infections concurrently.
Mange
Sarcoptic Mange (Scabies)
- Sarcoptes scabiei var. canis
- HIGHLY CONTAGIOUS; zoonotic
- Distribution: ear margins, elbows, ventrum
- Pinnal-pedal reflex: scratch reflex when rubbing pinna margin—highly specific
- Scraping often negative (low mite burden)
- Diagnosis often by treatment response
- Treatment: selamectin, ivermectin, isoxazolines
Demodectic Mange (Demodex)
- Demodex canis (normal commensal)
- NOT contagious; immune dysregulation
- Breeds: Shar Pei, Old English Sheepdog, Pit Bull, Doberman
- Deep skin scraping: many mites in follicles
- NEVER use systemic steroids (worsens disease)
- Treatment: afoxolaner (NexGard), fluralaner, ivermectin, amitraz
- Avoid ivermectin in MDR1 mutation breeds (Collies, Shelties, Aussies)
Dermatophytosis (Ringworm)
Microsporum canis (most common in cats and dogs), M. gypseum (soil, geographic), Trichophyton mentagrophytes (rodent source). Cats are major reservoir—often asymptomatic carriers. Wood's lamp: ~50% of M. canis fluoresces (apple-green)—T. mentagrophytes does NOT fluoresce. Gold standard: fungal culture on DTM (Dermatophyte Test Medium)—red color change. Treatment: systemic itraconazole or terbinafine + topical miconazole/chlorhexidine shampoo. Zoonotic.
Pemphigus Foliaceus (PF)
Most common autoimmune skin disease in dogs AND cats. Autoantibodies against desmoglein (desmosomal protein) → acantholysis → subcorneal pustules. Distribution in dogs: nasal planum, dorsal muzzle, ear pinnae, dorsal paws → progresses to trunk. Cats: face, ear pinnae, nail beds (paronychia is classic in cats).
Cytology of intact pustule: acantholytic keratinocytes (rounded-up keratinocytes in clusters) + neutrophils + NO bacteria. This is the most specific cytologic finding. Skin biopsy: subcorneal pustules with acantholysis (definitive). Treatment: immunosuppressive prednisolone + azathioprine (dogs) or chlorambucil (cats).
Equine Skin Conditions
Rain rot (Dermatophilosis): Dermatophilus congolensis; crusting "paintbrush" lesions on dorsum after prolonged wet conditions; treatment: penicillin/amoxicillin, remove crusts. Sweet itch (Culicoides hypersensitivity): Dorsal midline pruritus, mane/tail rubbing; Culicoides midges; management: mesh blankets, fly repellent, corticosteroids. Equine Sarcoid: Most common equine skin tumor; bovine papillomavirus 1&2; locally invasive, non-metastatic; treatment: cisplatin beads, surgical excision (high recurrence if incomplete).
Feline Eosinophilic Granuloma Complex
Three presentations: indolent ulcer (upper lip, ulcerated, often painless), eosinophilic plaque (ventral abdomen/medial thighs, intensely pruritic, raised), eosinophilic granuloma (oral cavity, thighs, footpads—linear form). Underlying trigger: usually allergy (flea allergy, food, atopy). Treat the underlying cause + prednisolone for the lesion. Eosinophils dominate on cytology of all three forms.