Vertically oriented, moderate to severe pyogranulomatous inflammation (neutrophils and macrophages) that track hair follicles in a sausage-shaped or nodular pattern without luminal folliculitis

Older lesions - discrete to confluent granulomas and pyogranulomas that obliterate adnexal structures and extend from the dermis down into the deep subcutis

Acanthosis and mild focal ulcerations may also be noted

• Cytopathic effect in keratinocytes +/- koilocytes
• More keratohyaline granules

• Neutrophilic: drug reaction, bacterial, ischemic dermatopathy, type 3 hypersensitivities such as purpura hemorrhagica (S.equi infection or vaccination), PDNS in feeders (PCV-2)
• Lymphocytic: drug reaction, dermatomyositis, rabies vaccine reaction
• Eosinophilic: arthropod hypersensitivity, parasitism
• Granulomatous: idiopathic septal panniculitis, granulomatous with polyangitis (formerly Wegners granulomatosis), CRH
• Cell poor vasculitis/vasculopathy: vaccine induced, photo aggravated. pastern vasculitis (horses), thermal

• Neutrophilic: bacterial, less common fungal
  • Impetigo- papules, pustules, collarettes
  • Subcorneal pustules with bacteria (Staph most common)
  • Bullous impetigo if larger (potentially immunosuppressed)
  • Superficial spreading pyoderma
  • Pustular dermatophytosis (Trichophyton spp)

• Seen in superficial spreading pyoderma-transient pustules to crusts- degenerative bacteria and cell debris

• Demodicosis
• Dermatophytosis
• Feline lymphocytic and mucinotic mural folliculitis
• Thymoma/Non-thymoma associated exfoliative dermatitis
• Pelodera Dermatitis
• Equine linear alopecia