A 26-year-old Hackney pony mare presented to Cornell’s Ambulatory Service with a two-week history of dermatologic issues. On presentation, the mare was pruritic with flaking skin; she was presumptively diagnosed with dermatophilosis and started on a 10-day course of trimethoprim sulfa (TMS).

One month later, the mare’s clinical signs had progressed to a generalized exfoliative and tender dermatitis with hair loss and crusting. Physical examination was impossible at that time due to extreme discomfort of the patient. Two days later, the mare was heavily sedated, blood was collected, and skin biopsies were taken. Her complete blood count was consistent with anemia of chronic inflammation, and her chemistry panel revealed changes consistent with inflammatory, exudative skin disease.

Clinical signs and blood work were strongly suggestive of pemphigus foliaceus. Other less likely differential diagnoses included dermatophytosis and cutaneous lymphoma. Skin biopsy revealed moderate, multifocal, chronic, neutrophilic epidermitis with acantholysis and serocellular crusts. These histologic changes were consistent with equine pemphigus foliaceus; however, results did not differentiate between an idiopathic cause or disease development secondary to a drug reaction to TMS. The mare was started on prednisolone at an immunosuppressive dose. Within two weeks, she had improved dermatologically. She continues to do well on a tapering dose of the steroid.

This report will review the clinical signs, pathophysiology, diagnosis, and treatment of pemphigus foliaceus in horses.