A 9-year-old, neutered male domestic shorthair developed a 1 cm area of alopecic, erythematous eroded skin on the left caudal thigh in May. Over the course of 10 months, it did not resolve with oral Clavamox or enrofloxacin. Surface culture grew Staphlococcus xylosus four months after initial presentation. Topical Baytril-SSD and Dermalone may have helped with infection. Cat was not pruritic and only licked after medication was applied. Fungal culture was negative. A 4 mm skin punch biopsy was collected, and tissue submitted for histopathology and deep tissue submitted for culture and sensitivity (including Mycobacteria spp, Nocardia spp, and Actinomyces spp). Deep tissue culture was negative. Surface cytology on the day of the skin biopsy found neutrophils, but no eosinophils or bacteria.
Figure 1. Erythematous and alopecic lesion on left caudal thigh.
Figure 4. Pancytokeratin IHC
Figure 5. Pancytokeratin IHC
Figure 6. Vimentin IHC
Figure 7. Melan A
August 2022 - What is the most likely diagnosis?