A focal area of hair loss developed in a Persian cat several days after application of a spot-on flea control product. Over 2 months, the area enlarged to 10 cm and was a well-demarcated, raised plaque-like lesion in the dorsal cervical area and interscapular region. Routine diagnostics did not identify the cause, and a skin biopsy was obtained. Histological findings revealed extensive and severe fibrosis characterized by collagen fibers and low cellularity. The lesion was compatible with scleroderma (morphea). Hair follicles were still visible on the histological section. After the lesion failed to respond to oral pentoxifylline, the area was treated with 5% minoxidil q12h for 60 days with complete resolution of signs. Thirty-six months after the episode, the initial lesion had not recurred and hair growth was still present.

Commentary
Morphea-like lesions would be included in the differential diagnosis of noninflammatory persistent alopecia in a cat or dog. As in many cases of focal noninflammatory alopecia, the most enlightening diagnostic test is a skin biopsy; however, the specimen should be sent to a veterinary dermatopathologist with history and, ideally, images. Little is known about morphea in dogs and cats, as it can be difficult to differentiate from scarring cicatricial alopecia.1 The pathogenesis of morphea is likely multifactorial involving underlying predisposition, environmental trigger (eg, trauma, medication, infections), and vascular injury.2 Of note, topical minoxidil has been reported to be toxic to cats, resulting in lethargy, dyspnea, hypothermia, pulmonary edema, pleural effusion, and death.3 Although this cat responded to treatment, the use of minoxidil is not recommended.—Karen Moriello, DVM, DACVD

Source
Morphea-like lesion following topical endectocide application in a cat. Seixas G, Taboada P. VET DERMATOL 23:244-e50, 2012.

1. Scarring alopecia. Mecklenburg L. In Mecklenburg L, Linek M, Tobin DJ (eds): Hairloss Disorders in Domestic Animals—Ames: Wiley-Blackwelll, 2009, p 189.
2. Update on morphea: Part 1. Epidemiology, clinical presenatation, and pathogenesis. Fett N, Werth VP. J Am Acad Dermatol 64:217-228, 2011.
3. Suspected toxicosis after topical administration of minoxidil in 2 cats. DeClementi C, Bailey KL, Goldstein SP, Orser MS. JVECC 14:287-292, 2004.