Topical Imiquimod Therapy for Localized Solar Dermatitis in a Dog

Chronic sun exposure in dogs can result in clinical changes in the skin referred to as solar or actinic dermatitis. A 5-year-old, neutered male, American Bulldog presented with localized unilateral erythematous macules, plaques, alopecia, comedones and hemorrhagic bullae involving the non-pigmented skin on left lateral ventral flank area. The presence of hyperplastic epidermis with keratinocyte dysplasia, superficial dermal elastosis, and multiple follicular cysts with occasional rupture (furunculosis) on histology, together with history and characteristic skin lesions was consistent solar dermatitis/actinic keratosis. Skin scrapings were negative and treatment for secondary pyoderma was initiated with oral clindamycin for 8 weeks. After 2 months of antibiotic therapy, the hemorrhagic bullae resolved; however, erythematous solar/actinic skin lesions with induration and comedones persisted. Topical application of imiquimod 5% cream 3 times weekly for 8 weeks resulted in the resolution of erythema, but some of the non-inflamed comedones remained. Staining for elastin and Ki67 revealed keratinocyte hyperplasia in hair follicle infundibulums and alterations in the elastic fibers around follicles, which may lead to closure and formation of follicular cysts. Imiquimod has been long suggested as a treatment option for solar dermatitis, but this is the first known case report detailing its efficacy in dogs.