Proliferative, lymphocytic, infundibular mural folliculitis and dermatitis with prominent follicular apoptosis and parakeratotic casts in four Labrador retrievers: preliminary description and response to therapy.

Objectives: To describe the clinical, histopathological and immunohistochemical lesions and the response to therapy of a novel skin disease in four dogs, and to compare the lesions with those of other similar conditions.

Methods: Clinical lesions, the histopathological findings in skin biopsy samples, immunohistochemistry for CD3 and cleaved caspase-3 and the response to therapy were evaluated.

Results: Clinical lesions included multifocal, coalescing, verrucous, crusted papules and plaques with erythematous borders and comedones or follicular casts. Lesions were in haired skin; they occurred at the edges of paw pads and claw beds in one dog. Histopathological lesions included ortho- and more prominent parakeratotic hyperkeratosis involving follicular infundibular epithelium, with cast formation and a papillary epidermal surface. Lymphocytic exocytosis affected all strata of follicular infundibular epithelium and epidermis. Variable numbers of acidophilic shrunken keratinocytes, often bordered by lymphocytes (satellitosis), occupied the more superficial strata of the follicular infundibular epithelium and epidermis. Immunohistochemistry revealed numerous CD3+ T lymphocytes and fewer cleaved caspase-3-positive apoptotic keratinocytes in the infundibular hair follicle epithelium and epidermis, with numerous CD3+ T lymphocytes and cleaved caspase-3-positive cells in the dermis. Two dogs responded completely to therapy with ciclosporin and remained lesion free off therapy; one dog responded to therapy with prednisone, azathioprine and ciclosporin, but relapsed; and one dog was not treated.

Conclusions And Clinical Importance: The cause of the lesions is unknown; the presence of intraepithelial CD3+ lymphocytes and cleaved caspase-3-positive apoptotic keratinocytes and the positive response to immunosuppressive therapy suggest an immune response directed towards unidentified antigens expressed on the surface of keratinocytes.