Treatment of Reactive Histiocytosis With Oclacitinib: A Retrospective Case Series of 10 Dogs

BACKGROUND: Canine reactive histiocytosis is a proliferative disorder of activated interstitial dendritic cells with cutaneous and systemic forms. An immune-mediated aetiology is likely, and systemic immunomodulatory agents such as corticosteroids, tetracycline/niacinamide, ciclosporin, azathioprine and leflunomide have been employed for its management. HYPOTHESIS/OBJECTIVES: The aim of this retrospective case series is to report the clinical features and therapeutic response to oclacitinib in dogs with reactive histiocytosis. ANIMALS: Ten privately owned dogs. MATERIALS AND METHODS: All dogs were diagnosed with reactive histiocytosis based on compatible clinical history, skin lesions and histopathological features, and were treated with oclacitinib as a sole therapy. Immunohistochemical investigation was used to characterise dermal cellular infiltrates for eight of 10 dogs. Clinical features and case outcomes are summarised. RESULTS: All 10 dogs presented with dermal nodules and/or erythematous plaques affecting the head, trunk or limbs; four dogs also had documented or suspected involvement of the nasal or oral cavities. Seven dogs had been treated previously with one or more immunomodulatory agents without durable disease control. All dogs were completely responsive to treatment with oclacitinib, at or slightly above the standard antipruritic dosage. Skin and mucosal lesions resolved within 2-12 weeks. Lesion remission was maintained with oclacitinib monotherapy for varying follow-up times, although four dogs had brief recurrences addressed with adjustments in oclacitinib dosing. CONCLUSIONS AND CLINICAL RELEVANCE: Although reactive histiocytosis can have a naturally waxing and waning course, oclacitinib appears to be rapidly effective for management, even in cases refractory to other immunomodulatory agents or with involvement of the oral or nasal cavities. ANIMAL USE STATEMENT: Animal use was conducted in accordance with the international, national and institutional guidelines for the humane treatment of animals, and with relevant legislation.