Lokivetmab therapy for pruritus in a dog with cutaneous mastocytosis

Background: Cutaneous mastocytosis (CM) is a rare disease of dogs characterized by rash, pruritus and proliferation of mast cells in the skin. Oral H1 antihistamines are recommended as the treatment to control pruritus.

Hypothesis/objective: To describe the effective treatment of pruritus associated with CM with lokivetmab in one dog.

Animal: A 4-year-old, spayed female cross-bred dog presented with severely pruritic, erythematous to pigmented macules and papules involving the ventral abdomen, interdigital skin, perivulval area and both pinnae; the pruritus had been unresponsive to treatment with antihistamines, prednisone and ciclosporin.

Methods and materials: Complete blood count and serum biochemistry, abdominal ultrasound, blood smear and skin cytological evaluation, PCR, histopathological and immunohistochemical examination of skin biopsies.

Results: Skin cytological evaluation revealed high numbers of uniform, heavily granulated mast cells; histopathological findings showed focal dermal proliferations of well-differentiated, uniform mast cells consistent with a low-grade mast cell tumour (MCT). Clinical staging revealed that the disease was confined to the skin. Mutations of c-Kit exon 8 and 11 were not detected. Treatment was initiated with anti-canine-interleukin (IL)-31 monoclonal antibody lokivetmab; antihistamines were continued. The dog's pruritus resolved within seven days and was maintained in remission over 15 months with once monthly lokivetmab injections; the skin lesions improved but did not resolve.

Conclusion and clinical importance: Lokivetmab treatment was effective in resolving and maintaining pruritus remission in this dog with widespread cutaneous mast cell disease. Whether CM in dogs represent a separate entity that should be distinguished from a low-grade MCT requires further investigation.