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The clinical and pathologic features of systemic mastocytosis in 16 dogs are reported. There was no apparent breed or sex predilection, and the median age at presentation was 9.5 years. In 14 of 16 cases there was a primary cutaneous mast cell tumor (MCT). When cutaneous tumor location was compared with previous reports, there was no association between location and systemic dissemination. The most common presenting signs associated with the cutaneous tumor were regional dissemination, edema, ulceration, and abscessation. They were present in 12 dogs (69%). Signs of systemic illness, including anorexia, vomiting, and diarrhea, were seen in eight dogs (50%). Other than the cutaneous tumors, the most consistent physical and radiographic abnormalities included lymphadenopathy, splenomegaly, and hepatomegaly. Eosinophilia and basophilia were seen in two and five dogs, respectively. Six dogs had increased numbers of mast cells in peripheral blood or buffy coat smears. Five of the nine dogs evaluated had increased numbers of mast cells in bone marrow aspirates. Bone marrow aspiration was superior to both peripheral blood and buffy coat smears in predicting mastocytosis. Coagulation abnormalities were seen in three of five dogs tested.

Using a conventional histomorphologic grading system, 10 of 13 (77%) tumors were classified as Grade III or undifferentiated and were overrepresented when compared with previous reports of cutaneous MCTs. Eighty-eight percent of the dogs either died or were euthanatized because of their tumors. Organs commonly involved at necropsy included lymph nodes, spleen, liver, and bone marrow; four dogs had gastroduodenal ulcers.