General Considerations

•visceral involvement reported in up to 50% of feline mastocytic MCT with the spleen the most common site for visceral MCT, but other sites include mediastinum (with pleural effusion), lymph node, and intestines

•splenic or lymphoreticular form is common and accounts for 15% of all splenic pathology in cats

•3 forms of splenic MCT: smooth, diffuse, and nodular

•dissemination and metastasis is more common in visceral MCT with splenic MCT disseminating to the liver (90%), visceral lymph nodes (73%), bone marrow (23%-40%), lung (20%), and intestine (17%)

•pleural effusion occurs in 15% of cats with visceral MCT

•peritoneal and pleural effusion rich in mast cells and eosinophils in up to 33% cats

•cats with primary visceral MCT rarely have cutaneous involvement, but splenic MCT is reported in 18% of cats with cutaneous MCT

•DDx: LSA, myeloproliferative disease, accessory spleen, HSA, hyperplastic nodules, and splenitis

Clinical Signs

•systemic illness with visceral or systemic forms: depression, anorexia, weight loss, and intermittent vomiting

Diagnosis

•splenomegaly ± peritoneal effusion for splenic MCT

•abdominal mass with diarrhea and possibly pyrexia in intestinal MCT

•mast degranulation is usually episodic with systemic mastocytosis and clinical signs include GI ulceration, uncontrollable hemorrhage, altered smooth muscle tone, hypotensive shock, and respiratory distress

•respiratory distress can also be caused by pleural effusion or anemia which is present in up to 33% of cats

•FNA of cutaneous mass, spleen, intestinal mass, or from pleural or peritoneal fluid: granules stain blue with Giemsa and purple with toluidine blue and appear more eosinophilic with hematoxylin and eosin stains

•tissue biopsy and histology required for diagnosis of histiocytic MCT

•disseminated disease: hematology, serum biochemistry, buffy coat smear, bone marrow aspirate, and coagulation profile

•anemia (33%) common in the splenic but not intestinal form due to increased splenic sequestration, red blood cell coating with antibodies, and endocytosis of red blood cells by mast cells

•cats with systemic mastocytosis will have eosinophilia, basophilia and peripheral mastocytosis (50%)

•mast cells can account for up to 25% of white blood cells in cats

•coagulation abnormalities reported in 90% of cats with splenic MCT, but rarely clinically significant

•methylated metabolites of histamine in urine may be a valuable diagnostic technique for mastocytosis

Treatment

•surgery: splenectomy for splenic MCT

•effectiveness of adjunctive therapy unknown

•combination chemotherapy protocols using prednisone, vincristine, cyclophosphamide, and methotrexate have not offered a survival advantage over surgery alone

Prognosis

•MST 12-19 months due to reduction of splenic suppressor cell activity allowing immune system control (hence corticosteroids are controversial in feline MCT)

•peripheral mastocytosis will decrease but rarely resolve, however, increase can be marker for progression

•poor prognostic factors include anorexia, significant weight loss, and male