+ 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
  • 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