Materials and Methods

Informed consent was obtained from all owners before cats were used in the study. The study was approved by the Institutional Animal Care and Use Committee of Kansas State University.

Healthy cats—Client-owned cats that were examined for annual vaccinations or for the specific reason of participation in the study were determined to be clinically normal on the basis of history; physical examination findings; and the absence of abnormalities in the results of a CBC, serum biochemical analyses, and urinalysis.

Cats ill with diseases unrelated to MCTs—Clientowned cats that were examined for any illness other than MCTs were eligible for the study. In addition to the clinicopathologic analyses, ill cats underwent one or more of the follow diagnostic tests: thoracic radiography, abdominal radiography, abdominal ultrasonography, echocardiography, ECG, bronchoscopy with bronchoalveolar lavage and microbial culture of collected fluid, cytologic examination of fine-needle aspirates, incisional biopsy, abdominal exploratory with biopsy, skin punch biopsies, assessment of thyroid hormones, bile acids evaluation, and fecal flotation.

Clinicopathologic analyses—Four milliliters of blood was collected from a jugular or medial saphenous vein in each cat; the blood was divided equally between 2 tubes (with and without EDTA). A urine sample was obtained via cystocentesis. Serum biochemical analyses, CBC, and urinalysis were performed for each cat. For the buffy coat analysis, 2 Wintrobe tubes were filled with blood from the EDTA tubes and then centrifuged at 14,000 × g for 5 minutes. The buffy coat layer of each tube was pipetted onto a slide, and a smear was prepared. The slides were stained with a modified Wright stain and evaluated microscopically by one of the investigators (CLC or BS) for the presence of mast cells. Two slides/cat were examined via microscopy. Each smear was scanned over its entirety at 200× magnification. Any cells that potentially were mast cells were evaluated at 500× and 1,000× magnification with oil immersion lenses for definitive identification.

Results

Mean age of the 40 clinically normal cats in the study was 6.5 years (range, 1 to 20 years). Of these 40 cats, 30 were domestic shorthair cats, 5 were domestic longhair cats, and 3 were Siamese; there was 1 each of Persian and Himalayan breeds. Twenty-one of the clinically normal cats were spayed females, 18 were castrated males, and 1 was a sexually intact male. Mean age of the 40 ill cats in the study was 10 years (range, 1 to 19 years). Of these 40 cats, 25 were domestic shorthair cats, 5 were domestic longhair cats, and 3 cats were Siamese; there was 1 each of Maine Coon, Persian, Himalayan, Balinese, Norwegian Forest cat, Burmese, and Manx breeds. Thirteen of the ill cats were spayed females, 1 was a sexually intact female, and 26 were castrated males.

Among the 40 ill cats, 14 (35%) had neoplasia (5 with gastrointestinal lymphoma; 2 with pulmonary carcinoma; and 1 cat each with nasal carcinoma, parathyroid carcinoma, carcinomatosis, carcinoma bone metastasis, pancreatic carcinoma, transitional cell carcinoma, or vaccine-associated sarcoma with lung metastasis). Eleven (27.5%) ill cats had endocrine disease (8 with hyperthyroidism and 3 with diabetes mellitus), and 6 (15%) had infectious disease (1 each with gastrointestinal histoplasmosis, pulmonary histoplasmosis, pulmonary aspergillosis granuloma, mycoplasma pneumonia, mesenteric abcess, or feline infectious peritonitis). Noninfectious inflammatory conditions were diagnosed in 4 (10%) cats (2 with lymphocytic plasmacytic inflammatory bowel disease, 1 with eosinophilic gastritis with focal peritonitis, and 1 with severe erythema multiforme); the diagnosis in the remaining 5 (12.5%) cats included gastrointestinal parasitism with Strongyloides spp, FeLV-induced neurologic disease, portosystemic shunt, hypertrophic cardiomyopathy, and severe traumatic skin slough. Eight (20%) cats had mild to moderate nonregenerative anemia as a concurrent problem. Three (7.5%) cats had concurrent renal failure. Thirteen (32.5%) cats had diseases involving the gastrointestinal tract, 7 (17.5%) cats had diseases involving the respiratory tract, and 2 (5%) cats had cutaneous conditions. Thus, 22 of 40 (55%) cats had problems in organ systems commonly associated with eosinophils and mast cells.

Of the 40 clinically normal cats, 11 (27.5%) had eosinophilia (range, 770 to 1,800 cells/μL; reference interval, 0 to 750 cells/μL). Four (10%) cats had mild basophilia (range, 130 to 260 cells/μL; reference interval, 0 to 100 cells/μL); only 1 of these 4 cats had concurrent eosinophilia. Of the 40 ill cats, 12 (30%) had eosinophilia (range, 850 to 2,700 cells/μL), whereas 8 (20%) cats had basophilia (range, 850 to 2,700 cells/ ML). Six of the ill cats with basophilia had concurrent eosinophilia. All of the clinically normal cats had neutrophil or band neutrophil counts that were within reference intervals (2,500 to 12,500 neutrophils/μL and 0 to 300 bands/μL, respectively). Eight (20%) of the ill cats had neutrophilia (range, 14,000 to 34,000 cells/ ML), and 2 of these cats had a high band neutrophil count (1,700 and 14,000 cells/μL). Three of the cats with neutrophilia had concurrent eosinophilia or basophilia. Thus, 14 (35%) clinically healthy cats and 19 (47.5%) ill cats had at least 1 hematologic abnormality indicative of inflammation. However, regardless of whether the cats were ill or clinically normal, no mast cells were detected in any of the buffy coat smears.

Discussion

Mast cell tumors are a common neoplasm in cats, occurring less frequently than only lymphoma and mammary adenocarcinoma in 1 study.¹⁴ The biologically malignant splenic form of MCT is the most common splenic disorder in cats, identified in 15% of pathologic specimens in a series of 455 cases.¹⁶ Additionally, cutaneous MCTs are the second most common skin tumor in cats.¹⁷ Although most dermal MCTs are benign, some may be an external sign of systemic disease or may metastasize systemically, reflecting malignant biologic behavior.^2,7,14,18-20 Mastocytemia is most frequently associated with the splenic form of the disease and occasionally with the cutaneous form; buffy coat smears are used to evaluate for and monitor progression of mastocytemia.^1,2,4-8 In cats with splenic MCTs, numbers of mast cells in the buffy coat appear initially to decrease after splenectomy, whereas systemic relapse may be predicted on the basis of increasing numbers of mast cells in the buffy coat. Such increases have been reported^1,5,13,a to signify progression of the disease and, usually, are indicative of a subsequently rapidly fatal course of disease. Response to chemotherapy in cats with systemic mastocytosis has been limited, but those reports are anecdotal.¹⁴ More recently, lomustine (1-[2chloroethyl]3-cyclohexyl-1-nitrosourea or CCNU) has been reported^4,21 to have antitumor activity in 2 cats with MCT. One of those cats had multiple cutaneous masses and achieved a partial remission after 1 dose. The other cat had an oral mass and achieved complete remission; remission was maintained at 14 months after diagnosis. Another cat with multiple cutaneous masses and mastocytemia achieved complete remission of the skin masses and a 10-fold decrease in the number of circulating mast cells during the 5-week evaluation period reported²⁰ after receiving treatment with the tyrosine kinase inhibitor imatinib mesylate.^b Early detection of MCT recurrence by identification of mastocytemia (via buffy coat examination) with subsequent initiation of chemotherapy may prove to be beneficial in cats with systemic MCTs. Before such information can be accumulated, the value of examination of the buffy coat smear as a tumor staging and monitoring test for cats with MCTs needs to be validated. The first step of such validation is to determine baseline mast cell numbers in the buffy coat smears in healthy cats and cats ill with diseases unrelated to MCTs.

In a study⁹ in healthy dogs, evaluation of buffy coat smears did not detect mast cells. However, in ill dogs with a wide range of diseases that were not associated with MCTs, mastocytemia was detected via buffy coat smear examination.^10-12 The lack of specificity of detection of mastocytemia to reflect MCTs in dogs makes its use as a screening or tumor staging tool of little value in that species. Because we were not aware of any similar study in cats, this investigation was undertaken. The lack of detectable mast cells in the buffy coat smears from 40 clinically normal, client-owned cats and 40 ill cats without MCT-related disease strongly supports the suggestions that circulating mast cells are rare to absent in these cat populations and that evaluation of buffy coat smears may be a useful tool in cats with MCTs. Further studies are needed to assess the prognostic value of buffy coat examination results as a tumor staging and follow-up test in cats with MCTs.