Prevalence of leukemic blood and bone marrow in dogs with multicentric lymphoma

Multicentric lymphoma was diagnosed in 53 dogs. A study was performed to evaluate the prevalence of leukemic involvement in blood samples, bone marrow aspirates, and bone marrow core biopsy specimens at the time of initial diagnosis. Data indicated that 57% (30/53) of the dogs were leukemic when all materials were considered relative to the presence of cellular atypia or immaturity and abnormal tissue distribution. In the 30 leukemic dogs, detection was made in the specimens with the following frequency: 15 in blood (50%), 18 in bone marrow aspirates (60%), and 29 in bone marrow core biopsy specimens (97%). Five cases (17%) were only detected by core biopsy examination, even when dogs with bone marrow lymphocytosis of greater than 15% of nucleated cells were considered leukemic. Nondiffuse histologic colonization patterns accounted for the lack of correlation between the type of bone marrow specimens. Clinical staging for treatment response and prognosis was best determined by evaluation of concurrently obtained blood samples, bone marrow aspirates, and bone marrow core biopsy specimens.     

Suspected myelodysplastic/myeloproliferative neoplasm in a feline leukemia virus‐negative cat

A 10‐year‐old castrated Domestic Short‐Haired cat was presented to a primary care veterinarian for a wellness examination and laboratory examination for monitoring of diabetes mellitus. The CBC revealed marked thrombocytosis, leukopenia and macrocytic, normochromic anemia. The cat tested negative for FeLV and feline immunodeficiency virus, but was positive for Mycoplasma haemominutum by PCR. Hematologic abnormalities were not responsive to therapy, so a repeat CBC and a bone marrow aspiration for cytology were performed. Additional blood smear findings included anisocytosis with megaloblastic erythroid precursors, large platelets, eosinophilic myelocytes and metamyelocytes, and rare unidentified blasts. The bone marrow smear was highly cellular, and the cytologic pattern was consistent with myelodysplastic syndrome with an erythroid predominance. At that time, 15% blasts were present. The cat was treated with a vitamin K₂ analog, doxycycline, and prednisolone, but without a clinical response. Within 3 months, euthanasia was elected due to declining quality of life, and a necropsy was performed. Postmortem bone marrow smears were highly cellular and dominated by monomorphic blasts of unknown line of origin (52%), persistent marked erythroid and megakaryocytic dysplasia, and ineffective erythropoiesis and granulopoiesis. Immunohistochemical, immunocytochemical, and cytochemical stains resulted in a diagnosis of acute myeloid leukemia of unclassified type. Additional histologic findings included mixed hepatitis with trematode infestation and lymphoplasmacytic interstitial nephritis with fibrosis. The marked thrombocytosis with myelodysplastic syndrome and the FeLV‐negative status of this cat were unusual. The difficulty in classifying the myelodysplasia and subsequent leukemia highlights a need for further reporting and characterization of these types of disease.     

BCR-ABL translocation in a dog with chronic monocytic leukemia

A 9-year-old female spayed mixed breed dog was evaluated at the University of Florida Small Animal Hospital for marked leukocytosis with no associated clinical signs. CBC abnormalities included marked leukocytosis (106,000/μL), marked monocytosis (78,000/μL), and the presence of 13% blast cells (13,832/μL), supporting a diagnosis of leukemia. Cytopenias and dysplastic changes in other cell lines were not present. Microscopic examination of bone marrow showed hypercellular uniparticles with a marginal increase in frequency of unclassified blast cells (2%), but was otherwise unremarkable. Flow cytometric immunophenotyping of blood cells determined that leukemic cells were CD45(+) , CD14(+) , and CD34(-) , and based on side scatter and CD45 reactivity the marrow contained 19% monoblasts. By immunocytochemical staining, the leukemic cells in the bone marrow were CD11b(+) , CD11c(+) , CD11d(+) , MHC-II(+) , MPO(+) , and CD34(-) . Fluorescence in situ hybridization (FISH) analysis of peripheral blood leukocytes documented a chromosomal translocation producing a BCR-ABL gene hybrid, similar to the "Philadelphia" chromosome abnormality recognized in human chronic myelogenous leukemia, as well as a phosphatase and tensin homolog (PTEN) gene deletion. Hydroxyurea therapy was attempted, but was ineffective; the dog died 7 months after initial presentation. Clinical and laboratory findings and the protracted course supported a diagnosis of chronic monocytic leukemia (CMoL) and, to our knowledge, this is the first case of CMoL with a BCR-ABL chromosomal abnormalitiy described in dogs. This may have clinical implications for treatment of dogs with chronic leukemias associated with particular genetic mutations. However, more case studies are needed to further characterize this disease.     

Chronic Idiopathic Neutropenia in a Cat

Persistent neutropenia (0-0.6 X 10(9) neutrophils/l) was documented during a 10-month period in a 4-year-old spayed female domestic shorthair cat that was presented for anorexia and depression. Salient abnormalities detected on physical examination were fever (40.3 degrees C), dehydration, and gingivitis. The cat was neutropenic (0.5 X 10(9) neutrophils/l) and enzyme-linked immunosorbent assay (ELISA) test for feline leukemia virus was negative. A bone marrow aspirate showed decreased numbers of mature granulocytic cells. In vitro bone marrow cultures for colony-forming units-granulocyte/macrophage (CFU-GM) were performed comparing bone marrow from the patient with that of a normal cat. The patient had fewer CFU-GM than the control. The number of CFU-GM increased when bone marrow mononuclear cells were cultured in the presence of 10(-5) and 10(-6) mol/l of hydrocortisone, but the cat did not respond to oral prednisolone therapy. The pathogenesis of the neutropenia in this cat remains obscure, but resembles the chronic idiopathic neutropenia syndrome of man.     

Lesions in chickens with spontaneous or experimental infectious hepato-myelopoietic disease (inclusion body hepatitis) in Germany.

A group of 83 two-to-eighteen-week-old chickens with acute infectious hepato-myelopoietic disease (a German form of inclusion-body-hepatitis) were observed to have the following histologic lesions: panmyelophthisis, small foci of liver necrosis, often with intranuclear inclusion bodies in hepatocytes (15 to 20% of chickens), involution-like atrophy of the bursa of Fabricius and thymus, loss of lymphatic tissue in spleen and cecal tonsils, and nonpurulent myocarditis. In 18 survivors 6 to 8 weeks after clinical signs of disease, nonpurulent myocarditis but normal lymphatic organs and bone marrow were present. A group of 75 chickens were infected after hatching with the field isolant "1942." Between the 3rd and 9th weeks postinoculation the same histologic changes-though mostly milder-were demonstrated. This syndrome differs somewhat from the syndrome described as inclusion body hepatitis in America and Europe.     

Radiographic findings in cats with mycobacterial infections

This study describes radiographic changes associated with mycobacterial infection in 33 domestic cats confirmed by culture or interferon-gamma testing. Infection was seen most frequently in adult (average age 5.7 years; range 1.5-12 years), non-pedigree (87%; 27/31), neutered male cats (69%; 22/32). The most common infections were Mycobacterium microti (60%; 18/30) and Mycobacterium bovis (37%; 11/30); Mycobacterium avium and Mycobacterium malmoense were infrequently cultured (3% of each; 1/30). Radiographs were available for the thorax (24 cats), abdomen (eight), appendicular skeleton (11) and head (three). Radiographic changes affected the thorax most commonly, consisting of bronchial (46%; 11/24), alveolar (38%; 9/24), nodular unstructured interstitial (38%; 9/24) or unstructured interstitial (25%; 6/24) lung patterns, which were often mixed. Perihilar or sternal lymphadenopathy were common (42%; 10/24), particularly perihilar lymphadenopathy (25%; 6/24). Skeletal changes were found in the distal antebrachium (three), pes (two), maxilla, scapula, spine, manus, femur, and tarsus (one each). Changes were typically osteolytic (73%; 8/11), often permeative osteolytic (64%; 7/11). Osteoproliferative changes were seen in three cats and soft tissue swelling in five cats, which were adjacent to the bony abnormality in four cats. Other changes included submandibular soft tissue swelling, marked aortic, aortic root and brachiocephalic trunk calcification, and soft tissue swelling with calcification in the distal antebranchium which was not involving bone. Abdominal changes were uncommon (seen in 2/8 cats) and consisted of hepatomegaly and hepatosplenomegaly. In summary, radiographic changes were varied, no lesion was pathognomic for mycobacterial infection, and pathology was seen most commonly in the thorax.     

Immunophenotypic comparison of blood and lymph node from dogs with lymphoma

Peripheral blood and lymph node tissue from 12 dogs with lymphoma was immunophenotyped. Additionally, the bone marrow was immunophenotyped in 6 dogs. The lymphomas were characterized as B-cell in 11 dogs and T-cell in 1 dog. Immunophenotypic patterns in the peripheral blood and bone marrow were variable. The trend in dogs with B-cell lymphoma was normal to increased percentage of IgG-positive cells, decreased percentage of pan-T-positive cells, decreased percentage of CD4-positive cells, and decreased CD4/CD8 ratio. Simultaneous immunophenotyping of lymph node, blood and bone marrow cannot be recommended routinely without further studies to document its value as an independent prognostic indicator. However, it is potentially useful for tumor staging and monitoring remission, especially in lymphoma patients with a leukemic phase.     

Multiple myeloma in 16 cats: a retrospective study

BACKGROUND: There is limited published information regarding feline multiple myeloma. Diagnostic criteria are derived from canine studies and to our knowledge, have not been critically reviewed for cats. OBJECTIVE: To evaluate the clinical and laboratory findings in cats with multiple myeloma and appraise diagnostic criteria. METHODS: Retrospective evaluation of medical records was performed. Inclusion required an antemortem diagnosis of multiple myeloma using 2 of 4 criteria: 1) >or=20% plasma cells in the bone marrow, or >or=10% if atypical plasma cells; 2) paraproteinemia; 3) radiographically-evident osteolysis; 4) light chain proteinuria. Alternatively, a postmortem diagnosis was based on the findings of multiple plasma cell neoplasms, with marrow involvement. RESULTS: Sixteen cats were diagnosed with multiple myeloma between 1996 and 2004, with a median age of 14.0 years; 9 of 16 (56%) were castrated males, and 7 of 16 (44%) were spayed females. Laboratory abnormalities included hyperglobulinemia (14/16, 87.5%), with 11/14 (78.5%) monoclonal and 3/14 (21.4%) biclonal gammopathies; hypoalbuminemia (4/16, 25%); light chain proteinuria, (4/9, 44.4%); hypocholesterolemia (11/16, 68.7%); hypercalcemia, (3/15, 20%); nonregenerative anemia, (11/16, 68.7%); regenerative anemia, (1/16, 6.2%); neutropenia (5/15, 33.3%); thrombocytopenia (8/16, 50%); and marrow plasmacytosis (14/15, 93.3%). Plasma cells were markedly immature, atypical, or both in 10 of 12 (83.3%) cats. Focal or multifocal osteolysis was noted in 6 of 12 (50%) cats for which radiographs were available for review; generalized osteopenia was found in 1 (8.3%) cat. Noncutaneous, extramedullary tumors were found in all cats assessed, 7/7 (100%), including spleen (6), liver (3), and lymph nodes (4). The disease in 1 of 2 cats with cutaneous tumors progressed to plasmacytic leukemia. CONCLUSIONS: Common findings in feline multiple myeloma include atypical plasma cell morphology, hypocholesterolemia, anemia, bone lesions, and multi-organ involvement. Based on the results of this study, we advocate modifying diagnostic criteria in cats to include consideration of plasma cell morphology and visceral organ infiltration.     

A retrospective study of the incidence and the classification of bone marrow disorders in the dog at a veterinary teaching hospital (1996-2004)

BACKGROUND: An 8-year retrospective study was conducted to evaluate the prevalence and the classification of canine bone marrow disorders in a clinical pathology service at a university referral hospital. ANIMALS: Dogs evaluated for bone marrow disorders at a veterinary teaching hospital. HYPOTHESIS: A better understanding of the spectrum and the prevalence of canine bone marrow disorders can be achieved with a multiyear retrospective study. METHODS: Bone marrow aspirate smears, core biopsy specimens, and case records from 717 dogs were reviewed. RESULTS: Bone marrow specimens were first categorized based on the presence or the absence of a primary bone marrow disorder. Nondysplastic and nonmalignant pathologic changes were placed into 14 subcategories. Frequently observed pathologic disorders included nonregenerative immune-mediated anemia, pure red cell aplasia, bone marrow necrosis, myelofibrosis, and hemophagocytic syndrome. Dysmyelopoiesis (n = 61) was subcategorized into myelodysplastic syndromes (n = 27), and congenital (n = 1) and secondary (n = 33) dysmyelopoiesis. One hundred twenty-six cases of neoplasia were divided into acute leukemia (n = 46), chronic leukemia (n = 7), stage 5 malignant lymphoma (n = 28), multiple myeloma (n = 25), malignant histiocytosis (n = 11), metastatic mast-cell tumor (n = 3), sarcoma (n = 5), and carcinoma (n = 1). CONCLUSIONS AND CLINICAL IMPORTANCE: This study provides a general indication of the spectrum and the prevalence of canine bone marrow disorders at a referral center in North America.     

High-grade canine T-cell lymphoma/leukemia with plasmacytoid morphology: a clinical pathological study of nine cases.

The aim of this study is to determine the clinical, morphological, and immunophenotypical presentation of 9 cases of a particular type of canine T-cell lymphoma/leukemia. The morphological presentation was a diffuse infiltration of small, medium-sized, or large blast cells with eccentric nuclei, hyperbasophilic cytoplasm, and a juxtanuclear, pale cytoplasmic area, giving a plasmacytoid appearance and suggesting a B-cell morphology. Surprisingly, all 9 cases were of T-cell phenotype (CD3+). Among the 7 immunophenotyped cases, 4 were CD4-/CD8+, 2 CD8+/CD4+, and 1 CD4+/CD8-. The median Ki-67 index was 65.7%, which placed this lymphoma in the high-grade group. This type of lymphoma/leukemia was found in dogs between 1 and 11 years of age, with a median age of 5.8. The male-female ratio was 0.8 for a reference population of 1.04. The most significant clinical findings were lymphadenopathy either generalized or localized in all cases, a mediastinal mass in 4 cases, bone marrow involvement in 7 cases, hypercalcemia in 4 cases, along with an aggressive clinical course and a poor response to chemotherapy in all cases, with a median disease-free survival time of 3 months.     

Adrenal lesions in the baboon (Papio spp)

A histological survey was conducted on 604 pairs of adrenal glands from yellow (Papio cynocephalus) and olive (Papio anubis) baboons used in drug safety evaluation studies. Spontaneous lesions were found in 372 glands--34 of which had more than one change. Cortical lesions consisted of accessory nodules (190), nodular hyperplasia (7), hepatoadrenal adhesion (18) and partial fusion (1), focal mineralization (20), ectopic bone marrow (6), and focal fatty change (60). Medullary lesions were confined to focal lymphocytic (66) and plasma cell (1) infiltrates.     

Monoblastic leukemia (M5a) with chronic basophilic leukemia in a cat

A cat was presented with depression and anorexia. The complete blood cell count (CBC) revealed non-regenerative anemia (PCV, 8.5%), marked thrombocytopenia (2,400/µl), and leukocytosis (32,090/µl). In the peripheral blood, proliferation of blast cells (85%; 27,276/µl) and basophils (7.7%; 2,460/µl) was observed. Bone marrow aspirate showed hyperplasia with 8.8% blasts and 90.2% basophils of all nucleated cells. The blast cells were negative for myeloperoxidase staining and positive for alpha-naphthol butyrate esterase staining, indicating the agranular blasts are monoblasts. Thus, acute monoblastic leukemia (M5a) with chronic basophilic leukemia was diagnosed. Basophils accounted for more than 40% of the bone marrow, and we diagnosed secondary basophilic leukemia. Secondary basophilic leukemia should be included in the differential list when abnormal basophil increases are observed in feline bone marrow.     

Multiple red cell transfusions in 27 cats (2003-2006): indications, complications and outcomes

The objectives of this study were (1) to evaluate the indications, complications and outcomes of multiple red cell transfusions (MrcTs) in cats; of these cats (2) to describe those that received massive transfusion and (3) compare them with those who received MrcTs over a longer time course. Twenty-seven cats were identified which received a total of 110 transfusions, with a median of three transfusions (range 3-15) per cat. The transfusions consisted of 47 units of whole blood and 63 units of packed red blood cells. The median age of cats was 6 years (range 6 months to 15 years). Cats were hospitalized for a median of 6 days, with a range of 1-38 days. No acute transfusion reactions were documented, although due to the critical nature of the cats, they may not have been appreciated. Sixteen cats survived to discharge and 11 died or were euthanased. Indications (and % survival) for transfusions included bone marrow failure (n=8; 50%); surgical loss (n=4; 100%), sepsis (n=3; 0%), neoplasia (n=3; 33%), acute renal failure (n=3; 66%), trauma (n=2; 100%), gastrointestinal bleeding (n=1; 100%), and cats with multiple disease processes (n=3; 33%). MrcTs are well-tolerated in cats and may be associated with a favorable outcome.     

Cytologic evaluation of primary and secondary myelodysplastic syndromes in the dog

Myelodysplastic syndromes are a heterogeneous group of acquired primary and secondary alterations of hematopoietic stem cells that result in cytopenias in blood and cytologic features of dysplasia in blood and/or bone marrow. To better understand the cytologic features that would permit differentiation of primary and secondary forms of myelodysplasia, we reviewed 267 consecutive bone marrow reports from dogs. These reports indicated that 34 dogs (12.7%) had dysgranulopoiesis, dyserythropoiesis, and/or dysthrombopoiesis in >10% of granulopoietic cells, erythroid cells, and/or megakaryocytes, respectively. Thirteen dogs had primary myelodysplastic syndromes, and 21 had secondary myelodysplastic syndromes. Of the 13 dogs with primary myelodysplasia, 4 were subclassified as myelodysplastic syndrome with refractory anemia (MDS-RA), and 9 were subclassified as myelodysplastic syndrome with excess blasts (MDS-EB). Secondary conditions associated with dysplasia in the bone marrow included malignant lymphoma (n = 5), myelofibrosis (n = 3), immune-mediated thrombocytopenia (n = 4), immune-mediated hemolytic anemia (n = 5), multiple myeloma with melphalan administration (n = 1), pyometra with estrogen administration (n = 1), polycythemia vera (n = 1), and thrombopathia (n = 1). MDS-RA was characterized by <5% myeloblasts in bone marrow, normal granulocyte maturation ratio, increased erythroid maturation ratio, and dysplastic changes in >15% of erythroid cells. MSD-EB was characterized by >/=5% myeloblasts in bone marrow, high granulocyte maturation and erythroid maturation ratios, >/=32% dysplastic granulocytes, and the presence of small atypical immature myeloid cells. Secondary myelodysplastic syndromes were characterized by <5% myeloblasts in bone marrow, variable granulocyte maturation and erythroid maturation ratios, and variable dysplastic features. These results indicate that morphology alone cannot be used to distinguish primary and secondary myelodysplastic syndromes in dogs.     

Primary bone marrow T‐cell lymphoma in a Golden Retriever

A 6.2‐year‐old 28‐kg (61.7 lb) intact female Golden Retriever was referred due to persistent and multiple cytopenias noted on a routine CBC prior to a mature ovariohysterectomy procedure. The patient's physical examination was unremarkable, and staging of the thorax and abdomen identified no abnormalities. At the referral hospital, moderate hypercalcemia, borderline anemia, and neutropenia were noted. Assessment of bone marrow samples by cytology, histology, immunohistochemistry, and flow cytometry indicated a T‐cell neoplasm. The patient was treated with a multi‐agent chemotherapy protocol for 6 months, which induced remission. Nine months after diagnosis, she relapsed with recurrence of hypercalcemia, cytopenias, and clinical illness. Single‐agent anthracycline (mitoxantrone) in combination with prednisone therapy was initiated for 3 months. Two months after completion, the patient relapsed again, and palliative therapy with prednisone was elected. The patient was euthanized 16 months after diagnosis due to progressive disease. Post‐mortem histopathologic evaluation showed extensive replacement of bone marrow by neoplastic cells, and infiltrates in multiple organs. The neoplasm was diagnosed as lymphoma rather than leukemia due to the lack of abnormal circulating cells throughout the course of disease. The neoplasm was detected only in marrow at the time of initial diagnosis, and the marrow was the most extensively effaced organ at the time of death. Therefore, leukemia or stage V lymphoma was considered unlikely. In patients with a cytopenia and lack of neoplastic leukocytosis or solid tissue masses, primary bone marrow lymphoma should be considered among the differential diagnoses.     

Acute myelomonocytic leukemia negative for alpha-naphthyl acetate esterase stain in a Holstein cow

A 4-year, 7-month-old Holstein cow presented with anorexia. Physical examination revealed masses in the interscapular region and vagina. Blast cells were detected in the masses and peripheral blood by fine needle aspiration cytology and hematological examination. By bone marrow aspiration, blast cells constituted up to 24.2% of all nucleated cells, and 22% and 2% of non-erythroid cells stained positive for myeloperoxidase and alpha-naphthyl acetate esterase (ANAE), respectively. Pathological examination revealed the mass lesions consisted of a proliferation of tumor cells, which were positive for monocytic markers (HLA-DR and Iba-1). The cow was diagnosed with acute myelomonocytic leukemia (AMML). Even when tumor cells are ANAE-negative, AMML cannot be completely ruled out and should be considered when diagnosing cattle with leukemia/lymphoma.     

Canine hemophagocytic histiocytic sarcoma: a proliferative disorder of CD11d+ macrophages

Histiocytic disorders of dogs include histiocytoma, localized histiocytic sarcoma (HS), disseminated HS (malignant histocytosis), and the reactive histiocytoses: cutaneous and systemic. A common element to these diseases is proliferation of dendritic cells (DC) of either Langerhans cell (epithelial DC) or interstitial DC lineage. In this report, 17 dogs with hemophagocytic HS are described. Breeds affected included Bernese Mountain Dog (6), Golden Retriever (4), Rottweiler (3), Labrador Retriever (2), a mixed-breed dog, and a Schnauzer, which were from 2.5 to 13 years old. The dogs presented with Coombs negative responsive anemia in 16/17 dogs (94%), thrombocytopenia in 15/17 dogs (88%), hypoalbuminemia in 16/17 dogs (94%), and hypocholesterolemia in 11/16 dogs (69%). All dogs died or were euthanized. The clinical course ranged from 2 to 32 weeks (mean 7.1 weeks). Diffuse splenomegaly with ill-defined masses was consistently present. Microscopic lesions were prevalent in spleen, liver, lung, and bone marrow. Metastasis occurred by insidious intravascular invasion with minimal mass formation. Histiocytes were markedly erythrophagocytic and accompanied by foci of extramedullary hemopoiesis. Cytologically, the histiocytes varied from well differentiated to atypical, with atypia more prevalent in spleen than bone marrow. These tumors arose from splenic red pulp and bone marrow macrophages, which expressed major histocompatibility complex class II and the beta2 integrin, CD11d. They had low and/or inconsistent expression of CD1 and CD11c, which are dominantly expressed by canine nonhemophagocytic HS of DC origin. Canine histiocytic proliferative diseases now encompass proliferation of all members of the myeloid histiocytic lineage: Langerhans cells, interstitial DC, and macrophages.     

Feline Hemoperitoneum 16 Cases (1986-1993)

Sixteen cases of feline, non-traumatic hemoperitoneum were evaluated retrospectively. The median age was 5.75 years (range 1.5 - 16 years). There were eight male and eight female cats. Common presenting complaints (n=13) were anorexia (37%), lethargy (31%), and recumbency (31%). Physical examination findings (n=11) included depressed mentation (100%), hypothermia (89%), pale mucous membranes (82%), and poor quality pulses (80%). The median initial peripheral packed cell volume (n=11) was 24% (range 17-55%). In four out of six cases where abdominocentesis was performed, the packed cell volume of the abdominal fluid ranged from 18% to 24%, and matched the peripheral packed cell volume (range 15 - 26%). Some common abnormalities in the serum chemistry screens 9n=6) were elevated alanine aminotransferase in 83% (5/6) of the cats (range 55-5828 U/l) and elevated alkaline phosphatase in 50% (3/6) of the cats (range 18-402 U/l). Ten cats (63%) were euthanized, three (19%) were presented dead on arrival, two (12%) are still alive, and one (6%) were euthanized, three (19%) were presented dead on arrival, two 912%) are still alive, and one (6%) died. The causes of hemoperitoneum were hepatic neoplasia (31%), hepatic necrosis (19%), hepatic amyloidosis (13%), non hepatic neoplasia (13%), hepatopathy (6%), hepatic rupture (6%), necrotic/hemorrhagic cystitis (6%), and ruptured bladder (6%).     

Thrombocytopenia in Cats: A Retrospective Study of 41 Cases

The prevalence of feline thrombocytopenia (<200,000 platelets/L) at North Carolina State University, College of Veterinary Medicine Teaching Hospital, from January 1985 to March 1990, was 1.2% (41/3300). Cats were divided into six categories based on clinical diagnoses: 29% (12/41) had infectious disease, 20% (8/41) had neoplasia, 7% (3/41) had cardiac disease, 2% (1/41) had primary immune-mediated disease, 22% (9/41) had multiple diseases, and 20% (8/41) had disorders of unknown etiology. The mean platelet count for all thrombocytopenic cats was 52,000/μL ± 46,000/μL (1 SD) with a range of 1000–190,000/μL. No significant differences were found between groups with respect to platelet count, packed cell volume, or white blood cell count, though anemia and leukopenia were common among the cats as a whole. Bleeding disorders (hemorrhage or thrombosis) were observed in 29% (12/41) of thrombocytopenic cats and were more likely to be associated with neoplasia, cardiac disease, and platelet counts less than or equal to 30,000/μL. Disseminated intravascular coagulopathy was diagnosed in 12% (5/41) of the cats. Infections and/or neoplasia affecting the bone marrow were the most common diseases associated with thrombocytopenia. Feline leukemia virus and myeloproliferative neoplasia accounted for approximately 44% (18/41) of the specific diagnoses in thrombocytopenic cats. (Journal of Veterinary Internal Medicine 1993; 7:261–265. Copyright © 1993 by the American College of Veterinary Internal Medicine.)