Overview

• Unregulated neoplastic proliferation of nonlymphoid cells originating in the bone marrow (granulocytic, monocytic, erythrocytic, and megakaryocytic cells) resulting in accumulations of differentiated cells.
• Includes polycythemia vera, chronic myelogenous leukemia, essential thrombocythemia, and possibly primary myelofibrosis.
• Believed to represent a spectrum of disorders in which the stem cell involved is a hematopoietic precursor capable of differentiating into all blood cell types except lymphocytes.

Signalment

• Cat and dog-more common in cat
• May be more common in large-breed dogs than small-breed dogs

Signs

• Pale mucous membranes
• Petechiation
• Lethargy
• Inappetance
• Weight loss
• Hepatosplenomegaly
• Peripheral lymphadenomegaly-occasionally
• Neurologic signs-disorientation, ataxia, seizure

Causes & Risk Factors

• Cats-most commonly associated with FeLV infection; when recovering from panleukopenia or hemobartonellosis, may be a relatively higher risk of developing a mutant cell line induced by FeLV.
• Dogs-has been experimentally induced with chronic low-dose radiation exposure.

Differential Diagnosis

• Acute lymphocytic leukemia-usually differentiated by special staining techniques (immunohistochemistry or immunocytochemistry for lymphoid markers) or PARR.
• Leukemoid response secondary to inflammation.
• Other causes of eosinophilia-parasitism; allergic disease; eosinophilic gastroenteritis; mast cell neoplasia; differentiate from eosinophilic leukemia.
• Relative and secondary absolute polycythemia.
• Reactive thrombocytosis-secondary to inflammation, hemolytic or iron deficiency anemia, splenectomy, rebound from immune-mediated thrombocytopenia, and drug-induced (e.g., vincristine).

CBC/Biochemistry/Urinalysis

• Severe, nonregenerative anemia
• Severe elevated hematocrit in cases of primary polycythemia vera
• Circulating nucleated red blood cells
• Megaloblastic erythrocytes
• Leukocytosis or leukopenia
• Thrombocytopenia with abnormal platelet morphology
• Thrombocytosis in cases of essential thrombocythemia
• Circulation of immature myeloid cells

Other Laboratory Tests

• Examination of bone marrow aspirate or core biopsy-reveals hypercellular bone marrow with abnormal morphology in all cell lines; neoplastic proliferation or absence of one cell line.
• Immunohistochemical or other special stains-may be necessary to determine cell lineage.

Imaging

Abdominal radiography and ultrasonography-hepatomegaly and splenomegaly common.

Diagnostic Procedures

Examination of bone marrow aspirates or core biopsy

Drug(s)

• Little information available in the literature regarding treatment.
• Cytosine arabinoside-may be used; 100 mg/m² SC divided q12h 4 days per week, or as a constant rate infusion over 6–8 hours at a dose of 400 mg/m².
• Hydroxyurea 30–45 mg/kg q24h for 7–10 days; then 30–45 mg/kg q48h or 15 mg/kg q24h; essentially, titrate dosage to patient response.
• Antibiotics-may be indicated to combat secondary infection.

Contraindications/Possible Interactions

Chemotherapy can be toxic; seek advice before treatment if unfamiliar with cytotoxic drugs.

Pregnancy/Fertility/Breeding

• Chemotherapy drugs are contraindicated in pregnant animals.
• It is not recommended to breed animals with neoplasia.

Abbreviations

• FeLV = feline leukemia virus
• PARR = polymerase chain reaction for antigen receptor rearrangement