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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • Rare tumors involving neoplastic myeloid hematopoietic cells (nonlymphoid) within bone marrow or extramedullary tissues
  • Results in pathologic ablation of normal tissue architecture and ultimately loss of bone marrow elements and myelophthisis
  • Malignant histiocytosis and myeloid leukemia are 2 forms of this disease
  • Myeloid leukemia further classified into monocytic and myelomonocytic leukemia, granulocytic leukemia, megakaryocytic leukemia, and primary erythrocytosis; eosinophilic myeloproliferative disorder is the other single-cell leukemia reported in horses
  • Malignant histiocytosis involves proliferation of mononuclear phagocytes intermediate in differentiation between monoblasts and tissue histiocytes
  • Familial megakaryocytic hypoplasia reported in Standardbreds, but other forms of leukemia of megakaryocytes not described
  • Primary erythrocytosis (polycythemia vera) is very rare

Signalment!!navigator!!

  • Age range for monocytic leukemia is 2–11 years
  • Other myeloproliferative disorders 10 months to 16 years

Signs!!navigator!!

  • Most clinical signs are nonspecific and relate to destruction of tissue by invasive neoplastic cells; fever, weight loss, signs of depression, and exercise intolerance
  • Dependent edema, lymphadenopathy, pallor of mucous membranes, petechial hemorrhages, and oral ulceration
  • Occasionally epistaxis, dyspnea, and colic
  • Icterus in cases with secondary immune-mediated hemolytic anemia

Causes and Risk Factors!!navigator!!

Remain undefined.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Lymphoma and lymphocytic leukemia
  • Anaplasmosis
  • Acute enteritis/colitis, Potomac horse fever, salmonellosis, coronavirus
  • Ventral and limb edema observed in vasculitis, impaired lymph drainage, hypoproteinemia, or purpura haemorrhagica
  • Immune suppression and secondary infections may occur in horses with myeloproliferative disorders and may complicate diagnosis

CBC/Biochemistry/Urinalysis!!navigator!!

  • Anemia, thrombocytopenia, or pancytopenia
  • Leukopenia, leukocytosis, or normal white cell count with or without abnormal circulating leukocytes depending on whether horse is leukemic, subleukemic, or aleukemic
  • Immune-mediated hemolytic anemia and immune-mediated thrombocytopenia
  • Gammopathy or mild hypoproteinemia

Other Laboratory Tests!!navigator!!

Blood gas analysis and erythropoietin concentration for diagnosis of primary erythrocytosis; decreased concentrations of erythropoietin with a normal PaO2 and elevated hematocrit are observed with or without thrombocytosis or leukocytosis.

Imaging!!navigator!!

  • Abdominal US; lymphadenopathy, hepatomegaly, or splenomegaly
  • Thoracic US and radiographs; respiratory infection or enlarged lymph nodes

Other Diagnostic Procedures!!navigator!!

  • Bone marrow aspirates or biopsies; elevated myeloid to erythroid ratio (normal 0.5–3.75) and neoplastic cells as a monomorphic population
  • Increased serum lysozyme (muramidase) concentrations; normally <5 μg/mL
  • Cytochemistry; nonspecific myeloid markers include Sudan black B, PAS stain, alkaline phosphatase, peroxidase, and chloroacetate esterase; esterase stains (e.g. α-naphthyl-butyrate or -acetate) differentiate granulocytic cells from monocytic and megakaryocytic cells; megakaryocyte stain used to differentiate monocytic and megakaryocytic cells

Pathologic Findings!!navigator!!

  • Gross findings include generalized lymphadenopathy, hepatomegaly, splenomegaly, and discoloration of myeloid tissue
  • Extensive hemorrhage in severe cases
  • Histopathology; immature myeloblastic cells in bone marrow, lymph nodes, spleen, liver, kidneys, lungs, and heart
  • Leukostasis; white cell accumulation causing blood vessel occlusion

Treatment ⬆ ⬇

TREATMENT

  • Limited response to therapy and timely euthanasia usually warranted
  • Inpatient care is required if treatment attempted
  • Client should be made aware of poor prognosis and limited treatments

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Most antineoplastic agents not effective and treatment options limited
  • Available drugs expensive with little guarantee of success
  • Cytarabine (10 mg/m2 every 12 h for 3 weeks) shown most promise
  • Antibiotics to treat any secondary infections during chemotherapy

Contraindications/Possible Interactions!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

CBC and biochemical profile performed periodically to assess side effects of chemotherapy such as bone marrow and organ dysfunction.

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

Secondary infections due to immunosuppression.

Expected Course and Prognosis!!navigator!!

  • Poor prognosis
  • Rapid progression with most horses dying within weeks of initial presentation

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Abbreviations!!navigator!!

  • PaO2 = partial pressure of oxygen in arterial blood
  • PAS = periodic acid–Schiff
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Munoz A, Riber C, Trigo P, Castejon F. Hematopoietic neoplasias in horses: myeloproliferative and lymphoproliferative disorders. J Equine Sci 2009;20:59–72.

Author(s) ⬆

Author: Krista Estell

Consulting Editors: David Hodgson, Harold C. McKenzie, and Jennifer L. Hodgson

Acknowledgment: The author and editors acknowledge the prior contribution of Laura Lee.