Canine Leukemia Study Guide

Overview and Clinical Importance

Leukemia is a malignant neoplasm of hematopoietic cells arising in the bone marrow (or occasionally the spleen), characterized by an abnormal proliferation of white blood cells. In dogs, leukemia represents an important but relatively uncommon category of hematologic malignancy. Understanding the classification, diagnosis, and management of canine leukemia is essential for the NAVLE.

Leukemia is classified by two key characteristics: the stage of cell maturation (acute versus chronic) and the cell lineage (lymphoid versus myeloid). This creates four main categories: Acute Lymphoblastic Leukemia (ALL), Chronic Lymphocytic Leukemia (CLL), Acute Myeloid Leukemia (AML), and Chronic Myeloid Leukemia (CML).

Classification of Canine Leukemia

Leukemia classification is critical because it determines treatment approach and prognosis. The classification system uses two main axes:

Cell Maturation Stage

Acute leukemia: Involves immature, undifferentiated cells called "blasts." These cells proliferate rapidly but cannot mature properly, leading to aggressive disease. Blast cells are large, with high nuclear-to-cytoplasm ratios, fine chromatin, prominent nucleoli, and basophilic cytoplasm.

Chronic leukemia: Involves mature, differentiated cells that accumulate over time. These cells can be morphologically identified on blood smear and are associated with slower disease progression.

Cell Lineage

Lymphoid leukemia: Arises from lymphocyte precursors (B-cells, T-cells, or rarely NK cells). Lymphoid leukemias are more common than myeloid leukemias in dogs.

Myeloid leukemia: Arises from myeloid precursors (granulocytes, monocytes, erythrocytes, or megakaryocytes). AML occurs approximately 10 times less frequently than lymphoproliferative diseases.

Leukemia Classification Summary

Acute Lymphoblastic Leukemia (ALL)

Pathophysiology

ALL is characterized by the rapid proliferation of immature lymphoid cells (lymphoblasts) in the bone marrow. The neoplastic cells arise from mutations in lymphoid progenitor cells that promote proliferation while preventing normal maturation. This results in myelophthisis (replacement of normal bone marrow elements), causing cytopenias.

Signalment and Breed Predisposition

ALL can occur at any age but is more common in younger to middle-aged dogs (median age 5.5-7 years). German Shepherds, Golden Retrievers, and Labrador Retrievers may be overrepresented.

Clinical Signs

• Lethargy, weakness, and exercise intolerance (anemia)
• Anorexia and weight loss
• Fever (secondary infections from neutropenia)
• Petechiae, ecchymoses, or hemorrhage (thrombocytopenia)
• Hepatomegaly and splenomegaly (greater than 70% and 50% of cases)
• Mild lymphadenopathy (NOT prominent - key distinction from lymphoma)

Diagnostic Findings

CBC and Blood Smear

• Marked lymphocytosis (often greater than 150,000/µL) with lymphoblasts
• Nonregenerative anemia (greater than 50% of cases)
• Thrombocytopenia (30-50% of cases)
• Neutropenia (bone marrow suppression)

Lymphoblast characteristics:

• Large cells (nucleus larger than neutrophil)
• High nuclear-to-cytoplasm ratio
• Fine, stippled chromatin (euchromatin)
• Prominent nucleoli
• Deeply basophilic (dark blue) cytoplasm
• Fragile cells ("basket cells" or "smudge cells")

Treatment

Prognosis: Poor. Median survival with treatment is approximately 55-120 days. Complete remission rates less than 30%.

Chronic Lymphocytic Leukemia (CLL)

Pathophysiology

CLL is characterized by the clonal proliferation of small, mature-appearing lymphocytes that accumulate in the bone marrow, blood, and spleen. CLL is the most common leukemia in dogs. T-cell CLL (specifically CD8+ cytotoxic T-cell) is most frequently diagnosed.

Signalment

CLL is typically a disease of older dogs (median age 10-11 years). German Shepherds and Golden Retrievers may be overrepresented.

Clinical Signs

Many dogs with CLL are asymptomatic and diagnosed incidentally on routine bloodwork:

• Mild lethargy
• Decreased appetite, weight loss
• Polyuria/polydipsia
• Mild splenomegaly (lymphadenopathy uncommon)

Diagnostic Findings

• Persistent lymphocytosis (typically greater than 20,000/µL, often 6,000 to greater than 100,000/µL)
• Normal lymphocyte count is less than 3,500/µL
• Small, mature lymphocytes with dense, clumped chromatin
• Anemia usually mild; severe cytopenias uncommon

Immunophenotype and Prognosis

When to Treat CLL

Not all CLL requires immediate treatment. Initiate treatment when:

• Clinical signs develop
• Anemia or thrombocytopenia develops
• Lymphadenopathy or hepatosplenomegaly worsens
• Lymphocyte count exceeds 60,000/µL

Treatment

Prognosis: Good to Fair. Median survival 1-3 years. Rarely, CLL transforms to aggressive lymphoma (Richter's syndrome).

Acute Myeloid Leukemia (AML)

Pathophysiology

AML involves the clonal expansion of myeloid stem cells (granulocytic, monocytic, erythroid, or megakaryocytic precursors). AML occurs approximately 10 times less frequently than lymphoproliferative diseases in dogs.

Clinical Signs

• Lethargy and inappetence (greater than 60%)
• Peripheral lymphadenopathy (74%) - more prominent than ALL
• Hepatosplenomegaly
• Hemorrhage/petechiae (thrombocytopenia)
• Mediastinal mass (can mimic thymic lymphoma)

Diagnostic Findings

• Circulating blasts (85% of cases; median blast count 35,700/µL)
• Anemia, thrombocytopenia, bicytopenia/pancytopenia
• Aleukemic leukemia: blasts confined to marrow
• Diagnosis requires greater than or equal to 20% myeloid blasts in bone marrow

Treatment

Prognosis: Very Poor. Median survival is only 19 days (range 1-121 days).

Diagnostic Approach

Step 1: CBC and Blood Smear

The first step is a CBC with blood smear examination by a clinical pathologist. In-house automated analyzers may misclassify neoplastic cells.

Step 2: Flow Cytometry (Gold Standard)

Flow cytometry determines cell lineage and identifies prognostic markers.

Key CD Markers

Additional Diagnostics

PARR (PCR for Antigen Receptor Rearrangement):

• Detects clonal populations (confirms neoplasia vs. reactive lymphocytosis)
• High specificity; negative result does not rule out neoplasia

Bone Marrow Aspirate:

• Essential when peripheral blood blasts are absent (aleukemic leukemia)
• Confirms diagnosis when greater than 20% blasts present

"Big Blasts = Bad" Rule:

If cells are big (blasts), it's acute leukemia with BAD prognosis. Small, mature cells = chronic = BETTER.

"CLL = Chlorambucil" Mnemonic:

Chlorambucil + Prednisone is first-line for CLL. Both start with "C".

Treatment by Type:

• ALL = CHOP (like lymphoma)
• AML = Ara-C + Anthracycline
• CLL = Chlorambucil + Prednisone
• CML = Hydroxyurea