Diagnosis
of Monocytic Leukemia in Dogs
Kristine K.
Yu; Kenneth S. Latimer, DVM, PhD; Heather L. Tarpley, DVM; and
Paula M. Krimer, DVM, DVSc
Undergraduate
Honors Biology Program, The University of Georgia, Athens, GA 30602 (Yu) and
Department of Pathology (Latimer, Tarpley, Krimer), College of Veterinary Medicine,
The University of Georgia, Athens, GA 30602
Leukemia is
a neoplasm of hematopoietic cells that originates within the bone marrow.
Although the term "leukemia" means "white blood," this neoplastic
condition can involve leukocytes, erythrocytes, megakaryocytes and platelets,
or mast cells singly or in combination. All of these cells originate from a
common pluripotential stem cell. The three main types of leukocytes are granulocytes
(including neutrophils, eosinophils, and basophils), monocytes, and lymphocytes.
Neutrophils and monocytes are derived from a common bipotential precursor cell,
a colony forming unit that produces granulocytes (neutrophils) and monocytes
(CFU-GM, Fig 1). Leukemic transformation of the bipotential CFU-GM precursor
cell can result in three forms of leukemia that can produce neutrophils, monocytes,
or a combination of both cell types. These types of neoplasia are called granulocytic
leukemia, monocytic leukemia, and myelomonocytic leumekia, respectively.
 |
| Figure
1. Overview of hematopoiesis. Neutrophils and monocytes are derived
from a bipotential stem cell designated granulocyte-macrophage colony forming
unit (CFU-GM). |
Clinical signs
of monocytic leukemia are nonspecific and include fever, lethargy, affects weight
loss, anorexia, and occasional changes in skin texture. Physical examination
also may disclose enlarged tonsils, lymphadenopathy, hepatosplenomegaly, and
hemorrhage. Laboratory findings include mild to moderate anemia (~26-34%), leukocytosis,
and moncoytosis that may range from mild to extreme (5,000 to > 800,000 monocytes
/µl of blood). With higher leukocyte counts, monocytes and monoblasts may account
for > 90% of the
leukocyte differential count.
Diagnosis of
monocytic leukemia is usually based upon the complete blood cell count and examination
of cellular morphology on Romanowsky-stained blood and bone marrow smears (Fig.
2). Cellular lineage usually can be identified easily in chronic monocytic leukemia;
however, acute monocytic leukemia is more difficult to diagnose. In acute monocytic
leukemia, the neoplastic cells are undifferentiated blasts that often resemble
lymphoblasts (Fig. 3). Definitive diagnosis of acute monocytic leukemia in dogs
ultimately is based upon cytochemistry, immunocytochemistry, and transmission
electron microscopy.
 |
 |
| Figure 2.
Chronic monocytic leukemia in a dog. These cells are easily recognized as
monocytes (Wright-Leishman stain). |
Figure 3. Acute monocytic leukemia in a dog. These cells are poorly differentiatted
and resemble lymphocytes (Wright-Leishman stain). |
A panel of
cytochemical stains is applied to blood and bone marrow smears to determine
the lineage of the leukemic cells. Monocytes usually exhibit fluoride sensitive
alpha naphthyl acetate esterase or alpha naphthyl butyrate esterase activities.
Neoplastic monocytes usually lack alkaline phosphatase, myeloperoxidase, and
chloroacetate esterase activities (Fig. 4). Antibodies to cell membrane antigens
may be used in conjunction with flow cytometry to characterize leukemic cell
populations.
 |
| Figure
4. Monoblasts have brown cytoplasm indicating reactivity for alpha
naphthyl actetate esterase activity (Alpha naphthyl actetate esterase technique
with fast green counterstain). |
Transmission
electron microscopy can be used to evaluate cytochemical staining reactions
and identify organelles ultrastructurally. Typical organelles of monocytes include
lysosomes, a few mitochondria, and microfilament bundles. Neoplastic cells may
have a smooth plasma membrane or scattered pseudopodia along the surface of
the cell membrane (Fig. 5 and Fig. 6). Cytoplasmic vacuoles also may be present.
 |
 |
Figure
5. Transmission electron micrograph of a neoplastic monocyte
with pseudopodia, few mitochondria, and infrequent lysosomes (osmiun tetroxide
and lead citrate staining). |
Figure
6. Transmission electron micrograph of a neoplastic monocyte. Microfilament
bundles (arrowheads) are present between nuclear lobes (osmiun tetroxide
and lead citrate staining) |
Diagnosis of
monocytic leukemia may require examination of Romanowsky-stained blood and bone
marrow smears, cytochemical staining, and transmission electron microscopy.
Precise identification of cellular lineage is necessary if treatment of leukemia
will be attempted.
References
Grindem CB:
Ultrastructural morphology of leukemic cells from 14 dogs. Vet Pathol 22:456-4621985.
Grindem CB:
Cytochemical reactions in cells from leukemic dogs. Vet Pathol 23:103-109, 1986.
Latimer KS,
Dykstra MJ: Acute monocytic leukemia in a dog. J Am Vet Med Assoc 184:852-854,
1984.
Mackey LJ:
Monocytic leukemia in the dog. Vet Rec 96:27-30, 1975.
Tolle DV, Seed
TM, Fritz TE, Lombard LS, Poole CM, Norris WP: Acute monocytic leukemia in an
irradiated beagle. Vet Pathol 16:243-254, 1979.
|