Page 1291 - Clinical Small Animal Internal Medicine

133 Lymphoid Leukemias, Myeloid Neoplasia, and Myelodysplastic Syndrome 1229

time, due to lack of useful clonality assays, eosinophilic Myelodysplastic Syndromes
VetBooks.ir leukemia and hypereosinophilic syndrome are difficult Myelodysplastic syndromes (MDS) are a heterogeneous
to differentiate. Essential thrombocythemia has been
described in the dog and is characterized by persistent
thrombocytosis of >600 000/μL without evidence of cir­ group of acquired alterations of hematopoietic stem cells
that have been described in humans, dogs, and cats.
culating blast cells. MDS can be primary or secondary/acquired. Primary
MDS result from clonal expansion of a defective pluripo­
Signalment tent stem cell and therefore are considered neoplastic.
There is no known age, breed, or sex predilection in any Some refer to myelodysplasia as preleukemia since it can
of these disorders. progress to acute leukemia in some cases.

History and Clinical Signs
A history of lethargy, inappetence, and/or weight loss is Etiology/Pathophysiology
common with any of the myeloid neoplasms. Clinical Several different aberrations can result from a dysfunc­
signs are generally vague and may include fever, pale tional hematopoietic system constituting MDS. In dogs,
mucous membranes, petechiation, hepatosplenomegaly, there are usually cytopenias in 2–3 cell lines in the
and lymphadenopathy. peripheral blood due to cell death caused by ineffective
hematopoiesis. Other blood abnormalities may include
Diagnosis dysplastic changes in several cell lines, metarubricytosis,
Basophilic leukemia must be differentiated from masto­ and macrocytic erythrocytes. The bone marrow is usu­
cytosis based on morphology. Although these cells are ally normocellular or hypercellular. Blast cells in bone
usually easily distinguished from each other, changes in marrow make up <30% of all nucleated cells but in peo­
morphology of the nucleus and granules in basophilic ple this threshold has been changed to <20%.
leukemia can make this distinction more difficult. Three subtypes of primary MDS have been proposed
Infiltration into other organs is possible, including the for dogs and cats: MDS with excessive blasts (MDS‐EB),
liver, spleen and lymph nodes, so evaluation of these in which blast percentages are >5% and less than 20%,
organs by abdominal ultrasound and/or cytology can MDS with refractory cytopenia (MDS‐RC) with blast
aid in the diagnosis. To make a diagnosis of eosinophilic percentages <5% and cytopenias in one or more
leukemia, disorders associated with eosinophilia such lineages, and MDS with erythroid predominance
as skin diseases, respiratory, and/or gastrointestinal (MDS‐ER) in which the myeloid:erythroid (M:E) ratio is
tract diseases and parasitism need to be ruled out before <1. Cases of canine secondary MDS consistently have
considering a neoplastic leukemic etiology. Dogs with <5% myeloblasts in the bone marrow. Secondary MDS
essential thrombocythemia can present with hepatos­ in dogs has been associated with several conditions
plenomegaly, nonregenerative anemia, and neutro­ including myelofibrosis, neoplasia (lymphoma and
philia. On examination of bone marrow, megakaryocytic multiple myeloma), immune‐mediated thrombocyto­
hyperplasia is the most common feature. Diseases that penia, immune‐mediated hemolytic anemia, drugs, and
are known to cause secondary thrombocytosis such ionizing radiation. In cats, secondary MDS can be
as chronic inflammatory diseases, rebound from associated with FeLV.
severe hemorrhage and iron deficiency anemia need
to be ruled out before making a diagnosis of essential
thrombocythemia. Epidemiology
Genetic abnormalities in hematopoietic cells such as
Treatment/Prognosis mutations in oncogenes and tumor suppressor genes
Treatment for basophilic leukemia with hydroxyurea has along with chromosomal deletions have been discovered
provided complete or partial remissions for several in human MDS patients and are thought to play a role in
months in case reports. The effective treatment of eosin­ veterinary patients.
ophilic leukemia is unknown but in one case report,
treatment with corticosteroids was effective. Information Signalment
on treatment for essential thrombocythemia is lacking
but in one case report, a dog was successfully treated MDS‐EB is the most common type of primary MDS and
with a chemotherapy protocol consisting of vincristine, occurs most frequently in middle‐aged to older dogs.
cyclophosphamide, Ara‐C, and prednisone. As with all Both MDS‐RC and MDS‐ER occur in older dogs. No
forms of leukemia, HSCT can be considered for suitable breed or sex predilection is known in any of the
dogs if feasible. subtypes.