Erythroleukemia

7-year-old domestic short-hair cat, Jazz

by Peter Kintzer, DVM, DACVIM, Boston Road Animal Hospital, Springfield, MA, and
Fred Metzger, DVM, DABVP, owner of Metzger Animal Hospital, State College, PA

Physical examination
T = 103.5 P = 200 bpm R = 60 rpm
Mucous membrane color was pale and the CRT was 1.5 seconds. Jazz was quiet, alert, responsive and hydrated. Cardiac auscultation revealed a 1/6 left-sided systolic murmur. She was obese.

 

Plan
CBC, chemistry profile, urinalysis, FeLV/FIV SNAP® Combo Test and survey radiographs. An intravenous catheter was placed and fluid therapy was started.

 

FeLV/FIV SNAP® Combo
Negative/Negative

Erythron—There is a severe macrocytic anemia; a reticulocyte count of less than 50-60 K/µL indicates that it is a nonregenerative anemia. Typically, macrocytosis is seen with regenerative anemia since the immature erythrocytes are larger than mature erythrocytes. Morphologic evaluation of erythrocytes on the peripheral blood film shows the presence of anisocytosis without significant polychromasia and the presence of large erythrocytes in circulation as well as large atypical metarubricytes. This morphologic finding is often associated with megaloblastosis and dysplastic erythrocyte production. This is most commonly seen with FeLV infection.

 

Leukon—The primary leukocyte abnormality is the presence of a mild lymphocytosis. Lymphocytosis in cats is most commonly associated with physiologic lymphocytosis/leukocytosis seen with excitement. Reactive lymphocytosis associated with systemic antigenic stimulation and atypical lymphocytosis associated with lymphoproliferative disease are possible also. Lymphocytes noted in this case are morphologically normal and physiologic lymphocytosis was most likely.

 

Thrombon—A significant thrombocytopenia is noted and enlarged as well as atypical platelets are noted in circulation during evaluation of the blood film.

 

Figure 1: Peripheral blood film from a cat, monolayer, 100x oil objective field of view, Wright’s stain. Note the severe decreased erythrocyte density supporting the anemia, the lack of any significant polychromasia even thought there is significant anisocytosis, the large megaloblastic metarubricyte in the lower center field of view and the large atypical platelet in the center right field of view. All these findings are supportive of the final diagnosis of a myeloproliferative disease.

 

Hematology Profile Summary—Nonregenerative severe anemia associated with megaloblastosis is highly suggestive of underlying FeLV-associated myeloproliferative disease. The thrombocytopenia is likely associated with inadequate production of bone marrow. In most animals, the finding of large platelet forms supports increased rate of production at the bone marrow level; however, this is commonly not the interpretation for the cat. The atypical forms of platelets seen in circulation in this case are commonly associated with FeLV infection; however, this is not an absolute diagnostic finding. Bone marrow evaluation is highly recommended as a next tier of diagnostic testing.

 

 

Chemistry Profile—Unremarkable

 

 

Urinalysis—Unremarkable

 

Diagnostic Imaging—Chest radiographs were unremarkable. Abdominal radiographs showed splenomegaly. An echocardiogram revealed mild left atrial enlargement and mild mitral regurgitation. An abdominal ultrasound revealed an enlarged hypoechoic spleen.

 

Presumptive Diagnosis—Anemia, thrombocytopenia (R/O immune-mediated, R/O bone marrow disease)

 

Diagnostic Plan
Jazz was blood-typed and a blood transfusion given. A bone marrow aspirate was collected and sent to the reference laboratory for cytological analysis and an FeLV IFA test.

 

Bone Marrow Analysis—Cellularity was adequate with moderate blood contamination. Megakaryocytes were only rarely seen. Only few myeloid precursors were observed and the Myeloid:Erythroid ratio was 0:1. The distribution of nucleated cells consisted approximately of 10% neutrophil forms, 56% erythroid precursors including metarubricytes and rubricytes, and 34% erythroblasts. Changes are compatible with erythroleukemia (AML-M6Er designation).

 

FeLV Antigen IFA on bone marrow slide: Positive

 

Final Diagnosis
Erythroleukemia (AML-M6Er designation); FeLV infection

 

Clinical Outcome
Due to the poor prognosis, the client elected euthanasia.

 

Discussion
Erythroleukemia is a rare form of leukemia in cats that carries a poor prognosis. The FeLV antigen IFA on a bone marrow slide was positive although the FeLV SNAP test on peripheral blood was negative. Although this is not a common finding, it is reported. Both tests are attempting to detect the same p27b core antigen; however, they are evaluating the presence of this antigen in different forms. The FeLV ELISA test in the FeLV SNAP® is detecting soluble antigen and the IFA on the bone marrow is testing for antigen within leukocytes.

 

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