Erythroid leukaemia is an aggressive malignant neoplasm of the red blood cell lineage in the bone marrow haematopoietic system. In cats, it appears to be one of the more common subtypes of myeloid leukaemia [1] and involves uncontrolled proliferation (clonal expansion) of red blood cell precursors in the marrow, often with suppression of normal haematopoiesis.

Common clinical signs include lethargy, inappetence, weight loss, pale mucous membranes and, in advanced cases, haemorrhage, ascites or pleural effusion [2,3]. Laboratory findings typically include severe, often initially non-regenerative anaemia; sometimes leucocytosis or thrombocytopenia; nucleated red blood cells (metarubricytes, rubricytes); and blasts (rubriblasts) in blood and bone marrow. Bone marrow aspirates reveal markedly increased erythroid precursor populations (often > 50%), with a relative reduction in the myeloid lineage. Furthermore, infiltration of neoplastic cells into other tissues may be observed in advanced disease [2].

In cats, there is a strong association between erythroid leukaemia and feline leukaemia virus (FeLV) infection. For example, two cats in the same household developed erythroleukaemia and were FeLV positive; Southern blot analysis showed clonal integration of FeLV [1]. It remains unclear whether FeLV is causal or contributory, but the virus may promote transformation of haematopoietic progenitors.

Diagnosis

Diagnosis requires bone marrow evaluation with demonstration of predominant erythroid precursors, and ideally quantification of proerythroblasts and rubriblasts. In the feline literature, one case showed a marrow myeloid:erythroid ratio of 0.02 and strong CD71 positivity in neoplastic cells [3].

In veterinary practice, diagnosis may be delayed because features overlap with anaemia, myelodysplasia or other leukaemia types. In addition, immunohistochemical markers for specifically identifying erythroid neoplastic cells in cats are limited.

Case results

A 6-year-old spayed female European domestic short-haired cat presented with deterioration in general condition, depression and slight tachypnoea for several weeks. Clinical examination revealed a gallop heart rhythm, a grade 3/6 left apical systolic murmur, pale mucous membranes and tachypnoea.

Laboratory findings

The complete blood count (CBC) and blood smear showed severe, non-regenerative, macrocytic, normochromic anaemia with agglutination and erythroid precursors. Observed blast cells were suspected to be erythroid precursor cells and/or other blast types. Bone marrow examination revealed hyperplasia secondary to infiltration by a blastic population, which could be erythroid or, less likely, lymphoid, with generalised dysmyelopoiesis and possible myeloid hypoplasia. The anaemia was most likely the result of defective erythropoiesis secondary to erythroleukaemia; less likely causes included myelophthisis and dysmyelopoiesis in the context of lymphoid leukaemia.

To rule out lymphoid leukaemia, immunophenotyping would be necessary, using markers such as CD45, CD34, CD3, CD21 and CD79a.

Fig. 2: WDF scattergram from the XN-V analyser. A) Healthy cat. B) Patient with ‘Monocytosis’.

Fig. 3: WNR scattergram from the XN-V analyser. A) Healthy cat. B) Patient with nucleated red blood cells (NRBC).

The blood smear revealed large, abnormal blastic cells, likely of erythroid origin (Fig. 4A), and various erythroid precursor cells (Fig. 4B).

Fig. 4 A, B: Blood smear microscopy images

Bone marrow cytology showed predominantly proerythroblasts. Erythroid blasts have uniformly round nuclei and deep blue, slightly grainy cytoplasm that may contain a perinuclear clear zone. Note the nucleated red blood cells (arrow), some of which are dysplastic (arrowhead) (Fig. 5).

Fig. 5: Bone marrow cytology, Wright’s stain, 50× objective

References

[1] Shimada T et al. (1995): Erythroleukemia in Two Cats Naturally Infected with Feline Leukemia Virus in the Same Household Journal of Veterinary Medical Science, 1995, Volume 57, Issue 2, Pages 199–204.

[2] Chimenes ND et al. (2022): Acute Erythroid Leukemia in a Cat Infected with Feline Leukemia Virus. Acta Scientiae Veterinariae, 50.

[3] Lim GH et al. (2025): A rare case of acute erythroid leukemia (AML M6-Er) with pleural metastasis and malignant pleural effusion in a cat. J Vet Med Sci. 2025 Jun 1;87(6):634–639.

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