Atypical chronic myelomonocytic leukemia of eosinophilic lineage in a FeLV-positive cat

Case reports in veterinary medicine · 2026

In plain language

The report describes a two-year-old cat with a positive FeLV test and an unusual form of leukemia, diagnosed through blood and bone marrow studies. The case did not fit the usual classification criteria. The cat died two days after diagnosis, but this single case cannot predict outcomes for other cats or attribute the death specifically to FeLV. It also does not show that FeLV caused the leukemia.

Scientific detail

This clinical report, published in Case reports in veterinary medicine in 2026 (doi: 10.1155/crve/6729159), describes a two-year-old cat with a positive FeLV test. The authors diagnosed atypical chronic myelomonocytic leukemia (CMML) of eosinophilic lineage through bone marrow examination and peripheral blood studies. The findings constitute level C evidence from a single case, with no reported control group; they do not independently validate a feline disease classification.

The authors reported diffuse alpha-naphthyl acetate esterase positivity in 21.3% of cells whose morphology appeared compatible with myeloid/neutrophilic lineage. They used cytochemistry to support monocytic identity despite that morphology. This single observation does not establish diagnostic sensitivity or specificity. Morphologically eosinophilic cells accounted for 45.4% of marrow cells and were negative for both myeloperoxidase and alpha-naphthyl acetate esterase. Lineage attribution in this finding is morphological: the negative stains do not independently establish eosinophilic clonality or an underlying genetic lesion.

The authors considered the case atypical because hypereosinophilia in blood and marrow prevented it from fitting classic WHO criteria. Separately tabulated blood and marrow blast percentages and serial measurements documenting persistent monocytosis were not reported. Karyotyping was not performed because the cat died two days after diagnosis, leaving chromosomal characterization unavailable. This observed survival cannot establish a general prognosis or attribute death specifically to FeLV.

Although the authors describe the leukemia as secondary to FeLV, the reported evidence documents the co-occurrence of a positive infection test and a myeloid neoplasm in one cat. No causal experiments, molecular analyses, or integration-site investigations linking FeLV to malignant transformation were reported. The case therefore does not establish that FeLV caused this leukemia or identify an oncogenic mechanism.

What we know

In a two-year-old cat with a positive FeLV test, the authors diagnosed atypical CMML of eosinophilic lineage from blood and bone marrow studies. The morphological and cytochemical findings are level C evidence limited to that case. The authors explicitly noted that hypereosinophilia prevented the case from fitting classic WHO criteria.

What we don't know

This case does not determine whether FeLV caused the leukemia or through what mechanism. Karyotyping was unavailable, and no molecular or integration-site studies were reported. Separately tabulated blood and marrow blast percentages and serial measurements documenting persistent monocytosis were also not reported. Eosinophilic clonality, an underlying genetic lesion, diagnostic sensitivity or specificity, independent validity of the feline classification, and a generalizable prognosis remain unestablished. The death cannot be attributed specifically to FeLV on the basis of this case.