Suspected chronic eosinophilic leukemia in a FeLV-negative cat

JFMS open reports · 2026

In plain language

This report describes one cat with suspected chronic eosinophilic leukemia, a condition that could not be reliably distinguished from hypereosinophilic syndrome. Infectious disease tests, which included FeLV, were negative. The authors suggested that one condition might have developed into the other, but did not demonstrate this. The cat received toceranib and prednisolone together, so the case cannot attribute the observed changes to toceranib alone or provide data on outcomes in FeLV-positive cats.

Scientific detail

The clinical report published in JFMS open reports in 2026 describes a 16-year-old spayed female domestic shorthair cat with suspected chronic eosinophilic leukemia (CEL), who received toceranib with concurrent prednisolone. This was a single uncontrolled case. All three claims recorded in the knowledge object are assigned evidence level C.

The authors considered possible transformation from hypereosinophilic syndrome (HES) of unknown significance to CEL as an explanation for the initially indolent course. This was a proposed explanation, not a demonstrated transformation: neither confirmed sequential diagnoses nor molecular evidence of clonality established oncogenic progression. Idiopathic HES could not be excluded because bone marrow biopsy was declined and only one myeloblast was observed. The authors note that reliable feline criteria distinguishing CEL from HES are lacking and morphology is not definitive; analogous feline cytogenetic markers had also not been identified. CEL therefore remained a suspected diagnosis rather than a confirmed clonal malignancy.

Gastrointestinal eosinophilic infiltration was not confirmed by biopsy. Liver and spleen cytology could have reflected blood contamination rather than true infiltration. Parasitism remained a theoretical differential diagnosis because of the lack of recent anthelmintic administration. In addition, the single uncontrolled case design and concurrent prednisolone prevent attribution of the observed changes to toceranib alone.

Infectious disease testing, which included FeLV, was negative; assay details were not reported. Although the discussion cites previous literature linking both HES and CEL to FeLV infection, the reported cat was not FeLV-positive. The case informs the differential diagnosis and uncertain classification of feline eosinophilic proliferative disorders, not FeLV-associated leukemogenesis, antiviral activity or outcomes in FeLV-positive cats.

What we know

In this single cat, infectious disease testing that included FeLV was negative. The authors proposed possible transformation from HES to CEL but did not demonstrate it. Idiopathic HES could not be excluded, and CEL remained a suspected diagnosis. These claims are assigned evidence level C; the uncontrolled case design and concurrent prednisolone prevent attribution of the observed changes to toceranib alone.

What we don't know

It was not established whether the cat had CEL or idiopathic HES, or whether transformation between them or clonal progression occurred. Gastrointestinal, hepatic and splenic eosinophilic infiltration was also unconfirmed, and parasitism remained a theoretical possibility. FeLV assay details were not reported. This case cannot determine the effect of toceranib alone and provides no evidence on FeLV-associated leukemogenesis, antiviral activity or outcomes in FeLV-positive cats.

Evidence

Provenance

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