Cutaneous mass in a ferret (Mustela putorius furo)

Antonio Meléndez-Lazo1 DVM MSc PhD DipECVCP MRCVS

Laura Vilalta Solé2 DVM DipECZM (Small mammals)


2Universidad Católica de Valencia (Spain)



A 5-year-old, neutered male ferret (Mustela putorius furo), presented for a routine health check. A small, 3mm in diameter skin nodule in the neck area was found.  The nodule was reddish and associated to small crusts (Fig.1). 


No associated clinical signs were detected. The ferret was otherwise healthy.

Figure 1. Cutaneous lesion.


Hematology and biochemistry

Hematological and biochemical examination revealed no abnormalities.


Fine needle aspiration (FNA) cytology of the axillary lymph node was performed (Figure 2).

Figure 2. FNA cytology cutaneous nodule (Modified Wright staining).

What is your interpretation of the cytology?

What is your differential diagnosis?

Which other stains would you use in cytology to further characterize the lesion?

What is your interpretation of the cytology?

A population of round cells was found. They were arranged singly or in variable sized non-cohesive groups. Cell borders were moderately distinct and round. Cytoplasm was light blue and microvacuolated. Nuclei were round to oval, occasionally indented, and central to paracentrally placed, with indistinct nucleoli. Anisocytosis and anisokaryosis were mild. Nuclear to cytoplasmic ratio was high. No inflammatory cells or microorganisms were detected. These findings were consistent with a round cell tumour.

What is your differential diagnosis?

The main differential diagnoses are mast cell tumor and histiocytoma.

Which other stains would you use in cytology to further characterize the lesion?

Additional stains include Giemsa (Fig. 2) and toluidine blue (Fig. 3). Both stains revealed several cytoplasmic granules that obscured the nuclei and were not visible with Modified Wright stain. If negative, immunocytochemistry with antibodies such as CD1 would be possible for the detection of histiocytic cells.

Figure 2. FNA cytology cutaneous nodule (Giemsa staining)
Figure 3. FNA cytology cutaneous nodule (Toluidine Blue staining)

DIAGNOSIS: Cutaneous mast cell tumor




Excisional biopsy of the nodule was done, and histopathological examination revealed a neoplastic round cell proliferation expanding the superficial dermis and reaching the deep dermis (Figure 4). The mass was non-delineated, non-encapsulated and infiltrative, and extended between the collagen bundles of the dermis. Tumour cellularity was moderate with growth pattern in sheets. Proliferating cells were round, with well-defined cytoplasmic margins and a moderate amount of basophilic cytoplasm. Nuclei were central, with finely stippled chromatin. Nuclear pleomorphism was absent and anisocytosis/anisokaryosis was mild. Additional staining with toluidine blue revealed no clear granulation in the cells (Figure 5). KIT immunostaining was positive (Figure 6). Six-month follow up revealed no further neoplastic growth.

Figure 4. Tissue section (H&E staining)
Figure 5. Tissue section (Toluidine Blue staining)
Figure 6. Tissue section (KIT immunostaining)


Mast cell tumours are one of the most common cutaneous neoplasms in ferrets. They have benign behaviour, but spontaneous regression may not occur as it does in dogs. Cytological examination may not be definitive, because cytoplasmic granules are not revealed by quick stains. Toluidine Blue and Giemsa should be used in cytologic smears when cutaneous mast cell tumour is suspected in ferrets. On tissue sections, Toluidine Blue is usually inconclusive, and KIT immunostaining is needed for confirmation.