Unusual cells in a blood smear from a cat

SIGNALMENT AND HISTORY

A 10 year old, male neutered, domestic short-haired cat presented to the veterinary practice with a history of chronic vomiting and weight loss of 2.2 kg despite polyphagia. No change in diet or increased exercise were recorded. Multiple skin nodules appeared at the same time.

 

PHYSICAL EXAMINATION

On presentation he was lethargic. His body condition score was 2/9. His abdomen appeared distended. He had multiple skin nodules of varying sizes (from a few millimetre to half centimetre) behind his left ear and on flanks, and some of these were very erythematous (Figure 1).

Figure 1. Multiple pink miliary masses expanding the dermis of the haired skin of both flanks.

INVESTIGATION

Haematology

Haematology was performed and identified the presence of large atypical cells (1.12.4x10^9/L, 15% of all nucleated cells) (Figure 2). Marked basophilia (2.4x10^9/L, reference interval, RI: 0-0.04x10^9/L) and mild lymphopenia (0.82x10^9/L, RI: 1-4x10^9/L) were also revealed.

Figure 2. Peripheral blood smear from a cat (A, B) (Wright-Giemsa stain, 50x objective).
Figure 2. Peripheral blood smear from a cat (A, B) (Wright-Giemsa stain, 50x objective).

Biochemistry

Biochemistry showed mild hypokalaemia (3.2 mmol/L, RI: 3.0-4.8 mmol/L) and low normal total protein with mild hypoalbuminaemia (23 g/L, RI: 26-56 g/L).

 

Imaging

Abdominal ultrasound revealed marked splenomegaly. The small intestine wall was thickened with abnormal layering (Figure 3A). Mesenteric lymph nodes were prominent (Figure 3B). Thoracic radiographs taken were unremarkable.

Figure 3. Abdominal ultrasound showing complete loss of layering in the jejunum (A) and prominent, hypoechoic mesenteric lymph nodes (B).
Figure 3. Abdominal ultrasound showing complete loss of layering in the jejunum (A) and prominent, hypoechoic mesenteric lymph nodes (B).

Cytology

Next, fine-needle aspiration (FNA) cytology of the skin nodule and spleen were performed (Figure 4).

Figure 4. FNA cytology of a cutaneous mass (A) and spleen (B) (Wright-Giemsa stain, 50x objective).
Figure 4. FNA cytology of a cutaneous mass (A) and spleen (B) (Wright-Giemsa stain, 50x objective).

INTERPRETATION OF THE RESULTS

A high number of large cells with magenta staining granules (mast cells) on the peripheral blood smear indicated the presence of marked mastocytaemia which was attributed to mast cell tumour. Marked basophilia was most likely paraneoplastic. Mild lymphopenia was likely due to stress.

 

Mild hypokalaemia was most likely related to gastrointestinal loss by vomiting and decreased potassium intake. Mild hypoalbuminaemia likely reflected decreased protein intake, cachectic state or gastrointestinal tract loss. Liver dysfunction and renal protein loss were also possible given the decrease in albumin with normal globulin.

 

Cytology of the skin nodule and spleen identified a high numbers of mast cells exhibiting features of atypia indicating the presence of a mast cell tumour.

DIAGNOSIS: DISSEMINATED MAST CELL TUMOUR WITH MASTOCYTAEMIA

 

MANAGEMENT AND FOLLOW-UP

Considering the prognosis, the current condition and possible side effects of chemotherapy, the owner elected euthanasia. Post-mortem examination was performed. Mast cells, either as discrete masses or as dense infiltrates, were present within the integument (Figure 5A), liver, jejunum, spleen (Figure 5B), presumptive mesenteric lymph node and heart.

 

SUMMARY

Mast cell tumour is a common neoplastic disease of cats. There are three typical forms of mast cell tumours in the cat: cutaneous, splenic/visceral and intestinal. In this case, given the presence of a systemic disease at the time of presentation it is not possible to determine the primary location of the tumour. The prognosis for a disseminated mast cell tumour is poor.

Mastocytaemia is defined as the presence of mast cells in the peripheral blood. It is a rare condition in the cat (prevalence of 0.05%) and unlike in dogs, is almost always attributed to feline mast cell tumour. It is worth noting that the sensitivity of routine blood smear examination for detection of circulating mast cells is low. As for that, buffy coat examination is the recommended technique of screening patients suspicious of mastocytaemia.

Figure 5. A. Histology of a cutaneous mast cell tumour from the cat showing unencapsulated and infiltrative masses within the dermis (haematoxylin and eosin stain, 4x objective).
Figure 5. B. Histology of a splenic mast cell tumour from the cat showing neoplastic cells exhibiting metachromatic staining cytoplasmic granules (toluidine blue stain, 100x objective).