Mast cell tumour in a Dutch Warmblood horse

Andrew Brooks 

Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2020;24(2):19. 

A biopsy was submitted to the AHL from a 3-year-old female Dutch Warmblood horse that presented with a firm painful cutaneous swelling on the back. Histopathology of the lesion revealed a subcutaneous mass composed of a monotypic population of round cells (Fig. 1). The round cells were arranged in densely cellular sheets and aggregates that were interspersed with eosinophils, necrotic tissue and fibrous stroma. The cells had uniform oval nuclei, abundant cytoplasm, and cytoplasmic granules that stained metachromatically with toluidine blue. No mitotic figures were observed in ten-400X high-powered fields. The round cells extended to the margin of the biopsied tissue. Immunohistochemistry for the tyrosine kinase receptor KIT (CD117) revealed positive cell membrane staining of the round cells (Fig. 2). The cell morphology, metachromatic granules and positive immunostaining for KIT supported a diagnosis of mast cell tumor. 

Mast cell tumors (MCT) in horses (also called equine cutaneous mastocytoma or mastocytosis) are uncommon and comprise between 2-10% of equine cutaneous and mucocutaneous neoplasms. A search of the AHL database over the past decade identified 20 equine pathology submissions with a diagnosis of MCT. These involved mostly Warmblood (n=8) and Arabian horses (n=6). Age at the time of diagnosis ranged from 3 to 24 years and the majority were geldings (n=14). Clinical descriptions were incomplete for most submissions, but many of the horses had solitary tumors involving the skin or subcutaneous tissue. Tumor location included the neck (n=5), limbs (n=5), trunk (n=3), nostril (n=2), anus (n=1), and lymph node (n=1). Some tumors located on the distal limbs contained mineralized foci. Two horses had multiple MCTs. One horse with a MCT on the limb also had a lymph node metastasis and developed another MCT on the limb a few years after the initial diagnosis.