MALIGNANT MAST-CELL NEOPLASIA WITH LOCAL METASTASIS IN A HORSE

A 12-year-old Arab stallion was presented with a chronically swollen r ight carpus resulting in profound lameness of the same leg. An incisio nal biopsy of subcutaneous tissue from the right carpus submitted for cytology and histopathology revealed large numbers of eosinophils inte rspersed by substantial numbers of variably sized and granulated mast cells. Fungal culture of a subcutaneous tissue sample taken from the r ight carpus was negative. Serial full blood counts revealed persistent mature eosinophilia, not accompanied by a mastocytaemia, neutrophilia without left shift and persistent hyperfibrinogenaemia. After humane destruction, dissection of the affected limb revealed a thick layer of connective tissue deposited around the right carpal joint. Within the connective tissue were embedded many small 0.25-1 cm diameter yellow gritty nodules, which consisted of dystrophic calcification and necrot ic cell debris. The tendons enveloped by the connective tissue mass ha d limited function. The right axillary lymph node was moderately enlar ged, yellow-brown and moist. Histopathological examination revealed a moderately well differentiated mast cell neoplasm with evidence of met astasis to the regional lymph node. In horses, malignant mast cell neo plasia is rare, while metastasis has only been reported in one other h orse. Eosinophilia associated with equine mast cell neoplasia has not been reported previously but is recorded in mast cell neoplasia in the dog.