Malignant mast cell neoplasia with local metastasis in a horse.

This research study focuses on the rare case of a horse developing malignant mast cell neoplasia, a type of cancer, which had metastasized locally. The disease, usually accompanied by eosinophilia (increased levels of a type of white blood cell), hadn’t been previously reported in horses.

Introduction

• The subject of the study is a 12-year-old Arabian stallion who had a chronically swollen right carpus (equivalent to human wrist). This swelling subsequently led to profound lameness in the horse’s right leg.

Analysis

• Initial diagnostic tests made on an incisional biopsy of the subcutaneous tissue from the affected right carpus. The findings showed substantial numbers of eosinophils (type of white blood cell involved in the immune response) and mast cells present. A fungal culture performed on the same sample recorded a negative result, ruling out the possibility of the swelling being caused by a fungal infection.
• Serial full blood counts were conducted, which consistently revealed an increased level of eosinophils, matured yet without a comparable increase in mast cells, an abnormal amount of neutrophils (another type of white blood cell), and a continuous excess of fibrinogen, a blood protein involved in clotting. These factors indicate a pathological condition.

Postmortem Investigation

• Following the horse’s death, postmortem dissection revealed a significant deposition of connective tissue around the carpal joint of the affected limb. This tissue embedded numerous small, gritty nodules consisting of calcification and necrotic cell debris.
• The tendons within this connective tissue mass had a limited function.
• The horse’s right axillary lymph node (near the underarm) was examined and noted to be moderately enlarged, with a yellow-brown, moist appearance.
• Further histopathological examination of these areas indicated the presence of a fairly differentiated mast cell neoplasm (cancer of the mast cells), which had metastasized to the regional lymph node. This discovery is remarkable since malignant mast cell neoplasia is rarely reported in horses, and its metastasis has been reported in only one other horse.

Conclusion

• The increase in eosinophils, commonly associated with mast cell neoplasia, is a common occurrence in dogs but had never been reported in horses until this study.