An 8-year-old, 3-kg male swift fox (Vulpes velox) housed at the Saskatoon Zoo was presented for examination of an ulcerated mass on the right hind leg. The fox was in good body condition and showed no lameness of the affected leg. The mass was on the medial aspect of the limb in the dewclaw area and was firm and well circumscribed. Blood and urine samples, radiographs of both hind limbs and of the thorax, fine needle aspirates, impression smears and two wedge biopsies of the mass were taken under isoflurane (Isoflurane, USP, Abbott Laboratories Limited, Montreal, QC, Canada) anesthesia. The mass was identified as a fibrosarcoma based on the biopsy results. Bloodwork showed mild abnormalities attributable to stress and chronic inflammation. No evidence of metastases was found radiographically.
Considering the location and behavior of this type of tumor (high rate of local recurrence after excisional biopsy), it was elected to amputate the affected limb. Following this, further histopathology of the tumor led to a revised diagnosis of malignant fibrous histiocytoma, giant cell variant, which was supported by immunohistochemistry. Electron microscopy results are pending.
A literature search found no reports of neoplasia in swift foxes, and only one report of malignant fibrous histiocytoma in a fox (genus was not specified).2 Canine cases have been reported, but this type of tumor appears to have a fairly low incidence in the pet dog population when compared to other soft tissue sarcomas.4 The diagnosis of malignant fibrous histiocytoma is particularly challenging and cannot be made by light microscopy alone, as much histologic overlap exists between different sarcomas and other neoplasms.1,3,4 Accurate diagnosis requires the use of more specialized techniques such as immunohistochemistry and electron microscopy. Malignant fibrous histiocytomas are locally invasive tumors with poor metastatic potential and a high rate of local recurrence after excision in domestic canids.3,4 Radical excision with wide margins of healthy tissue, cutting through bone if necessary, is recommended. Adjuvant radiation therapy or a second surgery should be considered following incomplete resection.4
1. Fletcher, C.D.M. 1987. Commentary. Histopathology. 11:433–437.
2. Fujimaki, Y., M. Sugiyama, and M. Isoda. 1985. Malignant fibrous histiocytoma in a fox. Jap J Vet Sci. 47:147–150.
3. Gross, T.L., P.J. Ihrke, and E.J. Walder. 1992. Veterinary Dermatopathology: A Macroscopic and Microscopic Evaluation of Canine and Feline Skin Disease. Mosby Year Book, St. Louis, Missouri. 446–450.
4. MacEwen, E.G., and S.J. Withrow. 1996. Soft tissue sarcomas. In: Withrow, S.J., and E.G. MacEwen (eds.). Small Animal Clinical Oncology, 2nd ed. WB Saunders, Philadelphia, Pennsylvania. 211–226.