Intramuscular epithelioid hemangiosarcoma in a horse

ABSTRACT: A 20-year-old, sexually intact male Dutch Warmblood horse was presented with a 5-day history of difficulty moving and non-flexion of the right pelvic limb, followed by subcutaneous swelling and permanent lateral decubitus. Despite attempts at palliative care, the horse was euthanized after clinical worsening and lack of treatment response. The main gross finding was a soft, red-to-black, lobulated mass involving the right semitendinosus, semimembranosus, and biceps femoris muscles. Soft, red nodules were also seen in all pulmonary lobes and adrenal glands. Histology revealed neoplastic proliferation of malignant polygonal cells, occasionally arranged in irregular anastomosing vascular channels filled with erythrocytes. Neoplastic cells were immunoreactive for factor VIII-related antigen, confirming the blood vascular origin. A final diagnosis of epithelioid hemangiosarcoma was made. These findings reinforced the inclusion of hemangiosarcoma as a differential diagnosis of equine muscular lesions and highlighted the histological features of the epithelioid hemangiosarcoma subtype.