Abdominal spindle cell sarcoma of probable myofibroblastic origin in a horse.

This research focused on the case of an 18-year-old Thoroughbred gelding which, after displaying signs of illness, was discovered to have a form of rare abdominal spindle cell sarcoma. The postmortem examination led to the assumption that the tumor probably originated from myofibroblasts.

Case Presentation

• The horse had a history of acute septic hepatocholangitis, from which it recovered five months prior, after which it began to display symptoms such as lack of appetite, fever, chronic weight loss, and abdominal distention.
• The horse also had developed intermittent dermal photosensitization involving lightly pigmented skin of its distal hind limbs.
• Upon examination, the horse also showed signs of depression, lethargy, patchy hair loss and scaling, abnormal protrusions on its skin, generalized muscle atrophy, and was in poor overall condition.
• Blood tests showed high protein and leukocyte count and other results consistent with chronic inflammation. A large nodular abdominal mass was identified, confirmed by ultrasound, but with no specific organ involvement.
• Fluid collected from the horse’s abdomen had high protein concentration but otherwise appeared normal. Cytologic exam of fluid collected from the protrusions revealed a monomorphic population of small lymphocytes and occasional erythrophagocytic macrophages.

Postmortem Examination

• The horse was euthanized due to its rapid and severe deterioration and suspected abdominal neoplasia.
• Necropsy revealed severe muscle atrophy, the loss of body fat stores, distinct bony processes and ribs, and abnormal fluid in the abdomen.
• A large cystic mass weighing around 82 kilograms and filled with reddish fluid and gelatinous material was found in the intestinal mesentery.
• Swollen mesenteric lymph nodes, fibrous adhesions, multilobulated masses on the prepuce were also observed.
• Other abnormalities included hepatic atrophy, spleen scars, lung congestion, atelectasis, and adrenal cortical nodular hyperplasia.

Microscopic Findings

• Corynebacterium spp. was found in both the lymph node and the fluid from the lumen of the abdominal mass.
• Microscopic examination of a variety of body tissues was conducted. Light microscopy of the abdominal mass showed signs of neoplasia.
• Neoplastic spindle cells indicative of sarcoma were found in the abdominal mass, arranged in interlaced sheets and whorls.
• The observation of other specific characteristics, such as acute hemorrhage within the neoplasm, minimal supportive stroma, mitoses occurring at a rate of 0-1/100 neoplastic cells, and monomorphic shape of the cells, further supported the diagnosis of neoplasia.
• Subcutaneous preputial masses were assumed to have been lymphoid tissue. Other findings included mild cholestasis, blood clotting in the liver, mineralized blood vessels in basal ganglia, axonal degeneration in the spinal cord, histiocytosis in the lymph nodes, and congestion and fluid accumulation in the lungs.

Immunohistochemistry

• Neoplastic cells tested positive for smooth muscle actin (a marker for myofibroblasts), desmin (indicative of muscle cells), and vimentin (a marker for mesenchymal cells). Special stains did not show presence of glycogen within the neoplastic cells.
• All these findings support the conclusion that the tumor cells were of myofibroblastic origin.

In conclusion, the study describes an unusual case of a horse with abdominal spindle cell sarcoma originating most likely from myofibroblasts. The detailed pathological and immunohistochemical evaluation contributed significantly to the definite diagnosis, providing insightful information on such rare cases of equine neoplasia.