Case Report Confirms Gastric Leiomyosarcoma in a 4-Year-Old Horse

A 4-year-old male Thoroughbred racehorse was found at necropsy to have a large intramural mass in the nonglandular region of the stomach. Histological and immunohistochemical analyses identified the lesion as a gastric leiomyosarcoma, with tumor cells closely associated with the gastric smooth muscle layer, strong alpha smooth muscle actin staining and no detectable CD117 expression. The tumor measured 20 by 11 by 11 centimeters and displayed malignant features, including cellular pleomorphism, nuclear atypia and neoplastic cells within intratumoral blood vessels. This single-animal case report documents an uncommon equine gastrointestinal tumor and its clinical, gross and microscopic findings.

The horse was a stallion used for flat racing and was evaluated in Uruguay from August through November 2023. Its initial history included intermittent low-grade fever, mild inappetence following competitions and subtle weight loss. During the early stages of illness, the horse remained generally bright, retained its appetite and produced feces of normal consistency. Over approximately three months, it experienced recurrent fever, progressive weight loss of about 25 kilograms and persistent hypoalbuminemia, prompting an extensive diagnostic evaluation for the cause of its clinical signs.

Serial complete blood counts and serum biochemical tests showed progressive hypoproteinemia and hypoalbuminemia without leukocytosis. Parasitological examination of feces detected a low burden of strongyle-type eggs, measured at 20 eggs per gram. Thoracic and abdominal ultrasonography revealed transient splenomegaly and mild pleural irregularities, but no abdominal mass or effusion was detected. Cytology of bronchoalveolar lavage fluid showed 59% macrophages, 39% lymphocytes and 2% eosinophils. Activated macrophages and multinucleated giant cells were considered consistent with chronic inflammatory changes, while no intracellular or extracellular bacteria were identified. Urine cultures, head radiographs and repeated viral and hemoparasitic screenings were negative.

Gastroscopy was performed approximately one week before the horse’s death, following a 24-hour fast. The examination documented incomplete gastric emptying, but the gastric mucosa appeared macroscopically normal, without a visible mass, ulceration or active bleeding. The horse was treated with antimicrobial therapy, beginning with ceftiofur and later enrofloxacin and metronidazole, along with supportive care and intravenous flunixin meglumine as needed for pyrexia. On November 16, it developed a higher fever, reaching 40 degrees Celsius, mild depression and decreased feed intake. Thoracic radiographs showed a bronchial-interstitial pattern, bronchiectasis and a large, well-defined structure consistent with a pulmonary abscess. The horse was euthanized and referred for necropsy at the veterinary pathology unit in Montevideo.

At necropsy, the mass was found within ingested feed, protruding from the wall of the nonglandular stomach. It was generally oval and soft, measuring 20 by 11 by 11 centimeters. Its external surface was brown, while the cut surface was grayish white with scattered black areas. Portions of the tumor were necrotic, its base was firmly attached to the stomach wall, and the surrounding wall was thickened. A portion of the specimen was fixed in 10% buffered neutral formalin and processed for routine histological examination. Sections were stained with hematoxylin and eosin, and additional 3-micrometer serial sections were prepared for immunohistochemistry using antibodies against alpha smooth muscle actin and CD117, also called c-Kit.

Microscopic examination showed that the tumor arose in close association with the normal smooth muscle layer. In these areas, the usual smooth muscle architecture was disrupted by neoplastic cell proliferation, connective tissue expansion and interstitial edema. Neoplastic cells accompanied by inflammatory infiltrates and edema extended into the underlying serosal layer, producing serosal thickening. The part of the tumor projecting into the gastric lumen was covered by squamous epithelium, whereas much of its exposed surface showed extensive necrosis and lymphocytic, plasmacytic and histiocytic inflammatory responses. Remnants of ingesta and clusters of bacteria were present in some necrotic areas.

The neoplastic population displayed considerable pleomorphism, including round, pleomorphic and spindle-shaped cells, with cytoplasm ranging from abundant to scant. Nuclei were round, oval or irregular, and mitotic figures were infrequent. Most cells were arranged in interwoven or interlacing fascicles. In several regions, spindle-shaped cells with abundant eosinophilic cytoplasm formed clusters resembling smooth muscle differentiation. Neoplastic cells were also observed within newly formed blood vessels inside the tumor, an observation that supported classification of the lesion as malignant. On serial sections, the tumor cells showed strong cytoplasmic staining for alpha smooth muscle actin. Blood vessels with positive labeling served as an internal positive control, while the neoplastic cells were negative for CD117. Together with the tissue architecture and the tumor’s location, this immunophenotype supported the diagnosis of primary gastric leiomyosarcoma.

Immunohistochemistry was important to the diagnosis because gastrointestinal stromal tumors and smooth muscle tumors can have overlapping appearances under routine microscopy. The source report notes that morphology alone can make these tumors difficult to distinguish, including in canine gastric tumors. CD117 staining detects most gastrointestinal stromal tumors, whereas alpha smooth muscle actin expression supports smooth muscle differentiation. The combination of alpha smooth muscle actin positivity and CD117 negativity in this horse, together with direct involvement of the gastric smooth muscle layer and smooth muscle-like cellular features, supported leiomyosarcoma rather than a gastrointestinal stromal tumor. The malignant designation was based on nuclear atypia and the presence of tumor cells in intratumoral vessels. Despite that vascular finding, pathologists identified no metastatic lesions in the liver, lungs or other organs examined.

The age of the horse makes the case unusual within the limited literature on equine gastric tumors. Gastric neoplasms have a reported incidence of 0.9% in horses, and horses with gastric neoplasms are reported to have a mean age of 18 years. Squamous cell carcinoma is the most frequently diagnosed form, while tumors derived from muscle are exceptionally rare. According to the report, only two cases of gastric leiomyoma and one earlier case of gastric leiomyosarcoma had been described in horses. The other reported gastric leiomyosarcoma occurred in a 12-year-old horse and was diagnosed from morphological findings without immunohistochemical confirmation. The present 4-year-old therefore provides a younger case documented with both histology and immunohistochemistry, but a single report cannot establish that these tumors are more common in young horses or reveal any incidence trend.

Location is another notable feature. The authors report that equine gastric smooth muscle tumors tend to arise in the nonglandular region, consistent with the site of this mass. The report also provides a cautious clinical interpretation: the authors presumed that the tumor was responsible for the horse’s persistent fever, protein-losing state and impaired gastric emptying. That explanation was not established experimentally, and the diagnostic work-up did not identify an abdominal mass on ultrasound or a visible tumor on gastroscopy. The horse’s euthanasia followed the development of a pulmonary abscess and respiratory abnormalities, rather than being an outcome directly demonstrated to result from the gastric tumor. As a single necropsy case with no separate dataset, the work cannot determine the tumor’s population frequency, risk factors or the full mechanism of systemic illness. Its central contribution is a detailed characterization of a rare gastric cancer in a young horse: a 20-centimeter smooth muscle-associated mass with alpha smooth muscle actin positivity, CD117 negativity, histologic evidence of malignancy and no detected distant metastases at necropsy.

Subject of Research: Veterinary case report of histopathological and immunohistochemical confirmation of gastric leiomyosarcoma in a 4-year-old horse.

Article Title: Case Report: Histological study of gastric leiomyosarcoma in a young horse

Article References: Guerrero, H., Varela, B., Morales, A., Larrañaga, C., Yozzi, V., Yamasaki, K., & Verdes, J. M. (2026). Case Report: Histological study of gastric leiomyosarcoma in a young horse. Frontiers in Veterinary Science, 13, 1979698. https://doi.org/10.3389/fvets.2026.1979698

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1979698

Keywords: equine oncology, gastric leiomyosarcoma, veterinary pathology, histopathology, immunohistochemistry, smooth muscle actin, CD117, horse, gastric tumor, case report, Case, Report