Squamous cell carcinoma of the stomach and the alimentary form of lymphosarcoma are the most common forms of neoplasia involving the GI tract in horses. Chronic weight loss may be the primary clinical sign. Chronic diarrhea and hypoalbuminemia may develop when lymphosarcoma has infiltrated the wall of the intestine.
Because the incidence of GI neoplasia is low, other causes of weight loss should be investigated first. Diagnosis is usually made by exclusion of other causes of weight loss and by histopathologic examination of the tissue collected by duodenal or rectal mucosal biopsy during exploratory laparotomy or at necropsy. Squamous cell carcinoma of the stomach can be diagnosed by gastroscopy. An endoscope 2–3 m long is necessary to examine the gastric mucosa of adult horses. In horses with lymphosarcoma, enlarged mesenteric lymph nodes or thickened bowel may be detected by rectal palpation or by ultrasonographic examination. Occasionally, neoplastic cells are identified by cytologic examination of abdominal fluid. Ultrasonography may reveal masses in the liver or spleen, as well as facilitate percutaneous biopsy of the masses. An exploratory laparotomy with biopsy of intestinal or other masses can provide a definitive diagnosis.
Treatment of GI neoplasia in horses is generally not attempted, and the prognosis is grave. There have been a few reports of surgical removal of the affected segment of bowel. Chemotherapy may be an option for some horses, and corticosteroid therapy may prolong survival time in some cases.