Small intestinal adenomatous polyposis resulting in protein-losing enteropathy in a horse

A 4-year-old Quarter Horse gelding was presented with a history of weight l oss of 6 months duration, along with extensive ventral subcutaneous edema. Clinicopathologic findings included a markedly low serum total protein (2.9 g/dl) and a low packed cell volume (24%). The mucosal surface of the dista l jejunum and entire ileum were carpeted with numerous polypoid, papillary, and glandular masses comprised of pseudostratified tall columnar cells and large numbers of interspersed goblet cells. Neoplastic change was diffuse throughout the mucosa of each mass, but abrupt demarcation occurred between neoplastic masses and adjacent mucosa. Immunohistochemical staining for pr otein of the p53 tumor suppressor gene revealed only occasional cytoplasmic reactivity within polyps and normal mucosa. Nuclear staining for papilloma virus antigens was not observed. Electron microscopic examination revealed features of well-differentiated intestinal epithelial cells, including apic al tight junctions and microvilli, desmosomes, and the presence of numerous goblet cells. Microorganisms were not detected. Small intestinal polyposis should be considered as a rare differential diagnosis for protein-losing e nteropathy in the horse.