Gastrointestinal biopsy in the horse: overview of collection, interpretation, and applications

Abstract

Evaluation of gastrointestinal (GI) biopsies is a multistep process that includes reviewing an appropriate history, determining sample quality, and evaluating histologic sections. Selected diagnostic parameters that, in combination with intestinal histopathology, can be useful to localize disease to the intestinal tract in the horse include hypoproteinemia and hypoalbuminemia, ultrasound evidence of increased thickness of the small intestinal wall, and alterations in glucose or D-xylose absorption tests. Biopsies may be acquired either endoscopically, or via laparoscopy or standing flank incisional approaches. GI sections should be evaluated using a systematic approach that includes both architectural changes and inflammatory cell infiltrates. Although strategies have been developed for assessment of GI biopsies from the dog and cat, a standardized approach to interpretation of the equine GI biopsy has yet to be developed. GI biopsies pose several challenges to the pathologist, especially for endoscopic biopsies in which the quality of the specimen and its orientation may vary greatly. Architectural changes are arguably the most critical changes to evaluate. In a horse with chronic GI inflammation, such as occurs in idiopathic inflammatory bowel disease (IBD), the cell types encountered frequently are macrophages, eosinophils, lymphocytes, and plasma cells. Increased numbers of these cell types are categorized loosely as mild, moderate, and severe. Specific forms of idiopathic IBD have been further classified by this infiltrate as granulomatous enteritis, eosinophilic enteritis, and lymphoplasmacytic enteritis; there is limited information on microscopic changes with each. Unfortunately, microscopic GI lesions are usually nonspecific, and determination of etiology requires further investigation.

Keywords: biopsy; gastrointestinal; horses; inflammatory bowel disease.

Figures 1–6.

Equine gastrointestinal biopsies. Figure 1. Gastric endoscopic biopsy showing only mucosa sectioned tangentially. H&E. Figure 2. Colon surgical biopsy, including full thickness of the colonic wall. H&E. Figure 3. Duodenal surgical biopsy. This biopsy is mostly free of artifact and well oriented, so that numerous villus-crypt units with associated lamina propria can be evaluated. H&E. Figure 4. Gastric endoscopic biopsy that includes only the superficial aspect of the mucosa with artifactually detached mucosal epithelium. H&E. Figure 5. Duodenal endoscopic biopsy with severe crushing artifact. H&E. Figure 6. Duodenal biopsy with muscularis and serosa, but missing mucosa and submucosa. H&E.

Figures 7–12.

Equine intestinal biopsies. Figure 7. Rectal biopsy from a horse with eosinophilic proctitis of unknown etiology. Observe loss and attenuation of the superficial epithelium and many eosinophils in the superficial lamina propria. H&E. Figure 8. Crypt hyperplasia in the small intestine of a horse, also with loss of goblet cell differentiation. The etiology of this lesion was not determined. H&E. Figure 9. Normal small intestinal mucosa with long, slender villi. H&E. Figure 10. Severe villus blunting in the jejunum of a horse with lymphoplasmacytic enteritis, giving the tissue the appearance of colon. H&E. Figure 11. Crypt dilation and necrosis in the colon; the crypt epithelium is attenuated and/or degenerate, and dead cells are present in the lumen of crypts. The cause of this lesion was undetermined. H&E. Figure 12. Fibrosis in the lamina propria of a horse with severe, chronic necrotizing colitis caused by Clostridioides difficile. H&E.

Figures 13–18.

Equine intestinal biopsies. Figure 13. Loss of goblet cells in the colonic mucosa of a horse with chronic diarrhea. This is a nonspecific lesion in the colon of horses with diarrhea. H&E. Figure 14. Small numbers of eosinophils in the deep lamina propria of the small intestine. This is considered a normal background finding in horses and should not be interpreted as a significant lesion. H&E. Figure 15. Eosinophilic proctitis; large numbers of eosinophils are intermixed with macrophages, lymphocytes, and plasma cells within the lamina propria. The cause of this lesion was not determined. H&E. Figure 16. Eimeria leuckarti in the lamina propria of the small intestine; this parasite is usually considered an incidental finding in horses. H&E. Figure 17. Ciliated protozoa in the lumen of the colon; these parasites are usually considered an incidental finding in horses. H&E. Figure 18. Granulomatous colitis in a horse with Rhodococcus equi infection. Macrophage infiltrates are present. Inset: small coccobacilli are present in the cytoplasm of most macrophages H&E.

Figures 19, 20.

Equine intestinal biopsies. Figure 19. Eosinophilic enteritis. Eosinophils and fewer lymphocytes, macrophages, and neutrophils are present in the lamina propria. Inset: higher magnification showing eosinophils in the superficial submucosa. The cause of this lesion was not determined. H&E. Figure 20. Lymphoplasmacytic enteritis consisting of infiltrates of lymphocytes and plasma cells in the lamina propria. Inset: higher magnification of the deep lamina propria showing several layers of lymphocytes and plasma cells. The cause of this lesion was not determined. H&E.