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

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.