Causes of infectious enteritis
• Giardiasis: Giardia lamblia is seen as a pear-shaped organism, which resides in the upper small intestine (duodenum and jejunum) and exists in two forms—trophozoite and cyst. The trophozoite form (7 μm wide, 14 μm long) shows two symmetrical nuclei with nucleoli and four pairs of flagella. On longitudinal sections, it appears as a long, curved organism.
Mycobacterium avium intracellulare infection: This opportunistic infection affects both the small and large bowel in immunocompromised hosts in a patchy distribution. Histologic examination shows numerous histiocytes in the lamina propria that contain numerous acid-fast bacilli highlighted by Kinyoun stain. Granulomas may not be identified.
• Whipple disease: Tropheryma whippelii infects the small intestine, cardiac valves, nervous system, and lymph nodes. Histologic examination shows expansion of lamina propria by positive periodic acid–Schiff (diastase resistant) Whipple bacilli that are negative with acid-fast bacilli stain. The other feature that points to Whipple infection is the dilated lymphatics in the lamina propria caused by obstruction of the lymphatic ducts by bacilli. Other tests include polymerase chain reaction (PCR) assay and electron microscopy.
• Other infections include cryptosporidium, disseminated histoplasmosis, Isospora belli, Microsporidium spp. (Enterocytozoon bieneusi, Enterocytozoon intestinalis), strongyloides, and Yersinia spp.
• Lymphangiectasia: Primary lymphangiectasia presents in the pediatric age group generally before 3 years. The biopsy sample shows dilated lymphatics in the superficial lamina propria. Secondary causes will show similar histologic findings and include local inflammatory or a neoplastic process.
• Ischemic enteritis: This is often the result of mechanical obstruction and, histologically, shows hemorrhage in the lamina propria or transmural hemorrhage with mucosal sloughing.
• GVHD: Histologic findings are graded as follows:
• Grade 1—Apoptosis (single cell necrosis) of the crypt epithelium
• Grade 2—Apoptosis with crypt abscesses
• Grade 3—Individual crypt necrosis or crypt drop-out
• Grade 4—Total surface denudation of areas of bowel
• Eosinophilic gastroenteritis: The biopsy shows villous blunting with numerous eosinophils in the lamina propria forming clusters or sheets. The etiologic factors include food allergies, parasites, drugs, hypereosinophilic syndrome, and idiopathic disease.
Small Intestinal Neoplasms
• Peutz-Jeghers polyps: The small intestine is the most common site for polyps in Peutz-Jeghers syndrome. Histologic examination shows arborizing smooth muscle bundles in the lamina propria without much expansion of lamina propria by inflammatory infiltrate. The overlying epithelium is that of small intestinal type and may show hyperplasia. Dysplasia can occasionally be seen in these polyps.
• Adenomas: Duodenum is the most common upper gastrointestinal (GI) site for an adenoma. The morphologic characteristics are similar to that in the colon: tubular, tubulovillous, or villous patterns are seen. Ampullary adenomas arise in the ampulla or periampullary region and are indistinguishable from each other based on morphologic examination.
• Adenocarcinomas: The primary adenocarcinoma of the small intestine is uncommon (2% of GI tract tumors), and the duodenum is the most common site. Usually, these arise from a sporadic adenoma. Histologic examination resembles colonic adenocarcinoma. Other predispositions include familial adenomatous polyposis (FAP), hereditary nonpolyposis colorectal cancer (HNPCC), or hamartomatous polyp syndromes. Risk factors include chronic inflammatory conditions such as celiac disease, Crohn’s disease, ileostomy, and protein-losing enteropathy.