Infectious enteritis is a common cause of acute diarrheal illness in the United States and can be caused by viruses, bacteria, or, less commonly, protozoa. The most common viral causes include norovirus (formerly Norwalk virus) and rotavirus, with the most common bacterial causes including Salmonella, Escherichia coli, and Clostridium perfringens. Protozoa that typically involve the small bowel include Cryptosporidium, Microsporidium, Isospora, Cyclospora, and Giardia lamblia. However, there is a limited role for radiologic imaging studies in the diagnosis and management of infectious enteritis, as most patients do not seek medical attention, and, in those who do, diagnosis can generally be made from the history, physical exam, stool studies, and, when needed, endoscopy. Imaging studies are only necessary in rare cases or special situations, such as that of the immunocompromised patient. The proximal small bowel is the typical site of infection by Giardia lamblia, the most common parasite to cause infection in the United States. Findings on CT are fairly nonspecific, with diffuse bowel-wall thickening and mesenteric lymphadenopathy.
Infectious enteritis affecting the distal ileum, often in combination with the cecum, can cause a confusing clinical picture, mimicking appendicitis. Frequent pathogens causing an infectious ileocecitis include Salmonella, Yersinia, Shigella, and Campylobacter.[44,45]
Clostridium difficile, a well-recognized cause of colitis, is increasingly recognized as a cause of enteritis as well. As in the colon, histologic changes include extensive ulceration and sloughing of the bowel mucosal surface. In contrast to cases of pseudomembranous colitis, bowel-wall thickening is not as prominent of a feature in cases of C. difficile enteritis, sometimes only seen in a very short segment of small bowel. Distended,fluid-filled small-bowel loops, ascites, and mesenteric-fat stranding were the predominant features in one small published series and were described in several other reports as well. Abnormalities may also be seen in the cecum, possibly heralding early involvement of the colon as well.
Important causes of enteritis in patients with HIV include Mycobacterium avium intracellulare (MAI), cytomegalovirus, and Crypto sporidium. MAI more frequently affects the jejunum, causing bowel wall thickening that can be nodular. The finding of low-density, bulky mesenteric and retroperitoneal lymphadenopathy is helpful in suggesting the diagnosis, although Whipple disease and lymphoma can have the same appearance. Cytomegalovirus enteritis is often hemorrhagic, secondary to mucosal ulcerations, which can sometimes be seen onCT. Cryptosporidium infection results in bowel wall thickening, also typically involving the proximal small bowel, which can be indistinguishable from MAI and giardiasis.
Appl Radiol. 2012;41(8):6-17. © 2012 Anderson Publishing, Ltd.