The vast majority of small bowel infections consist of self-limited viral or bacterial enteritis, which usually do not necessitate imaging. However, a plethora of bacteria, viruses, fungi, and parasites have demonstrated an ability to cause chronic intestinal infections, leading to nausea, vomiting, diarrhea, fever, and weight loss. The AIDS epidemic increased the incidence of chronic small bowel infections, including tuberculosis, MAI, CMV, and cryptosporidium.

Imaging findings are extremely variable, as would be expected from the grocery list of causative agents. These include bowel wall and fold thickening, luminal narrowing, mural nodules, ulceration, and intraluminal filling defects. An interesting connection has been made between Giardia, nodular lymphoid hyperplasia, and hypogammaglobulinemia.

Given its relative abundance of lymphoid tissue, the terminal ileum is host to many of the chronic intestinal infections. Tuberculosis can affect any part of the small bowel, but is most common in the ileocecal region. Here, it can be virtually indistinguishable from Crohn’s disease, with fold thickening, luminal narrowing, and ulcerations. Yersinia, Campylobacter, and Salmonella also affect the terminal ileum.

An interesting (although exceedingly rare in the US) presentation of small bowel infection is intraluminal filling defect. The most notorious culprit for such a finding is Ascariasis. Caused by the roundworm Ascaris lumbricoides (35cm), Ascariasis represents the third most common infection worldwide, with an estimated 80-100 million cases. Infection with Ascaris involves primarily three systems:  pulmonary, intestinal, and biliary. Pulmonary manifestations include cough, fever, hemoptysis, asthma, pneumonia, and eosinophilia (>50%). Intestinal manifestations include abdominal pain, nausea, vomiting, constipation, obstruction, appendicitis, and eosinophilia (5-12%). Biliary manifestations include jaundice, cholecystitis, pancreatitis. Chest X-Rays demonstrate patchy, often transient, pulmonary infiltrates (Loeffler’s syndrome).  Abdominal plain films may show an obstructive pattern. Fluoroscopy may show Ascaris worms in the intestine as solitary or multiple filling defects. CT, ultrasound, and ERCP are used in the evaluation of abdominal complications of Ascariasis, such as SBO, appendicitis, cholecystitis, pancreatitis. 

Ascariasis.

SBFT (left ) demonstrates a linear filling defect (arrows). Enteroclysis (right) shows multiple long, tubular filling defects (arrow).