Enteritis in the Immunocompromised Host

Content below is from a textbook reference chapter (Sabiston Ch50 - Infectious Enteritis & Small Bowel Neoplasms), not primary literature — treat as background/orientation rather than critically-appraised evidence.

The AIDS epidemic, along with widespread use of immunosuppressive agents after organ transplantation, has resulted in a number of rare and exotic pathogens infecting the GI tract. Almost all patients with AIDS have GI symptoms during their illness, most commonly diarrhea. A surgeon may be asked to evaluate the immunocompromised patient with abdominal pain, acute abdomen, or GI bleeding; a range of protozoal, bacterial, viral, and fungal organisms may be responsible.

Protozoa

Most frequent class of pathogens causing diarrhea in AIDS patients — Cryptosporidium, Isospora, and Microsporidium, with the small bowel the most common site of infection. Diagnosis via acid-fast staining of stool/duodenal secretions, with specific antigen tests and immunochromatography cards for rapid stool protein detection improving diagnostic capability (sensitivity/specificity >90%). Symptoms most commonly relate to diarrhea, which may be intractable. Treatment regimens have not been entirely effective, but prophylactic cotrimoxazole and highly active antiretroviral therapy appear to elicit a response to HIV-related diarrheal illness.

Bacteria

Infections by enteric bacteria are more frequent and more virulent in HIV-infected individuals than in healthy hosts. Salmonella, Shigella, and Campylobacter are associated with higher rates of bacteremia and antibiotic resistance in the immunocompromised patient.

• Shigella/Salmonella: diagnosed by stool cultures.
• Campylobacter: harder to establish by stool culture (often negative), but PCR of stool/serum shows promising diagnostic results in culture-negative cases. These enteric infections present with high fever, abdominal pain, and diarrhea that may be bloody; abdominal pain may mimic an acute abdomen. Bacteremia/serious infections should be treated with IV imipenem; ciprofloxacin is an attractive choice if organisms are multiply resistant; erythromycin is safe in pregnancy. Incidence of Campylobacter infection among AIDS patients treated with rifabutin prophylaxis was reported decreased vs. untreated controls.
• Clostridium difficile: more common in AIDS patients because of increased antibiotic use in this population vs. healthy hosts. Diagnosis by standard stool enterotoxin assays; treatment with metronidazole or vancomycin is usually effective (see also Infectious Colitis for the fuller treatment ladder in the general population).

Mycobacteria

Frequent cause of intestinal disease in immunocompromised hosts — secondary to Mycobacterium tuberculosis or Mycobacterium avium complex (MAC, an atypical mycobacterium related to the type causing cervical adenitis/scrofula). Usual route of infection is by swallowed organisms directly penetrating the intestinal mucosa. MAC infection notably causes massive thickening of the proximal small intestine. Clinically, patients with MAC present with diarrhea, fever, anorexia, and progressive wasting.

The most frequent site of M. tuberculosis intestinal involvement is the distal ileum and cecum (~90% of cases); gross appearance can be ulcerative, hypertrophic, or ulcerohypertrophic. Bowel wall appears thickened, with an inflammatory mass often surrounding the ileocecal region; strictures and even fistula formation are noted. Serosal surface normally covered with multiple tubercles; mesenteric lymph nodes frequently enlarged and thickened with caseous necrosis on sectioning. Mucosa is hyperemic, edematous, and sometimes ulcerated. On histology, the distinguishing lesion is a granuloma, with caseating granulomas found most commonly in lymph nodes. Most patients complain of chronic abdominal pain that may be nonspecific, plus weight loss, fever, and diarrhea.

Diagnosis: identification of the organism in tissue by direct visualization with acid-fast stain, culture of excised tissue, or PCR assay. Radiographic exams usually reveal a thickened mucosa with distorted mucosal folds and ulcerations. CT may show thickening of the ileocecal valve and cecum.

Treatment of M. tuberculosis is similar in immunocompromised or nonimmunocompromised hosts — usually responsive to multidrug antimicrobial therapy. Therapy for MAC infection is evolving; drugs successfully used in vivo/in vitro include amikacin, ciprofloxacin, cycloserine, and ethionamide. Clarithromycin has also been successfully used in combination with other agents. Surgical intervention may be required for intestinal tuberculosis, particularly M. tuberculosis; obstruction and fistula formation are the leading indications for surgery, though with current treatment most fistulas now respond to medical management. Surgery may be necessary for ulcerative complications when free perforation, perforation with abscess, or massive hemorrhage occurs; treatment is usually resection with anastomosis.

Viruses

CMV is the most common viral cause of diarrhea in immunocompromised patients. Clinical manifestations include intermittent diarrhea accompanied by fever, weight loss, and abdominal pain. Enteric CMV infection’s manifestations result from mucosal ischemic ulcerations, accounting for the high rate of perforations seen with CMV. Patients may present with abdominal pain, peritonitis, or hematochezia from diffuse ulcerating intestinal involvement. Diagnosis is by demonstrating viral inclusions — classically a “so-called owl’s eye” intranuclear inclusion, often surrounded by a halo (Fig. 50.27); cytoplasmic inclusions also occur. Cultures for CMV are usually positive when inclusion bodies are present, but these cultures are less sensitive/specific than histopathologic identification. Once diagnosed, treatment is usually effective with ganciclovir; foscarnet is an alternative (pyrophosphate analogue inhibiting viral replication). Other less common viruses reported include adenovirus, rotavirus, and novel enteric viruses such as astrovirus and picornavirus.

Fungi

Fungal infections have been recognized in AIDS patients. Gastrointestinal histoplasmosis occurs in the setting of systemic infection, often with pulmonary and hepatic disease; diagnosis by fungal smear/culture of infected tissue or blood; treated most commonly with amphotericin B. Coccidioidomycosis of the intestinal tract is rare and, like histoplasmosis, occurs in the context of systemic infection.

Open items / gaps

• No primary literature yet in the wiki independently appraising treatment efficacy for any of these organisms — content above is entirely textbook-reference-derived.
• Cross-link to Infectious Colitis for the general-population (non-immunocompromised) treatment of C. difficile, Campylobacter, Salmonella, Shigella once the overlap between the two pages is worth spelling out explicitly.