CHAPTER 25
Bacterial, viral, and toxic causes of diarrhea, gastroenteritis, and anorectal infections

Gail A. Hecht¹, Jerrold R. Turner², Phillip I. Tarr³, and Mitchell B. Cohen⁴

¹ Loyola University Medical Center, Maywood, IL, USA

² Brigham and Women’s Hospital and Harvard Medical School, Boston, MA, USA,

³ Washington University, St Louis, MO, USA

⁴ University of Alabama at Birmingham, Birmingham, AL, USA

The spectrum of organisms that can infect and cause disease in the human colon includes bacteria, viruses, and protozoa. Reviewed in this chapter are the bacterial and viral pathogens of the large intestine. Although the symptoms associated with infection by enteric bacterial pathogens are essentially indistinguishable (including abdominal pain, diarrhea, and fever), the range of pathological appearances is somewhat more varied. For example, colonic biopsies of Campylobacter colitis often have features similar to those seen in inflammatory bowel disease, including crypt abscesses (Figure 25.1), but typically lack changes associated with chronicity.

Figures 25.2 and 25.3 portray the results of cholera infection and equipment used by patients to cope with the symptoms. Infection with enterohemorrhagic Escherichia coli (EHEC) typically induces histopathology that overlaps with ischemic colitis (Figure 25.4). The colon can be quite severely affected, as demonstrated by contrast and computed tomography studies of affected children (Figure 25.5) and by the colon at laparotomy (Figure 25.6). Gastroenterologists need to be cognizant of the clinical progression of this infection, as patients can present at multiple different points during their illness, although it is most often the bloody diarrhea that prompts evaluation (Figure 25.7).

The histological features of colitis associated with enteroinvasive colitis and Shigella are typically identical (Figure 25.8). This stems from the fact that the genes conferring the invasive phenotype are identical for these two pathogens. Despite these highlighted differences, colonic histology is usually not specific enough to conclusively determine the causative agent. The one exception to this statement is Clostridioides difficile‐associated pseudomembranous colitis. C. difficile is the number one cause of healthcare‐associated diarrhea because hospitals and long‐term care facilities serve as reservoirs, and establishment of infection in the colon by this spore‐forming pathogen is dependent upon disruption of the resident colonic microflora by antibiotics (Figure 25.9). Although the diagnosis of C. difficile‐associated colitis is usually determined by assays that identify the presence of toxin A or B in the stool, the gross appearance of pseudomembranes seen at sigmoidoscopy, as are evident quite vividly following resection (Figure 25.10), and the characteristic histological volcano lesions (Figure 25.11) are virtually pathognomonic for this infection. If a barium enema is performed, which is not recommended, then the presence of pseudomembranes may be demonstrated (Figure 25.12).

Infections of the anus and rectum are most commonly seen in homosexual men and heterosexual women who engage in anoreceptive intercourse. Primary anorectal syphilis appears as a chancre of the squamous epithelial lining of the anal canal or rectum (Figure 25.13). Condyloma lata represents the secondary phase of syphilis (Figure 25.14). Biopsy of anorectal lesions from patients infected with Treponema pallidum may reveal spirochetes (Figure 25.15). However, nonpathogenic spirochetes can also reside in the rectum, thus reducing the significance of this finding.

More commonly seen are condyloma acuminata (anal warts) caused by infection with human papillomavirus. These verrucous lesions are generally easy to differentiate from the flat, fleshy lesions of condyloma lata, but histology easily distinguishes between the two and is recommended to confirm the diagnosis (Figure 25.16).