Intestinal spirochetosis in the modern era: a decade-long clinicopathologic study

Intestinal spirochetosis (IS) is infrequently identified in about 1% of human colonic biopsy specimens. On routine sections, it is diagnosed by identifying a thick band of basophilia on the apical surface of colonic mucosa. Intestinal spirochetosis is caused by Brachyspira pilosicoli and Brachyspira aalborgi, but whether it represents a commensalism or a pathogenic process remains debated.

We studied 175 cases of IS diagnosed at our institution over a 10-year period. The clinicopathologic features were reviewed using hematoxylin and eosin (H&E)-stained slides, along with the results from histochemical and immunohistochemical analyses.

All cases were diagnosed by colonoscopic biopsy, except for 1 case. Indications for colonoscopy were unexplained diarrhea (48.6%), colorectal cancer screening (36%), rectal bleeding (6.3%), abdominal pain (4%), follow-up for ulcerative colitis (4%), Crohn disease (0.6%), and constipation (0.6%). Associated significant clinical conditions included HIV-positive status (14.3%), diabetes mellitus (1.7%), post-liver transplant (0.6%), retroperitoneal liposarcoma (0.6%), prostate cancer (0.6%), and Kaposi sarcoma (0.6%). Pathologic changes in background colonic mucosa included unremarkable (89%), active inflammation (6%), hyperplastic polyp (1.7%), tubular adenoma (1.7%), and Trichuris trichiura infestation (0.6%). On H&E, all cases had the characteristic basophilic hue on the epithelial surface of colonic mucosa.

Intestinal spirochetosis in our study had a striking male predominance and affected both immunocompetent and immunosuppressed individuals. Nearly two-thirds of the patients were symptomatic, indicating that IS is likely a pathogenic infection of low virulence. Histologically, IS can involve normal, inflamed, and dysplastic epithelium.