Human intestinal spirochetosis presenting as colonic ulceration in an immunocompetent 65-year-old man

Purpose: To report an unusual case of symptomatic human intestinal spirochetosis (HIS) in an immunocompetent individual without known risk factors, and a potential zoonotic source (domestic pig). Methods: A 65-year-old heterosexual man with no gastrointestinal or immunological history presented with acute diarrhea, abdominal pain, vomiting, and rectal bleeding. Colonoscopy revealed two ulcers in the distal transverse colon; biopsies were taken from ulcerated and normal mucosa. Histopathology included Warthin-Starry staining. Results: Histology showed dense spirochetal colonization forming a "false brush border," consistent with HIS. The ileum was unremarkable. The patient was treated with oral metronidazole 500 mg q6h for 10 days, resulting in complete resolution of symptoms. Follow-up colonoscopy at 13 months confirmed full mucosal healing, and the patient remained asymptomatic. Conclusion: This case highlights the possible pathogenic role of HIS in gastrointestinal symptoms and colonic ulceration, even in immunocompetent individuals without traditional risk factors. It underscores the diagnostic value of histology in atypical colonic lesions and suggests zoonotic transmission as a plausible route of infection outside classical risk groups.