Intestinal Spirochetosis Due to Syphilis as a Cause of Chronic Diarrhea in an Immunocompetent Patient

Syphilis is a sexually transmitted infection caused by Treponema pallidum , a spirochete that penetrates through mucous membranes, reaches regional lymph nodes, and rapidly disseminates throughout the body. The infection can mimic various diseases, often hindering proper diagnosis and treatment. Lower gastrointestinal syphilis is a rare manifestation of the disease, typically transmitted through anal sexual intercourse, and may cause chronic diarrhea that is frequently misdiagnosed as inflammatory bowel disease in previously healthy individuals. This case concerns a 36-year-old male, homosexual, previously healthy, who sought specialized care in infectious diseases due to diarrhea persisting for five months, about four episodes per day, with mucus but no blood or undigested food, and mild abdominal cramps. A relevant history included unprotected receptive anal intercourse two months before symptom onset. He initially sought care at a primary health unit, where he received multiple empirical antimicrobial treatments (albendazole, metronidazole, ciprofloxacin) without improvement. Complete blood count and biochemical tests were unremarkable. Serologic testing for HIV, syphilis, and viral hepatitis, as well as colonoscopy, were ordered. The VDRL test was reactive at a titer of 1:32, while other serologies were negative. Colonoscopy showed sigmoid and rectal mucosa with erythema, friability, and shallow erosions, suggestive of ulcerative colitis. Biopsy revealed chronic colitis with neutrophilic cryptitis, presence of spirochetes along the luminal surface of the colonic and rectal mucosa, regenerative epithelial changes, and reactive lymphoid aggregates, suggesting intestinal spirochetosis. The patient was treated with a single dose of benzathine penicillin 2,400,000 IU, with complete resolution of symptoms. Proctitis may occur during the secondary stage of syphilis, following initial infection and systemic dissemination of T. pallidum . Intestinal syphilis should be considered in men and women with high-risk sexual behaviors who present with lower gastrointestinal symptoms or mucosal abnormalities. Low clinical suspicion often leads to unnecessary empirical antimicrobial treatments, delays appropriate therapy, and poses a public health concern due to ongoing disease transmission. Taking a detailed sexual history and performing serologic testing remain crucial tools for the prompt and accurate management of chronic, unexplained cases such as the one presented here.