S3672 Refractory Lymphocytic Colitis in a Patient With Chronic Diarrhea: Unmasking Protozoan Infection Refractory Lymphocytic Colitis in a Patient With Chronic Diarrhea: Unmasking Protozoan Infection

Introduction: Lymphocytic colitis, a subtype of microscopic colitis, presents with chronic watery diarrhea despite a normal colonoscopic appearance. Histologically, it is marked by increased intraepithelial lymphocytes.1 Budesonide is the first-line treatment and usually results in rapid clinical remission.2 However, in rare cases of refractory disease, alternative etiologies such as enteric infections should be considered, particularly in patients with a recent history of travel. Case Description/Methods: A 59-year-old man developed persistent watery diarrhea shortly after returning from Santo Domingo. He described 3–4 loose, non-bloody stools daily, occasionally waking him from sleep, but denied fever, weight loss, or other systemic symptoms. Initial stool studies for bacterial and parasitic infections were negative. Fecal calprotectin was normal, and both upper endoscopy and colonoscopy excluded celiac disease and confirmed lymphocytic colitis, and he was started on budesonide 9 mg daily, with significant initial improvement. Symptoms recurred during tapering and persisted despite restarting the full dose, prompting further evaluation. Medication review revealed no contributing agents, and he had no further travel. His stool osmolar gap was consistent with secretory diarrhea, and repeat stool studies showed Blastocystis species and Chilomastix mesnili. A repeat colonoscopy with biopsy was planned to assess histologic response and evaluate for coexisting pathology. Discussion: This case highlights the importance of re-evaluating patients with lymphocytic colitis who experience relapse despite appropriate budesonide therapy, particularly when initial infectious studies are negative but clinical suspicion for an underlying etiology remains high. Celiac disease, which may coexist with microscopic colitis, warrants exclusion. In addition, Crohn’s disease, ulcerative colitis, and medication-related triggers must also be considered. While Blastocystis is often a commensal, it may contribute to inflammation and steroid resistance under certain circumstances. Identifying reversible factors is key to optimizing management and preventing unwarranted immunosuppression.