Isolated Colonic Strongyloidiasis Presenting as Iron Deficiency Anemia with Negative Duodenal Biopsies and Stool Studies

Background Strongyloides stercoralis is a parasitic nematode capable of lifelong autoinfection, often resulting in delayed diagnosis because of nonspecific symptoms and the low sensitivity of stool testing. Isolated colonic involvement is uncommon and may be overlooked in the absence of hyperinfection. Case Presentation A 67-year-old man with longstanding iron deficiency anemia presented with fatigue and intermittent eosinophilia. Upper endoscopy with duodenal biopsies was unrevealing. Colonoscopy demonstrated diffuse polypoid lesions with surrounding erythema and edema throughout the colon. Histopathologic examination of colonic biopsies revealed invasive Strongyloides stercoralis larvae with marked eosinophilic inflammation. Stool ova and parasite examinations were negative on two occasions, whereas Strongyloides IgG serology was positive. No evidence of hyperinfection or disseminated disease was identified. The patient was treated with ivermectin. Conclusion This case highlights a rare presentation of isolated colonic strongyloidiasis causing chronic iron deficiency anemia. Negative stool studies and unrevealing duodenal biopsies did not exclude infection, with diagnosis established only through colonic histopathology. Strongyloidiasis should be considered in patients with unexplained eosinophilia, iron deficiency anemia, or eosinophilic colitis despite negative stool studies and unrevealing upper gastrointestinal biopsies.