Cystoisospora belli infection in an immunocompetent patient.

Cystoisospora belli, also known as Isospora belli, is an opportunistic pathogen which predominantly affects individuals with compromised immunity, including transplant recipients and patients with AIDS, HTLV-1 infections, or hematologic malignancies. The primary symptom of C. belli infection is diarrhea, but the rarity of C. belli infection makes C. belli an uncommon etiology in patients with diarrhea. Only sporadic cases of C. belli infection have been documented in the past century. Diagnosis of C. belli infection in non-endemic regions poses a challenge and delays can escalate mortality risks. This report detailed a unique case of C. belli infection from Guangxi, China involving a male with an intact immune system and no underlying tumor who presented with prolonged diarrhea and bile duct dilatation. C. belli was identified in bile based on acid-fast staining, a rare diagnosis in mainland China and globally. The patient was treated with sulfamethoxazole tablets following diagnosis and responded favorably with significant clinical improvement. This case underscores the importance of considering C. belli in the differential diagnosis of patients with unexplained diarrhea and hepatic ductal dilatation, even though the patient had normal immune function. This finding highlights the need for increased awareness and diagnostic vigilance in both endemic and non-endemic regions.