A fishy situation – expanding the differential diagnosis of the fish-mouth papilla

Hydatid disease (echinococcosis) is a parasitic infection that presents with hepatic cysts and pulmonary involvement. Hepatic hydatid disease can lead to an obstruction of the biliary tree in the case of cyst rupture [ 1 ]. This case report provides endoscopic images of a hepatic hydatid cyst mimicking the fish-mouth appearance of papilla, usually pathognomonic for main duct intraductal papillary mucinous neoplasm (MD-IPMN [ 2 ]). A 35-year-old man, from North Africa, presented with acute on chronic epigastric pain. Initial work-up revealed increased alanine aminotransferase (648 IU/L) and total bilirubin (99 μmol/L) levels. A computed tomographic scan showed a multilobulated, septated, calcified lesion in the inferior left liver, suggestive of a hepatic cyst ([ Fig. 1 ]). He was later lost to follow-up. Fig. 1 A CT scan showed a multi-lobulated, septated, calcified lesion in the inferior left liver, suggestive of a hepatic cyst. CT, computed tomography. Nine years later, he returned to the emergency room with work-up confirming cholangitis. MRCP showed poorly defined obstructive filling defects within a slightly dilated bile of 8 mm in diameter ([ Fig. 2 ]). Interval growth of the hepatic cyst and left intrahepatic biliary ductal dilatation was noted. Fig. 2 MRI revealed interval growth of the hepatic cyst and left intra-hepatic biliary ductal dilatation. MRI, magnetic resonance imaging. ERCP demonstrated a white, soft, mucinous substance protruding from the ampulla ([ Video 1 ]). The endoscopic images were suggestive of a fish-mouth papilla ([ Fig. 3 ]). After biliary sphincterotomy, the spontaneous discharge of thick, white membranes occurred. Download Video Fish-mouth papilla appearance of the ampulla, and discharge of thick, white membranes after sphincterotomy.Video 1 Fig. 3 ERCP: a white, soft, mucinous substance protruding from the ampulla, suggestive of a fish-mouth papilla. Evaluation of the biliary aspirate with wet mount iodine microscopy identified numerous hooklets ([ Fig. 4 ]) and confirmed the diagnosis of a compressive echinococcal cyst. He was first treated with albendazole and then referred to hepatobiliary surgery for resection. Both ERCP and left hepatectomy conferred a significant clinical and biochemical improvement. Fig. 4 Wet mount iodine microscopy: biliary aspirate with numerous hooklets, confirming echinococcosis. A ruptured hydatid cyst can lead to the fish-mouth papilla, which is typically pathognomonic for MD-IPMN. ERCP was effective in the treatment of cholangitis in the context of biliary obstruction from hydatid cyst rupture and biliary aspirate confirmed the diagnosis. Endoscopy_UCTN_Code_CCL_1AB_2AG_3AD Publication History Article published online: 08 January 2026 © 2026. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution License, permitting unrestricted use, distribution, and reproduction so long as the original work is properly cited. (https://creativecommons.org/licenses/by/4.0/). Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany