Pancreatobiliary Parasites

CT: Linear high-density foci within dilated biliary tree

US: Tubular echogenic structure, 3-6 mm in diameter, with central anechoic area (digestive tract of worm)

• Clonorchiasis: Flukes typically involve peripheral intrahepatic ducts, not GB, CBD, or PD

CT/MR: Preferential dilatation of small peripheral intrahepatic ducts with intraductal high-density foci

US: Flukes appear as echogenic filling defects in bile ducts, which float with changes in position

• Fascioliasis: Flukes usually involve large intrahepatic ducts, extrahepatic duct, and GB (after liver involvement)

CT: Low-density abscesses forming tract from entry site at Glisson capsule into liver parenchyma

– High-density foci within duct lumen represent trematodes with associated mild ductal dilatation

• Echinococcosis: Communication of hepatic hydatid cyst with small biliary radicles or rupture of cyst into bile ducts

CT: High-attenuation material within dilated bile ducts, often contiguous with wall defect in hepatic hydatid cyst

– Air-fluid level within adjacent hydatid cyst may indicate cyst infection or biliary communication

US: Linear/round filling defects within dilated bile ducts

CLINICAL ISSUES

• Ascariasis most prevalent helminth infection worldwide

• Clonorchiasis endemic in Asia and present in Western world secondary to travel and immigration

• Echinococcosis endemic in underdeveloped grazing regions (Mediterranean, Africa, South America)

• Fascioliasis affects temperate sheep-rearing areas including South America, Europe, China, Africa, and Middle East

(Left) Specimen photograph shows an Ascaris worm retrieved from a bile duct with multiple stones. These biliary parasites may grow up to 30 cm in length. If multiple, they may fill the entire bile duct and produce the so-called spaghetti sign.

(Right) Liver flukes are short, flat, somewhat transparent, and taper anteriorly. They have prominent oral and ventral suckers. (Courtesy J. Doss, MD.)

(Left) T-tube cholangiogram of a woman who recently immigrated from China and presented with RUQ pain and symptoms of cholangitis demonstrates a tubular filling defect in the proximal common bile duct from a biliary Ascaris worm.

(Right) Ultrasound of the porta hepatis shows an Ascaris worm within the dilated common duct . The worm was mobile at real-time examination. Central hypoechogenicity is thought to represent the digestive tract of the worm. (Courtesy A. Dasyam, MD.)

TERMINOLOGY

Synonyms

• Biliary ascariasis, biliary clonorchiasis, biliary fascioliasis, biliary echinococcosis

Definitions

• Biliary and pancreatic duct (PD) involvement by parasitic infections (e.g., ascariasis, clonorchiasis, echinococcosis, fascioliasis)

IMAGING

General Features

• Best diagnostic clue

Ascariasis/clonorchiasis: Longitudinal filling defect in bile ducts or PD on ERCP/MRCP

Echinococcosis: Dilated, debris-filled biliary ducts adjacent to ruptured hepatic hydatid cyst on CT, US, MR

Fascioliasis: Clustered low-density hepatic lesions forming tract from liver capsule into parenchyma on CT/MR

• Location

Ascariasis may involve entire biliary tract and PD

Clonorchiasis typically involves peripheral intrahepatic ducts, not gallbladder (GB), common bile duct (CBD), or PD (except in heavy infections)

– Small or medium-sized peripheral ducts typically diffusely dilated, while more central intrahepatic or extrahepatic ducts are normal in caliber

Echinococcosis: Any portion of biliary tree can potentially communicate with hydatid cyst

Fascioliasis: Usually large intrahepatic ducts, extrahepatic duct, and GB

• Size

Ascaris: 2-30 cm (3-6 mm thick)

Other parasites (Fasciola, Clonorchis) are smaller

• Morphology

Linear or rounded

CT Findings

• Ascariasis/clonorchiasis: Intraductal high-density foci within dilated biliary tree due to biliary worms/flukes or debris

Imaging evidence of complications, including peripancreatic inflammation due to pancreatitis, intrahepatic abscess, and abnormal biliary tree wall enhancement or heterogeneous parenchymal enhancement due to cholangitis

Characteristic pattern of biliary dilatation in clonorchiasis with preferential dilatation of small peripheral intrahepatic ducts (with sparing of more central ducts)

– Bile duct walls may be thickened due to parasite-related infection/inflammation

• Echinococcosis: High-attenuation material within dilated bile ducts, often continuous with wall defect in hepatic hydatid cyst

Air-fluid level within adjacent hydatid cyst may indicate cyst infection or biliary communication

• Fascioliasis: Clustered low-density hepatic abscesses forming a tract from entry site at Glisson capsule into parenchyma

High-density foci within duct lumen represent trematodes, usually with associated mild ductal dilatation and ductal wall thickening/hyperenhancement

MR Findings

• Ascariasis: MRCP or T2WI demonstrate dilated ducts with linear low-signal filling defects

Ascaris worms have characteristic 3-lines appearance on T2WI/MRCP with central high-signal intensity line between 2 low-signal intensity lines

• Clonorchiasis: Preferential peripheral biliary dilatation with low-signal filling defects on T2WI or MRCP

• Echinococcosis: Low-signal linear or rounded filling defects within dilated ducts, often with adjacent deformed hydatid cyst

Direct communication between adjacent irregular hydatid cyst and biliary tree may be demonstrated

• Fascioliasis: Liver lesions are low signal on T1WI and high signal on T2WI with extension from liver capsule into deeper liver

T2WI and MRCP demonstrate mild ductal dilatation with low-signal filling defects

Ultrasonographic Findings

• Grayscale ultrasound

Ascariasis: Ultrasound very sensitive for worms in biliary system, but insensitive for worms in duodenum or ampulla (sensitivity for pancreatobiliary ascariasis only 50%)

– “Bull’s-eye” appearance due to echogenic filling defect

– Tubular echogenic structure, 3-6 mm in diameter, with central anechoic area (digestive tract of worm)

– Motility of worms may be evident, and worms that have not moved for 10 days are usually dead

– May fill entire bile duct when multiple, producing spaghetti or railway track sign

– No acoustic shadowing