Hepatic hydatid infection

Hepatic hydatid disease is a parasitic zoonosis caused by the Echinococcus tapeworm. In the liver, two agents are recognized as causing disease in humans:

• Echinococcus granulosus

• Echinococcus multilocularis

For a general discussion, and links to other system-specific manifestations, please refer to the article on hydatid disease. For a more specific discussion related to the invasive pattern attributed to the E. multilocularis infection,  please refer to the article on alveolar echinococcosis.

Pathology

The parasite E. granulosus is endemic in North America and Australia and is commonly seen in the liver. It typically forms a spherical, fibrous-rimmed cyst with little, if any, surrounding host reaction. Classically it has a large parent cyst within which numerous peripheral daughter cysts are present. Satellite daughter cysts (outside the parent cyst) are seen frequently (~16% cases).
There are two forms of E. granulosus:

• pastoral: the most common form; the domestic dog is the main host 

• sylvatic: wolf or dog is the main host

The E. multilocularis definitive host (adult parasite) is the red fox (Vulpes vulpes) (sometimes cats and dogs as well), with humans serving as the accidental intermediate host. It is widely distributed throughout the Northern hemisphere.

Radiographic features

This article will discuss the most common presentation of the hepatic hydatid disease, characterized by well-defined encapsulated cystic or multicystic masses related to E. granulosus. For a specific discussion on the less common invasive form, caused by E. multilocularis, please refer to the article on alveolar echinococcosis. 

Plain radiograph

May show a curvilinear or ring calcific shadow overlying the liver due to calcification of the pericyst.

Ultrasound

Septated cyst with "daughter" cysts and echogenic material between the cysts. Appearances can vary. May show a double echogenic shadow due to the pericyst. The stage of the cyst may be classified on ultrasound, see: World Health Organization 2001 classification of hepatic hydatid cysts.

CT

Fluid density cyst, with frequent peripheral focal areas of calcification, usually indicates no active infection if completely circumferential. Septa and daughter cysts may be visualized. The water-lily sign indicates a cyst with a floating, undulating membrane, caused by a detached endocyst. May also show hyperdense internal septa within a cyst showing a spoke wheel pattern. The fluid is of variable attenuation, depending on the amount of proteinaceous debris. May show dilated intrahepatic bile ducts due to compression or rupture of the cyst into bile ducts.

MRI

• T1: mixed low signal (depending on the amount of proteinaceous cellular debris)

• T2: mixed high signal (depending on the amount of proteinaceous cellular debris), septa and daughter cysts are well visualized (especially on single-shot T2 sequences)

• T1 C+ (Gd): enhancing walls and septa

Treatment and prognosis

Complications

Hepatic cystics can rupture into the:

• biliary tree

• peritoneal space (if exophytic)

• bloodstream

• lung ⁵

^{See also}

• Hydatid cyst signs