Hepatic hydatid infection

Hepatic hydatid disease is a parasitic zoonosis caused by the Echinococcus tape worm. (see Hydatid disease for a general discussion). In the liver, two agents are recognised as causing disease in the human: 

• Echinococcus granulosus 
• Echinococcus alveolaris

Echinococcus granulosus

E. granulosus is endemic in North America and Australia, and is commonly seen in the liver. It typically forms 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).

Radiographic features

Plain film

May show a curvilinear or ring calcific shadow overlying the liver due to calcification of the pericyst ³. 

Ultrasound

Appearences can vary. May show a double echogenic shadow due to the pericyst ³.

CT

Fluid density cyst, with frequent peripheral focal areas of calcification. Septation and daughter cysts may be visualised. Fluid is of variable density depending on the amount of proteinacous debris. May also show hyperdense internal septa representing a spoke wheel pattern within a cyst.

MRI

• T1 - mixed low signal (depending on the amount of proteinacous cellular debris)
• T1 C+ (Gd) - the walls and septae enhance
• T2 - mixed high signal (depending on the amount of proteinacous cellular debris), septae and daughter cysts are well visualised (especially on single shot T2 sequences)

Complications

Rupture

• into biliary tree
• to peritoneal space (if exophytic)
• into blood stream 

Hepatic alveolar echinococcosis (HAE)

Hepatic alveolar echinococcosis (HAE) (sometimes referred to as E. alveolaris - although although this is not the species name) is a rare cause of hepatic hydatid disease caused by Echinococcus multilocularis. It mimics a slow-growing tumour, as in contrast to E. granulosus it does not form a well defined encapsulated mass, but rather infiltrates the liver and its surrounding structures, especially at the porta hepatis (hepatic veins, inferior vena cava (IVC), and the biliary tree). 

Its definitive host (adult parasite) is the Red Fox (Vulpes vulpes), (sometimes cats and dogs as well) with humans as the accidental intermediate host. It is widely distributed throughout the northern hemisphere.

Radiographic features

It usually presents as a large ( ~ 10 cm) multiloculated / confluent necrotic mass without a fibrous capsule. It has irregular margins with nodular components, but does not demonstrate central enhancment. Up to 38% of cases have peripheral liver reactive enhancement.

Differential diagnosis of HAE

• hepatic metastases
• hepatocellular carcinoma (HCC)