Hydatid disease

Hydatid cysts result from infection by the Echinococcus worm, and can result in cyst formation anywhere in the body. 

Notable sites include 

• hepatic hydatid infection : commonest organ ¹
• pulmonary hydatid infection : 2^nd commonest organ
• splenic hydatid infection : 
• CNS hydatid infection :
• retroperitoneal hydatid infection :
• renal hydatid infection :

There are two main strains which are

• Echinococcus granulosus : commoner 
  • defnitive host : dog / occasionally another carvinore
  • intermediate host : sheep
• Echinococcus alveolaris / multilocularis : less common but more invasive

Pathology

Cyst structure

The cysts usually have three components

• pericyst : composed of inflammatory tissue of host origin
• exocyst : 
• endocyst : scolices (the larval stage of the parasite) and the laminated membrane are produced here

Cyst classification

Based on morphology the cyst can be classified into 4 different types ²

• type I : simple cyst with no internal architecture
• type II : cyst with daughter cyst(s) + matrix
  • type IIa : round daughter cysts at periphery
  • type IIb : larger, irregularly shaped daughter cysts occupying almost the entire volume of the mother cyst
  • type IIc : oval masses with scattered calcifications and occasional daughter cysts
• type III : calcified cyst (dead cyst)
• type IV : complicated cyst : e.g. ruptures cyst

Markers

• Casoni skin test

Radiographic features

Refer to individual articles