Hydatid disease

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

Pathology

There are two main strains which are 1,2:

  • Echinococcus granulosus: commoner
    • pastoral: dog is a main host; most common form 
    • sylvatic: wolf is a main host 
  • Echinococcus alveolaris/multilocularis: less common but more invasive; fox is a main host

Definitive hosts are carnivores (e.g. dogs, foxes, cats), and the intermediate hosts are most commonly sheep. Humans are accidental host, and the infection occurs by ingesting food contaminated with Echinococcus eggs 3.

Cyst structure

The cysts usually have three components 1,2:

  • 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 four different types 2:

  • type I: simple cyst with no internal architecture
  • type II: cyst with daughter cyst(s) and 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. ruptured cyst

For hepatic hydatid infection on ultrasound also refer to World Health Organization 2001 classification of hepatic hydatid cysts.

Location
Markers

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