Hydatid cysts result from infection by the Echinococcus, and can result in cyst formation anywhere in the body.
There are two main species of the Echinococcus tapeworm 1,2:
- pastoral: dog is a main host; most common form
- sylvatic: wolf is a main host
- 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.
The cysts usually have three components 1,2:
- pericyst: composed of inflammatory tissue of host origin
- endocyst: scolices (the larval stage of the parasite) and the laminated membrane are produced here
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.
- hepatic hydatid infection: most common organ, 76% of cases involve the liver 1,5
- pulmonary hydatid infection: second most common organ
- splenic hydatid infection
- CNS hydatid infection
- spinal hydatid infection
- retroperitoneal hydatid infection
- renal hydatid infection
- musculoskeletal hydatid infection
A chest film or other plain films can be the first diagnostic modality when echinococcosis is suspected, depending on clinical indications.
CT and MRI imaging are indicated when considering surgical treatment, particularly in regions like the brain, spine and locations inaccessible for conventional radiography or ultrasound, or in case of diagnostic uncertainty.
The Gharbi ultrasound classification consists of five stages 4:
- stage 1: homogeneously hypoechogenic cystic thin-walled lesion
- stage 2: septated cystic lesion
- stage 3: cystic lesion with daughter lesions
- stage 4: pseudo-tumor lesion
- stage 5: calcified or partially calcified lesion (inactive cyst)
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