Hydatid cysts result from infection by the Echinococcus tapeworm species and can result in cyst formation anywhere in the body.
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Epidemiology
Cystic echinococcosis has a worldwide geographical distribution. The Mediterranean basin is an important endemic area 6,7.
Pathology
Two main species of Echinococcus tapeworm result in human disease 1,2:
-
Echinococcus granulosus
more common
pastoral: the dog is the main host; most common form
sylvatic: the wolf is the main host
-
Echinococcus alveolaris/multilocularis
less common but more invasive
fox is the main host
Definitive hosts are carnivores (e.g. dogs, foxes, cats), and the intermediate hosts are most commonly sheep. Humans are accidental hosts, and the infection occurs by ingesting food contaminated with Echinococcus eggs 3.
Cyst structure
The cysts usually have three components 1,2,10,11:
pericyst: dense fibrous capsule produced by the host
exocyst (laminated membrane): semi-permeable acellular mucin-based membrane produced by the parasite
endocyst: germinal layer of the parasite, which produces daughter cysts containing protoscolices (larval parasite) on its internal surface and laminated membrane on its outer surface
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 the 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 Organisation 2001 classification of hepatic hydatid cysts.
Location
hepatic hydatid infection: most common organ (76% of cases) 1,5
pulmonary hydatid infection: second most common organ (15% of cases)
splenic hydatid infection: third most common organ (5% of cases) 8
mediastinal hydatid infection (very rare) 9
Markers
Radiographic features
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.
Ultrasound
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-tumour lesion
stage 5: calcified or partially calcified lesion (inactive cyst)
Treatment and prognosis
Four treatment options are available 7:
surgical excision
PAIR (puncture, aspiration, injection of protoscolicidal agent and reaspiration)
chemotherapy with an anti-helminthic agent (albendazole, mebendazole)
watch and wait for inactive and silent cysts
Treatment outcomes were improved when surgery or PAIR was combined with benzimidazole given before and after surgery 7. Regarding medical management, higher scolicidal and anti-cystic activity was seen in combination therapy with albendazole plus praziquantel and was more likely to result in cure or improvement 7.