Microbiology & parasitology



Topic Completed: 1 January 2015

Minor changes: 29 March 2021

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PubMed Search: Echinococcus[TI] cestodes[TIAB]

Haind Fadel, M.D.
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Cite this page: Fadel H. Echinococcus. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/parasitologyechinococcus.html. Accessed January 19th, 2022.
Definition / general
  • Most common cause of hepatic cysts worldwide; in US, usually in immigrants from the Middle East, Greece, Australia, North Africa, parts of South America
  • Also called echinococcal cyst (hydatid cyst)
  • Relatively common in endemic areas in liver, lung
  • Rare in bone, breast, spleen
  • Disease due to dog tapeworm in larval or cystic stage: E. granulosus (causes cystic hydatid disease), less commonly E. multilocularis (alveolar hydatid disease) or E. vogeli (polycystic hydatid disease)
  • Humans become infected by eating food contaminated with tapeworm eggs, becoming intermediate hosts (Wikipedia: Echinococcus [Accessed 5 January 2018])
Pathophysiology / etiology
  • Foxes, wolves and dogs are definitive hosts
  • Intermediate hosts include several genera of small rodents
  • Human infection occurs in liver, where hydatid develops as an invasive cyst that insinuates itself within tissue in an alveolar pattern
  • Although the germinal membrane proliferates in the human liver, protoscoleces fail to develop
Diagrams / tables

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Life cycle

Clinical features
  • By hydatid serology
  • Reliable antibody detection tests exist for E. multilocularis and E. granulosus
Radiology images

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Large primary hydatid cyst

Prognostic factors
  • Unilocular cysts appropriately treated have an excellent prognosis
  • If spillage occurs at aspiration, has recurrence rate up to 50%
  • High death rate unless radical hepatic resection eliminates all disease
Case reports
  • For simple cystic echinococcosis, open surgical removal of cysts combined with chemotherapy using albendazole or mebendazole before and after surgery is recommended (Wikipedia: Echinococcosis [Accessed 5 January 2018])
  • If cysts are in multiple organs or tissues or in risky locations, chemotherapy or PAIR (puncture, aspiration, injection, reaspiration) is recommended
    • Percutaneous or open aspiration of cyst with injection of hypertonic 20% saline (destroys scolices and cysts within 10 minutes), followed by repeat aspiration
  • Plane of dissection to excise cysts is the laminated, chitinous layer; the adventitial (outer layer of cyst) should be left intact
Gross description
  • Cysts of E. granulosis (cystic hydatid disease):
    • Cysts tend to be filled with clear fluid, solitary, unilocular, white
    • Usually involve right lobe of liver
    • May be subcapsular and pedunculated
    • Cysts can rupture into abdomen or gallbladder or through diaphragm into pleural space
    • Viable cysts are filled with a colorless fluid that contains daughter cysts and brood capsules with scolices
    • Occasionally daughter cysts are present outside the fibrous laminar layer of the cyst, referred to as extracapsular or satellite cysts
  • Cysts of E. multilocularis (alveolar hydatid disease):
    • Simulate malignant neoplasm or cirrhosis
    • Numerous small, irregular cysts, all < 2 cm
    • Border with uninvolved liver appears infiltrative
Gross images

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Fused to liver

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Daughter cysts

Microscopic (histologic) description
Microscopic (histologic) images

Contributed by Bobbi Pritt, M.D.

Classic protoscoleces with an "armed" rostellum

Numerous free hooklets in background of degenerating protoscoloces

Positive stains
  • Laminated (middle) chitinous layer stains strongly with PAS, GMS, Best carmine

Bird's eye view of Echinococcus

Differential diagnosis
  • Nonparasitic cysts
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