Microbiology & parasitology

Parasites-hepatobiliary

Echinococcus

Author: Haind Fadel, M.D.

Topic Completed: 1 January 2015

Minor changes: 29 March 2021

Copyright: 2003-2022, PathologyOutlines.com, Inc.

PubMed Search: Echinococcus[TI] cestodes[TIAB]

Page views in 2021: 4,963

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Table of Contents

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

Epidemiology

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

Clinically may resemble carcinoma
Parasitic flatworm (tapeworm) of class Cestoda (i.e. a cestode) (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
Member of family Taeniidae due to presence of an "armed rostellum", i.e. 2 rows of hooklets (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
Echinococcus granulosus is a complex of closely related organisms (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
- E. granulosus sensu stricto is most common species causing human disease worldwide
- E. canadensis and E. ortleppi also cause human disease
- Species are differentiated via molecular means
Alveolar echinococcosis is caused by infection with larval stage of E. multilocularis, a 1 - 4 millimeter long tapeworm found in foxes, coyotes, dogs (definitive hosts)
- E. multilocularis human infection is less common but has more aggressive and invasive growth that resembles a tumor and is not contained within a large parent cyst (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
E. oligarthra and E. vogeli are rare causes of human echinococcosis in South and Central America (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])

Diagnosis

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

25 year old man with abdominal pain, syncope and hypotension (N Engl J Med 2015;372:265)
Echinococcus found in a single liver cyst (Pritt: Creepy Dreadful Wonderful Parasites Blog - Case of the Week 514 [Accessed 2 November 2018])
Objects seen in liver cyst aspirate (Pritt: Creepy Dreadful Wonderful Parasites Blog - Case of the Week 537 [Accessed 19 April 2019])

Treatment

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

Daughter cysts

Microscopic (histologic) description

Rostellum: knob-like protrusion at extreme anterior end of tapeworm as an extension of the tegument (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
Echinococcus has 2 rows of hooklets, 1 large and 1 small (i.e. armed rostellum) (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
Cysts of E. granulosis wall has 3 layers:
- Germinal layer (innermost) contains nuclei and gives rise to brood capsules
- Future heads of the adult tapeworm (protoscolices) develop within brood capsules
- Protosolex consists of a double row of refractile, birefringent, acid fast hooklets and 4 round suckers
- Daughter cysts are detached brood capsules
- Calcification in germinal layer indicates that cyst is dead
- Hydatid sand: mixture of disintegrating protoscoleces with free hooklets and calcareous corpuscles (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
- Laminated membrane (middle layer) is avascular, eosinophilic, refractile and chitinous and does not contain nuclei
- Advential layer (outer; pericyst) consists of dense fibrovascular tissue with variable numbers of inflammatory cells
- Adjacent liver often shows changes of pressure atrophy with a moderate portal infiltrate with increased eosinophils
- Differentiate from E. multilocularis because of abundant protoscoleces / hooklets and presence within a single cyst (Pritt: Creepy Dreadful Wonderful Parasites Blog - Answer to Case 514 [Accessed 1 November 2018])
Cysts of E. multilocularis:
- Has a thin, laminated wall
- Germinal layer is absent or very attenuated; forms small vesicles that invade adjacent structures, result in an infiltrative multilocular fibrotic mass that has the capability of "metastasizing"
- Inner portion may be necrotic
- Calcification in 70%
- Brood capsules and protoscolices seen in < 10%