Table of Contents
Definition / general | Epidemiology | Pathophysiology / etiology | Diagrams / tables | Clinical features | Diagnosis | Radiology images | Prognostic factors | Case reports | Treatment | Gross description | Gross images | Microscopic (histologic) description | Microscopic (histologic) images | Positive stains | Videos | Differential diagnosis | Additional referencesCite this page: Fadel H. Echinococcus. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/parasitologyechinococcus.html. Accessed May 8th, 2021.
Definition / general
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
Images hosted on other servers:
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
Images hosted on other servers:
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
Images hosted on other servers:
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%
Microscopic (histologic) images
Contributed by Bobbi Pritt, M.D.
Classic protoscoleces with an "armed" rostellum
Numerous free hooklets in background of degenerating protoscoloces
Videos
Bird's eye view of Echinococcus
Differential diagnosis
- Nonparasitic cysts
