Echinococcus/Echinococcosis

Echinococcosis is a parasitic disease caused by Echinococcus tapeworms. Infection most often occurs from the ingestion of Echinococcus eggs in food or water contaminated with dog feces. Signs and symptoms are caused by hydatid cyst development in visceral organs and depend on the species. E. granulosus causes cystic echinococcosis, which can involve any organ. The most notable presentations involve the liver or lungs, resulting in RUQ abdominal pain, hepatomegaly, cough, or dyspnea. E. multilocularis causes alveolar echinococcosis, which typically involves the liver. Serology and imaging may be used for the diagnosis, the latter of which can show characteristic findings of hydatid cysts. Management depends on the size and complexity of the cysts but can involve observation, anthelmintic therapy, percutaneous drainage, or surgery.

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General Characteristics and Epidemiology

General features of Echinococcosis

Echinococcosis is a parasitic disease caused by Echinococcus tapeworms. Features include:

Eggs:

  • Small
  • Round
  • Thick-shelled
  • Contain a 6-hooked (hexacanth) embryo

Adults:

  • Small (1.2–7 mm long, depending on the species)
  • Ribbon-like body shape:
    • Scolex: 
      • Hook-filled rostellum
      • 4 lateral suckers
    • Neck
    • Strobila with 3–6 proglottids:
      • Segmented
      • Contain sets of reproductive organs
  • Can reproduce by self-fertilization
  • Absorb nutrients:
    • No mouth or anus
    • No digestive system

Clinically relevant species

  • Cystic echinococcosis: E. granulosus (most common)
  • Alveolar echinococcosis: E. multilocularis
  • Polycystic echinococcosis:
    • E. vogeli
    • E. oligarthrus

Epidemiology

E. granulosus:

  • Geographic distribution:
    • South America
    • Middle East
    • Eastern Mediterranean
    • Sub-Saharan Africa
    • Western China
    • Australia and New Zealand
  • Prevalence: 2%–6% in endemic regions
  • Incidence: Approximately 50 cases per 100,000 persons per year in endemic regions
  • Average age: 30–40 years

E. multilocularis:

  • Geographic distribution:
    • Northern and Central Europe
    • Siberia
    • Asia
    • North America
  • Incidence: 1–20 cases per 100,000 persons per year in endemic regions
  • Average age: > 50 years

E. vogeli and E. oligarthrus:

  • Geographic distribution: Central and South America
  • Rare

Pathogenesis

Hosts

E. granulosus:

  • Definitive hosts: dogs
  • Intermediate hosts:
    • Sheep
    • Horses
    • Goats
    • Deer
    • Camels
  • Humans are incidental hosts.

E. multilocularis:

  • Definitive hosts: 
    • Dogs
    • Foxes
    • Coyotes
  • Intermediate hosts: rodents
  • Humans are incidental hosts.

Transmission

Transmission occurs through ingestion of eggs, usually from food or water contaminated with animal feces.

Life cycle

E. granulosus:

  • Adult tapeworm dwells in the definitive host’s small intestine → releases eggs → passed in feces
  • Ingested by an intermediate host → eggs hatch in the small intestine
  • Oncospheres penetrate the intestinal wall → bloodstream → reach visceral organs
  • Development of hydatid cyst → produces protoscolices and daughter cysts
  • Definitive host ingests infected organs.
  • Protoscolices leave cyst → attach to the intestine → develop into adult worms → cycle continues
Echinococcus granulosus Life Cycle Echinococcosis

The life cycle of Echinococcus granulosus

Image: “Life Cycle of Cystic Echinococcosis” by CDC. License: Public Domain

E. multilocularis:

  • Adult tapeworm dwells in the definitive host’s small intestine → releases eggs → passed in feces
  • Ingested by an intermediate host → eggs hatch in the small intestine
  • Oncospheres penetrate the intestinal wall → bloodstream → reach visceral organs
  • Development of mutilocular, thin-walled (alveolar) hydatid cyst → proliferation by outward budding → production of protoscolices
  • Definitive host ingests infected organs. 
  • Protoscolices leave cyst → attach to the intestine → develop into adult worms → cycle continues
Echinococcus multilocularis Life Cycle Echinococcosis

The life cycle of Echinococcus multilocularis

Image: “Alveolar Echinococcosis (Echinococcus multilocularis)” by CDC. License: Public Domain

Pathophysiology

  • Human ingestion of eggs → release of oncospheres in the intestine
  • Migration to visceral organs → hydatid cysts
  • Cyst rupture → protoscolices are released → secondary cysts can develop in other sites

Clinical Presentation

Both cystic and alveolar echinococcosis are characterized by asymptomatic incubation periods (months to years).

Cystic echinococcosis

Symptoms depend on:

  • Parasite load
  • Site of cysts (any organ can be infected)
  • Size of cysts (can cause complications from mass effect and obstruction)
  • Note: Rupture of cysts can cause anaphylactic shock.

Liver (75% of cases):

  • General signs and symptoms:
    • Abdominal pain 
    • Nausea and vomiting
    • Hepatomegaly
    • Abdominal mass
    • Jaundice
    • Fever
  • Complications of cyst rupture into the biliary tree:
    • Biliary colic
    • Obstructive jaundice
    • Cholangitis
    • Pancreatitis
  • Complications of mass effect on bile ducts or veins:
    • Cholestasis
    • Portal hypertension
    • Budd–Chiari syndrome

Lungs (5%–15% of cases):

  • General signs and symptoms:
    • Chronic cough 
    • Dyspnea 
    • Chest pain
    • Hemoptysis
  • Complications of cyst rupture:
    • Pneumothorax
    • Pleural effusion
    • Secondary bacterial infection

Brain: 

  • Headache
  • Dizziness 
  • Decreased level of consciousness
  • Seizures

Other organ systems:

  • Heart: pericardial tamponade
  • Kidneys: hematuria
  • Bones: pathologic fractures
  • Eyes: ocular cysts

Alveolar echinococcosis

  • Malaise
  • Weight loss
  • RUQ pain
  • Hepatomegaly
  • Jaundice
  • Cholangitis
  • Portal hypertension
  • Budd–Chiari syndrome

Diagnosis

Imaging

Imaging of hydatid cysts is the mainstay of diagnosis.

Ultrasonography: 

  • 90%–95% sensitivity
  • Findings for cystic echinococcosis:
    • Round, anechoic, smooth cyst
    • Internal septations → daughter cysts
    • Fine, echogenic contents (“hydatid sand”) → protoscolices
    • “Eggshell calcifications” → cysts with calcified rim
  • Findings for alveolar echinococcosis:
    • Irregular cysts without well-defined walls
    • Central necrosis
    • Irregular wall calcifications

CT:

  • Best for determining the number, size, and location of cysts
  • Better than ultrasound in:
    • Detecting extrahepatic cysts
    • Assessing for complications (e.g., rupture)

MRI:

  • Not usually required
  • No major advantage over CT

Serology

Serologic tests are used for diagnosing echinococcosis and for monitoring after treatment.

  • More sensitive for E. multilocularis than for E. granulosus
  • Options:
    • Indirect hemagglutination 
    • ELISA 
    • Indirect immunofluorescence 
    • Immunoblot
    • Latex agglutination

Management and Prevention

Management

There are 4 management strategies: observation, medical therapy, percutaneous drainage, and surgery.

Observation is appropriate:

  • With inactive liver cysts
  • In the absence of complications

Medical therapy:

  • Definitive therapy for small, single-compartment cysts
  • Often used as adjunctive therapy with percutaneous drainage and surgery
  • Options:
    • Albendazole (preferred)
    • Mebendazole
    • Praziquantel

Percutaneous drainage:

  • Involves drainage of fluid and injection of hypertonic saline (scolicidal) into the cyst cavity
  • Risk of anaphylaxis

Surgery:

  • Treatment of choice for complicated cysts
  • Goal is to remove the whole cyst.
  • Hypertonic saline is injected into the cyst before attempting surgical excision.

Prevention

  • Do not allow dogs to feed on livestock or rodents.
  • Control stray dog populations.
  • Avoid contact with foxes, coyotes, and stray dogs.
  • Wash hands after contact with dogs.
  • Improve water sanitation.
  • Avoid consumption of contaminated food.

Comparison of Tapeworm Species

Table: Characteristics and diseases of different tapeworm species
OrganismDiphyllobothrium latumTaenia saginataEchinococcus granulosus
Characteristics
  • Approximately 10 m long
  • No hooks
  • Bothria present
  • > 3000 proglottids
  • < 5 m long
  • No hooks
  • No neck
  • Approximately 1000 proglottids
  • 2–7 mm long
  • Hooks present
  • 3–6 proglottids
TransmissionEating raw infected fishEating raw infected beefFecal–oral (ingestion of contaminated food or water)
DiseaseDiphyllobothriasisTaeniasisCystic echinococcosis
Clinical
  • Abdominal discomfort
  • Weight loss
  • Vitamin B12 deficiency
  • Bowel obstruction
  • Usually asymptomatic
  • Mild GI symptoms
Depends on location and size of hydatid cysts
DiagnosisEggs or proglottids in stoolEggs or proglottids in stool
  • Imaging
  • Serology
Management
  • Praziquantel
  • Niclosamide
  • Praziquantel
  • Niclosamide
  • Albendazole
  • Percutaneous drainage
  • Surgical excision
Prevention
  • Freeze fish.
  • Thoroughly cook fish.
  • Water sanitation measures
Beef should be cooked thoroughly.
  • Personal hygiene
  • Avoid contact with stray dogs.
  • Avoid potentially contaminated food.
  • Improve water sanitation.

Differential Diagnosis

  • Ascariasis: infection caused by the parasitic roundworm Ascaris lumbricoides: Transmission occurs from ingestion of water or food that is contaminated with Ascaris eggs. Patients may be asymptomatic or may experience cough and hemoptysis. A large worm burden can cause intestinal obstruction and impair growth in children. Examination of the stool may show the presence of worms or eggs.  Management is with anthelmintic therapy.
  • Hepatocellular carcinoma: the most common primary liver cancer: usually arises in a chronically diseased or cirrhotic liver: Constitutional symptoms are rare and RUQ pain does not often occur. Imaging will show a well-defined mass with enhancement during the arterial phase and washout during the venous phase. The mainstay of treatment is liver resection.
  • Pyogenic liver abscess: polymicrobial infection that arises from contiguous or hematogenous spread: Patients can present with a triad of fever, malaise, and RUQ pain. Imaging will reveal solitary or multiple lesions on ultrasonography or CT scan. These lesions are generally well defined and rim enhancing on contrast imaging.  Diagnosis requires aspiration with Gram stain and culture. A combination of drainage and IV antibiotic therapy is the primary method of treatment.
  • Cirrhosis: late stage of hepatic necrosis and scarring: Symptoms of cirrhosis are often nonspecific (e.g., fatigue, anorexia, weight loss). Decompensation occurs late in the disease, with manifestations including jaundice, ascites, portal hypertension, and liver failure. Unlike in echinococcosis, ultrasonography will show nodularity of the liver. Diagnosis often requires liver biopsy. Management is mostly supportive, with liver transplantation being the only curative treatment.
  • Lung cancer: malignant transformation of lung tissue: Symptoms include cough, dyspnea, weight loss, and chest discomfort. Regional and metastatic spread cause additional symptoms and complications depending on the location and organs affected. Definitive diagnosis and staging are made by biopsy, genetic mutation with biomarker testing, and imaging. Management is guided by the cancer stage and associated molecular profile.
  • Tuberculosis: disease caused by Mycobacterium tuberculosis: Symptoms include fever, productive cough, night sweats, and weight loss. Cavitary lung lesions, which could resemble a hydatid cyst, may be seen on imaging. Diagnosis is made with identification of acid-fast bacilli on sputum culture. Multiple antimicrobial medications are required for management, including isoniazid, rifampin, pyrazinamide, and ethambutol.

References

  1. World Health Organization. (n.d.). Echinococcosis  https://www.who.int/health-topics/echinococcosis/
  2. Ravis, E., Theron, A., Lecomte, B., Gariboldi, V. (2018). Pulmonary cyst embolism: a rare complication of hydatidosis. Eur J Cardiothorac Surg 53(1):286–287. http://reference.medscape.com/medline/abstract/28977397
  3. Wang, N., Zhong, X., Song, X., et al. (2017). Molecular and biochemical characterization of calmodulin from Echinococcus granulosus. Parasit Vectors 10(1):597. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5716380/
  4. Siracusano, A., Delunardo, F., Teggi, A., Ortona, E. (2012). Host-parasite relationship in cystic echinococcosis: an evolving story. Clin Dev Immunol 2012:639362. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3206507/
  5. Wang, K., Zhang, X., Jin, Z., Ma, H., Teng, Z., Wang, L. (2013). Modeling and analysis of the transmission of Echinococcosis with application to Xinjiang Uygur Autonomous Region of China. J Theor Biol 333:78–90. http://reference.medscape.com/medline/abstract/23669505
  6. Torgerson, P. R. (2013). The emergence of echinococcosis in Central Asia. Parasitology 140(13):1667–1673. https://www.zora.uzh.ch/id/eprint/78236/
  7. Moro, P. L. (2019). Epidemiology and control of echinococcosis. In Baron, E. L. (Ed.), UpToDate. Retrieved April 16, 2021, from https://www.uptodate.com/contents/epidemiology-and-control-of-echinococcosis
  8. Moro, P. L. (2019). Clinical manifestations and diagnosis of echinococcosis. In Baron, E. L. (Ed.), UpToDate. Retrieved April 16, 2021, from https://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-echinococcosis
  9. Moro, P.L. (2020). Treatment of echinococcosis. In Baron, E. L. (Ed.), UpToDate. Retrieved April 16, 2021, from https://www.uptodate.com/contents/treatment-of-echinococcosis
  10. Pearson, R. D. (2020). Echinococcosis. MSD Manual Professional Version. Retrieved April 16, 2021, from https://www.msdmanuals.com/professional/infectious-diseases/cestodes-tapeworms/echinococcosis
  11. Huzaifa, M., Sharman, T. (2020). Echinococccus. StatPearls. Retrieved April 16, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK557623/
  12. Centers for Disease Control and Prevention (2019). Parasites—Echinococcosis. Retrieved April 19, 2021, from https://www.cdc.gov/parasites/echinococcosis/biology.html
  13. The Australian Society for Parasitology Inc. (n.d.). Echinococcus. Retrieved April 19, 2021, from http://parasite.org.au/para-site/text/echinococcus-text.html

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