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Echinococcosis (Hydatid Disease)

by Richard D. Pearson, MD

Echinococcosis is infection with larvae of Echinococcus granulosus or E. multilocularis (alveolar hydatid disease). Symptoms depend on the organ involved—eg, jaundice and abdominal discomfort with liver cysts or cough, chest pain, and hemoptysis with lung cysts. Cyst rupture can cause fever, urticaria, and serious anaphylactic reactions. Diagnosis is with imaging, examination of cyst fluid, or serologic tests. Treatment is with albendazole, surgery, or both or with cyst aspiration and instillation of a scolicidal agent.

Etiology

E. granulosus is common in sheep-raising areas of the Mediterranean, Middle East, Australia, New Zealand, South Africa, and South America. Canines are definitive hosts and herbivores (eg, sheep, horses, deer) or humans are intermediate hosts. Foci also exist in regions of Canada, Alaska, and California.

E. multilocularis worms are present in foxes, and the hydatid larvae occur in small wild rodents. Infected dogs and other canines are the main link to occasional human infection. E. multilocularis occurs mainly in Central Europe, Alaska, Canada, and Siberia. Its range of natural infection in the continental US extends from Wyoming and the Dakotas to the upper Midwest.

Rarely, E. vogelii or E. oliganthus causes polycystic hydatid disease in humans, primarily in the liver. These species occur in Central and South America.

Pathophysiology

Ingested eggs from animal feces (which may be present on the fur of dogs or other animals) hatch in the gut and release oncospheres (immature forms of the parasite enclosed in an embryonic envelope). Oncospheres penetrate the intestinal wall, migrate via the circulation, and lodge in the liver or lungs or, less frequently, in the brain, bone, or other organs.

In tissue , E. granulosus oncospheres develop into cysts, which grow slowly (usually over many years) into large unilocular, fluid-filled lesions—hydatid cysts. Brood capsules containing numerous small infective protoscolices form within these cysts. Large cysts may contain >1 L of highly antigenic hydatid fluid as well as millions of protoscolices. Daughter cysts sometimes form in or outside primary cysts. If a cyst in the liver leaks or ruptures, infection can spread to the peritoneum.

E. multilocularis produces spongy masses that are locally invasive and difficult or impossible to treat surgically. Cysts occur primarily in the liver but can occur in the lungs, or other tissues. The cysts are not large, but they invade and destroy surrounding tissue and can cause liver failure and death.

Symptoms and Signs

Although many infections are acquired during childhood, clinical signs may not appear for years, except when cysts are in vital organs. Symptoms and signs may resemble those of a space-occupying tumor.

Liver cysts eventually cause abdominal pain or a palpable mass. Jaundice may occur if the bile duct is obstructed. Rupture into the bile duct, peritoneal cavity, or lung may cause fever, urticaria, or a serious anaphylactic reaction.

Pulmonary cysts can rupture, causing cough, chest pain, and hemoptysis.

Diagnosis

  • Imaging

  • Serologic testing

  • Examination of cyst fluid

Pulmonary cysts are usually discovered on routine chest x-ray as round, often irregular pulmonary masses.

CT, MRI, and ultrasound findings may be pathognomonic if daughter cysts and hydatid sand (protoscolices and debris) are present, but simple hydatid cysts may be difficult to differentiate from benign cysts, abscesses, or benign or malignant tumors. The presence of hydatid sand in aspirated cyst fluid is diagnostic.

Serologic tests (enzyme immunoassay, immunofluorescent assay, indirect hemagglutination assay) are variably sensitive but are useful if positive and should be done. CBC may detect eosinophilia.

Treatment

  • Surgical removal or percutaneous aspiration followed by instillation of a scolicidal agent and reaspiration

  • Sometimes albendazole

Treatment varies depending on the type, location, and size of the cyst and on complications. Surgery, sometimes via laparoscopy, can be curative. Albendazole is often given before surgery to prevent metastatic infections that can occur if cyst contents spill during the procedure. In some centers, percutaneous aspiration under CT guidance is done, followed by instillation of a scolicidal agent (eg, hypertonic saline) and reaspiration (PAIR [percutaneous aspiration-injection-reaspiration]).

Pearls & Pitfalls

  • Take care to avoid leakage of cyst contents during aspiration or surgery because metastatic infection can occur.

For E. granulosis, albendazole 400 mg po bid for 1 to 6 mo (7.5 mg/kg bid in children up to a maximum of 400 mg bid) is curative in 30 to 40% of patients and can be used to suppress growth in inoperable cases.

Prognosis for patients with E. multilocularis infection is poor unless the entire larval mass can be removed. Surgery is indicated if it is feasible, which depends on the size, location, and manifestations of the lesion. Albendazole in the above doses can suppress growth of inoperable lesions. Liver transplantation has been lifesaving in a few patients.

Key Points

  • Echinococcosis occurs when ingested tapeworm eggs hatch, releasing oncospheres, which migrate into the liver or lungs or, less frequently, to the brain, bone, or other organs and develop into cysts; no worms are present in the GI tract.

  • The cysts develop slowly (usually over many years) into large (up to 1 L), fluid-filled cysts (hydatid cysts), which contain numerous infective protoscolices.

  • Feces from infected dogs (and other canines) are the main source of human infection.

  • Liver cysts cause pain and sometimes jaundice; lung cysts can cause pain, cough, and hemoptysis.

  • E. multilocularis does not produce large cysts but invades and destroys surrounding tissue and can result in liver failure and death.

  • Diagnose by analysis of cyst fluid and serologic testing.

  • Treatment varies depending on the organism, cyst size and location, and complications; it may include surgery, cyst aspiration and instillation of a scolicidal agent, and/or prolonged treatment with albendazole.

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