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Hydatid Lung Disease (Pulmonary Echinococcosis/Cystic Echinococcosis)

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1. DEFINITION

• Pulmonary hydatid disease (cystic echinococcosis, CE) is a parasitic infection of the lung caused by the larval (metacestode) cyst stage of the tapeworm Echinococcus granulosus.
• The lungs are the second most common organ involved (20%) after the liver (50-80%).
• In children, pulmonary involvement is more frequent than hepatic, due to greater lung elasticity permitting faster cyst growth.

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2. ETIOLOGY

• Caused by zoonotic cestodes of the genus Echinococcus. Four species cause human disease: E. granulosus, E. multilocularis, E. vogeli, and E. oligarthus.
• Most common pulmonary cause: E. granulosus (dog tapeworm), causative agent of CE or hydatid disease.
• E. granulosus is now recognized as a complex of 10 distinct genotypes, some elevated to distinct species.
• Geographic distribution: Worldwide; endemic in pastoral communities of South America, Australia, eastern Europe, north Africa, the Middle East, central Asia, western China, Mediterranean basin (Italy, Spain, Albania, former Yugoslavia), Central/South America, sub-Saharan Africa, China, Russia, and former Soviet countries.
• Definitive hosts: Domestic dogs, foxes, wolves, other canids.
• Intermediate hosts: Sheep, pigs, cattle, goats; humans are accidental intermediate hosts.

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3. PATHOGENESIS / LIFE CYCLE

• Definitive host (canids) ingest hydatid cysts in infected viscera of intermediate host → cyst matures into adult tapeworm in canine intestine → adult sheds embryonated eggs in feces.
• Humans are infected by ingestion of eggs in contaminated food/water or by close contact/handling of dogs.
• In the human stomach, oncospheres are released → penetrate intestinal wall → spread via bloodstream → lodge in liver or lungs (or other organs: kidney, bone, brain) → mature into metacestode cysts.
• The maturing cyst forms a space-occupying lesion with three layers:
  1. Outer layer (pericyst): Host-derived, composed of compressed lung tissue and granulomatous inflammation that progresses to fibrosis.
  2. Middle layer (laminated layer): Acellular, derived from the parasite.
  3. Inner layer (endocyst/germinal layer): Produces protoscolices, daughter cysts, and deposits hydatid sand into the enlarging cyst.
• The cyst grows slowly over years to decades until it causes symptoms by mass effect.
• Eventually the cyst may: (a) degenerate into a calcified mass, or (b) acutely rupture into a bronchus, pleural space, or adjacent structures.
• Lung disease is more common in children whose lungs accommodate faster cyst growth.

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4. CLINICAL FEATURES

Asymptomatic Presentation

• Pulmonary hydatid cysts are often asymptomatic and found incidentally on imaging.
• Children are more likely to be asymptomatic even with very large cysts.

Symptomatic - Intact/Enlarging Cyst

• Cough (most common)
• Chest pain
• Dyspnea (from mass effect of enlarging cyst)
• Hemoptysis (less frequent)
• Cysts can erode into adjacent structures causing bone pain, hemorrhage, or airway compression.

Symptomatic - Cyst Rupture (Abrupt onset of new/worsening symptoms)

• Rupture into a bronchus: Fever, cough with abrupt onset; patient may expectorate macroscopic fragments of the parasite; can become secondarily infected with bacteria and/or fungi (lung abscess).
• Rupture into the pleural space: Hypersensitivity responses (fever, urticaria, wheezing); frank anaphylaxis can develop (rarely fatal); empyema with or without bacterial superinfection.

Epidemiology of Cysts

• Usually unilateral (50-80%), solitary (60%), with predilection for the lower lobes.
• Up to 35% of individuals have concomitant hepatic involvement.
• 10-15% of individuals have more than one organ involved.
• Cysts may be ruptured (two-thirds) or unruptured (one-third) at presentation.

Laboratory

• Peripheral eosinophilia seen in fewer than 25% of cases (non-specific).

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5. DIAGNOSIS

A. Imaging

Plain Chest Radiograph

• Uncomplicated (unruptured) cysts: Well-defined, round or oval homogeneous masses of uniform density, 1-20 cm in diameter, with smooth borders and normal adjacent lung tissue.
• Cyst rupture into bronchus: Air-fluid level within the cyst due to partial discharge of cyst contents (endocyst detachment).
• "Water lily" sign (camalote sign): Pathognomonic of a collapsed cyst. Air enters the cyst → detachment of inner endocyst from outer pericyst → endocyst collapses to float on remaining fluid in the partially filled pericyst → appearance of a "water lily floating on a pond." Rare but diagnostic.
• "Meniscus sign": Air crescent between pericyst and endocyst; resembles the air crescent of a mycetoma.
• Aggressive vascular invasion may cause massive hemoptysis and hemorrhage.

CT / HRCT FINDINGS (Point-Wise)

1. Uncomplicated cysts: Well-defined, round or oval, near-water-density (0-20 HU) unilocular cyst with a smooth wall; no contrast enhancement of cyst contents; mass effect on surrounding lung.
2. Daughter cysts: CT may reveal daughter cysts (smaller cysts) within a larger parent cyst - this finding distinguishes hydatid cysts from other pulmonary cysts and is highly characteristic.
3. Pericyst wall: Smooth, well-defined outer wall (may calcify in degenerated/old cysts). Contrast enhancement can be seen around the cyst wall in secondary infection.
4. Air crescent sign: Crescent of air between pericyst and endocyst, mimicking a mycetoma on CT.
5. Air-fluid level: Present when cyst ruptures into bronchus with partial fluid evacuation.
6. "Water lily" / "camalote" sign on CT: Collapsed, floating endocyst membrane seen as a floating structure within residual cyst fluid - appears as a wavy, serpentine or lily pad-shaped membrane floating on the cyst fluid.
7. Ruptured cyst with secondary infection: Contrast enhancement around the cyst wall; communication with the bronchial tree visible; associated lung consolidation or abscess.
8. "Serpentine sign": Detached, collapsed inner membrane forming a serpentine or wavy line within the cyst cavity.
9. Multiorgan involvement: CT permits identification of concomitant hepatic cysts (dome of liver/sub-diaphragmatic region).
10. Pleural complications: Pleural effusion (from rupture into pleural space); empyema.
11. Cyst location: Predilection for lower lobes; may be bilateral; multiple cysts in 10% of cases.
12. Cardiac/vascular involvement (rare): Cysts in mediastinum, heart, and pulmonary arteries are rarely seen on CT.

B. Serology

• ELISA, indirect hemagglutination, or latex agglutination as adjunctive confirmation.
• Lower sensitivity in lung vs. hepatic disease: 50-60% vs. 85-98%.
• Positive results must be confirmed with Western blot (purified antigens) due to cross-reactivity with other helminth diseases, malignancies, and autoimmune disease.
• Cyst complications or multiorgan involvement are associated with higher seropositivity rates.

C. Microscopy

• Direct microscopy of protoscolices or other parasite features from aspirated cyst fluid or surgical histopathology is diagnostic.
• Ruptured cysts: scolices and cyst fragments may be detected in sputum or pleural fluid.

D. WHO Guidelines

• WHO expert consensus guidelines (last updated 2009) summarize key diagnostic and management recommendations.

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6. TREATMENT

A. Surgery (Mainstay)

• Surgery is the mainstay of treatment and can both confirm diagnosis and treat local complications.
• Primary goal: Remove as much of the cyst as possible while avoiding intraoperative spillage and dissemination (rupture causes anaphylaxis and parasite dissemination with subsequent relapse).
• Surgical approaches:
  • Cyst enucleation
  • Cystotomy with drainage
  • Complete cyst excision after aspiration
  • Lung parenchyma-preserving surgery preferred; recurrence rates < 2%.
• More extensive resection (lobectomy) reserved for giant cysts or those complicated by secondary bacterial abscesses.
• Intraoperative precautions: Hypertonic saline-soaked surgical drapes to protect the operative field; a helminthicidal agent such as hypertonic saline or 1% formaldehyde is left in place in the cyst lumen for ≥15 minutes before further manipulation to minimize consequences of spillage.
• Cavity marsupialization or surgical drain placement after removal is often required.

B. Drug Treatment (with Dosages)

From Fishman's (TABLE 136-1 - Treatments for Helminthic Pulmonary Diseases)

• Preoperative albendazole: Reduces consequences of dissemination if intraoperative spillage occurs.
• Perioperative/postoperative albendazole combined with praziquantel shows promise for improved efficacy (data more robust for hepatic disease; limited for pulmonary).
• Albendazole alone is curative in only a minority of cases when used as sole therapy.
• Key side effects of albendazole: Hepatotoxicity, leukopenia, hair loss, GI distress (monitor LFTs and CBC).

From Murray & Nadel's

• Surgical removal is the principal therapeutic approach in patients who can tolerate the procedure.
• Preoperative albendazole: Recommended to reduce consequences of dissemination.
• Praziquantel: Added to albendazole particularly if the cyst has ruptured, because it has a scolicidal effect.
• Intraoperative helminthicide: Hypertonic saline or 1% formaldehyde in cyst lumen ≥15 minutes before further manipulation.
• In patients who cannot tolerate surgery: Prolonged albendazole may improve symptoms but is curative in only a minority; albendazole + praziquantel combination may have greater efficacy than either drug alone.
• Serial serologic assays are of no value for monitoring treatment response; monitoring must be based on clinical findings and imaging.

C. PAIR Procedure (Not Recommended for Lung)

• The PAIR procedure (Percutaneous Aspiration, Injection of cysticidal agent, Re-aspiration) - used for hepatic cysts - is NOT recommended for pulmonary cysts due to higher complication rates.

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SUMMARY TABLE

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• Fishman's Pulmonary Diseases and Disorders, 2-Volume Set - Chapter 136 (Helminthic Pulmonary Diseases), pp. 2424-2426, Table 136-1
• Murray & Nadel's Textbook of Respiratory Medicine, 2-Volume Set - Chapter 58 (Cestodes: Echinococcosis), pp. 1349-1350