Echinococcal Infections (Echinococcosis / Hydatid Disease)

1. Species and Forms of Disease

2. Life Cycle

1. Ingestion of embryonated eggs from dog feces (fecal-oral route; contaminated vegetables, soil, water)
2. Eggs hatch in the intestine → oncospheres penetrate the intestinal wall
3. Oncospheres travel via portal blood → liver (first filter; majority lodge here)
4. Those traversing the liver enter pulmonary circulation → lungs (second filter)
5. A small number pass into systemic circulation → brain, heart, bones, kidneys, spleen

• Survivors form a cyst with an outer laminated cuticle (acellular) and inner germinal membrane (cellular)
• Cyst fills with clear fluid and expands: ~1 cm over 5–6 months; can reach >10 cm over years
• Germinal layer gives rise to brood capsules → protoscolices (immature tapeworms)
• Daughter cysts form within the mother cyst; degenerated protoscolices/membranes settle as hydatid sand

3. Epidemiology

• Cystic echinococcosis (E. granulosus): Global distribution — highly endemic in eastern/southern Europe, northern Africa, Central Asia, southern South America, and sheep-raising regions of Australia. Predominantly a pastoral-cycle disease; highest rates where dogs feed on raw animal viscera. Up to 6.6% infection prevalence detected in endemic areas by community ultrasound surveys. — Goldman-Cecil Medicine
• Alveolar echinococcosis (E. multilocularis): Northern hemisphere — western China, Tibet, central Asia, alpine Europe, and subarctic North America. Has recently emerged in western Canada. Less common but more dangerous.
• Both are rare in the US; most US cases occur in Alaska, the southwestern sheep-raising states, and among foreign-born individuals from endemic regions.

4. Pathology and Cyst Structure

1. Pericyst (adventitia) — host-derived fibrous outer layer
2. Ectocyst (laminated membrane) — acellular, outer parasitic layer; lamellar appearance on H&E
3. Germinal (inner/endogenous) layer — the living cellular layer; produces brood capsules, protoscolices, and daughter cysts

Echinococcal cyst in lung — the white collapsed laminated membrane is visible within the cyst. (Sherris & Ryan's Medical Microbiology, 8th Ed.)

Microscopy of the cyst wall: the lamellar laminated membrane is visible above lung parenchyma, with protoscolices (darkly stained circular structures) budding from the germinal layer. (Sherris & Ryan's Medical Microbiology, 8th Ed.)

• E. granulosus: thick outer laminated membrane + brood capsules with protoscolices — contained within a cyst
• E. multilocularis: infiltrative growth; back-to-back brood capsules without a thick containing laminated wall; protoscolices are usually absent in human infection

5. Clinical Manifestations

Cystic Echinococcosis (E. granulosus)

• Incubation/latency: 5–20 years (intervals up to 75 years reported) — most patients are asymptomatic until the cyst causes mechanical effects
• Liver (~70% of cases): upper abdominal discomfort/pain, nausea, hepatomegaly, palpable mass, jaundice (biliary compression/rupture), portal hypertension, ascites, IVC compression
• Lung (~25% of cases): cough, chest pain, hemoptysis; rupture into bronchi → vomiting of hydatid material (brood capsules, scolices), salty taste, pneumothorax, pleural effusion
• Brain (~4% of primary hepatic/pulmonary cases): seizures, blindness, raised ICP
• Bone: pathological fractures (bone cysts do not have a pericyst wall)
• Heart: cardiac mass, pericardial effusion, conduction disturbances, embolic metastases
• Rupture: fever, pruritus, urticaria, anaphylactic shock (major complication); seeding of protoscolices can cause secondary/disseminated echinococcosis
• Intrabiliary rupture: acute cholangitis, jaundice, signs mimicking acute cholecystitis
• Circulating immune complexes: rare membranous glomerulonephritis

Alveolar Echinococcosis (E. multilocularis)

• Almost invariably involves the liver
• Presents as a slowly expanding tumor-like mass over decades
• Symptoms: epigastric pain, obstructive jaundice, weight loss, malaise, liver failure
• Can metastasize hematogenously to lung and brain, closely mimicking hepatocellular carcinoma
• Fatal if untreated; 10-year mortality can exceed 90% without treatment

6. Diagnosis

Laboratory Tests

• Routine labs are nonspecific
• Eosinophilia: usually low-grade or absent unless cyst has ruptured (may reach up to 60% with rupture)
• Alkaline phosphatase elevation if biliary involvement

Imaging

Multimodal imaging of disseminated echinococcosis: (a) liver ultrasound with septations; (b) splenic ultrasound with floating membranes; (c) T2 MRI showing "water-lily sign" (floating endocyst membrane); (d–f) CT showing multiple liver, spleen, and peritoneal cysts. — Focal Liver Lesions textbook

Multi-stage imaging: (a) CE1 — unilocular hypodense CT lesion; (b) CE2 — daughter cysts in "rosette" pattern; (c) CE3 — water-lily sign on ultrasound; (d) CE5 — calcified inactive cyst on X-ray.

Cerebral echinococcosis MRI: (A) multiple cysts; (B) large unilocular cyst with daughter cysts; (C) contrast-enhanced T1 showing thin outer enhancing rim, indicating active cyst. — Grainger & Allison's Diagnostic Radiology

Serology

• Useful as confirmatory, not primary diagnostic tool
• Sensitivity depends on cyst location: hepatic > pulmonary > splenic/brain/bone
• Indirect hemagglutination/latex agglutination: ~90% sensitivity for hepatic, ~60% for pulmonary lesions
• Generally negative in inactive cysts (CE4/CE5); titers rise after cyst disruption (surgery/PAIR)
• E. multilocularis patients are usually seropositive on E. granulosus assays; E. multilocularis-specific assays available in Europe
• False positives in neurocysticercosis (up to 25%)

PCR

• Can detect picogram quantities of Echinococcus genomic DNA from fine-needle biopsy material
• Valuable when serology is equivocal

Caution re: aspiration

• Blind needle aspiration is contraindicated due to risk of anaphylaxis and dissemination; however, ultrasound-guided PAIR is a therapeutic procedure (see below)

7. Treatment

A. "Watch and Wait" (Inactive Cysts)

• Inactive liver cysts (CE4/CE5) free of complications can be monitored without intervention, with serial imaging

B. Chemotherapy (Albendazole — drug of choice)

• Dose: 10–15 mg/kg/day orally (continuous, no interruptions)
• Best for: CE1 and CE3a cysts (small/medium) — induces solidification
• Limited efficacy: giant CE1/CE3a (>10 cm), CE2 and CE3b (little effect alone)
• Adverse effects: hepatotoxicity, leukopenia, thrombocytopenia, alopecia
• Avoid in pregnancy (teratogenic risk theoretical, but standard precaution)
• For E. multilocularis: indefinite albendazole if not resectable; post-curative-surgery: 2 years to reduce relapse risk

C. PAIR (Percutaneous Aspiration–Injection–Re-aspiration)

• Indications: CE1 and CE3a cysts
• Procedure: ultrasound-guided percutaneous puncture → aspiration of cyst fluid → injection of scolicidal agent (hypertonic saline, ethanol) → re-aspiration
• Risk of anaphylaxis: 0.03% lethal; 1.7% reversible allergic reaction
• Always combined with albendazole (≥30 days post-procedure)
• PAIR is generally ineffective for multivesicular cysts (CE2, CE3b) — tend to relapse
• Various catheter drainage modifications exist for complex cysts

D. Surgery

• Complicated cysts (rupture, biliary fistula, compression of vital structures, superinfection, hemorrhage)
• Large cysts with many daughter vesicles not amenable to PAIR
• High risk of rupture
• E. multilocularis (primary modality when feasible)

E. Summary by Cyst Stage

8. Prognosis

• Cystic echinococcosis: significant morbidity, death rare — long-term follow-up (>5 years) needed to assess for recurrence
• Alveolar echinococcosis: fatal if untreated; with benzimidazole treatment, survival approaches that of the age/sex-matched general population. Prognosis is poor if not diagnosed early.

9. Prevention and Control

• Deworming of dogs (praziquantel) in endemic areas
• Avoiding feeding raw viscera to dogs
• Hand hygiene; avoiding contamination from dog feces
• Food hygiene (washing vegetables; avoiding raw berries in fox-endemic areas)
• Surveillance and slaughterhouse control programs
• Vaccination of sheep (EG95 vaccine) has shown promise in breaking the pastoral cycle

Recent Evidence (PubMed)

• Goldman-Cecil Medicine, International Edition — Chapters on Cystic and Alveolar Hydatid Disease
• Sherris & Ryan's Medical Microbiology, 8th Edition — Chapter 56: Cestodes
• Tietz Textbook of Laboratory Medicine, 7th Edition — Chapter 88: Echinococcus
• Grainger & Allison's Diagnostic Radiology — Chapters on CNS and Renal Hydatid Disease