Echinococcosis (Hydatid Cyst Disease)

Echinococcosis is a chronic zoonotic helminthic infection caused by larval stages (metacestodes) of Echinococcus tapeworms. Three clinically important species: E. granulosus sensu lato (causing cystic echinococcosis CE — most common, worldwide distribution especially Mediterranean, Middle East, Central Asia, South America, sheep-raising areas), E. multilocularis (causing alveolar echinococcosis AE — Northern Hemisphere, fox/dog hosts, more aggressive tumor-like behavior), and E. vogeli/oligarthrus (causing polycystic echinococcosis PE — Latin America, rare). Lifecycle: definitive hosts (dogs, foxes, wolves) shed eggs in feces; intermediate hosts (sheep, cattle, humans) ingest eggs which hatch in intestine releasing oncospheres that penetrate intestinal wall and migrate to liver, lungs, or other organs forming cysts. Humans are accidental dead-end hosts.

Cystic echinococcosis (CE): cysts contain three layers (outer adventitia from host, intermediate laminated layer, inner germinal layer producing brood capsules with protoscolices). WHO ultrasound classification of CE: CE1 (active, anechoic with snowstorm sign), CE2 (active, multivesicular), CE3a (transitional, detached membranes), CE3b (transitional, multivesicular with daughter cysts in solid matrix), CE4 (inactive, heterogeneous with no daughter cysts), CE5 (inactive, calcified). Alveolar echinococcosis (AE): tumor-like infiltrative growth with multiple small vesicles, mimics malignancy, hepatic involvement most common, can extend to diaphragm, lungs, vascular structures. Clinical features depend on organ and size: hepatic CE often asymptomatic until complications (rupture, biliary obstruction, infection), pulmonary CE with cough, hemoptysis, dyspnea, chest pain, vomiting hydatid material, anaphylaxis from rupture, brain CE with focal deficits and seizures.

Diagnosis is by ultrasound (initial test for hepatic CE — characteristic features include septations, daughter cysts, water lily sign of detached membranes, calcifications), CT and MRI for staging, characterization, alveolar echinococcosis evaluation (irregular borders, infiltrative growth), serology (ELISA for IgG against Echinococcus antigens, immunoblot, indirect hemagglutination — sensitivity 80-100% for hepatic, lower for pulmonary), Casoni skin test (historical), eosinophilia in some cases. Treatment depends on cyst type: CE1 and CE3a — albendazole alone or PAIR (Puncture, Aspiration, Injection of scolicidal agent, Reaspiration) under ultrasound guidance with prophylactic albendazole; CE2 and CE3b — surgery preferred (cystectomy, pericystectomy, hepatectomy, lobectomy) with albendazole pre/post-op; CE4 and CE5 — observation; complicated cysts (rupture, infection, biliary fistula) — surgery; pulmonary CE — surgery (cystectomy or lobectomy) usually preferred; AE — radical liver resection if possible, lifelong albendazole if not, liver transplantation in selected cases. Albendazole 10-15 mg/kg/day in 28-day cycles with 14-day breaks. Newer continuous regimen also acceptable. Prevention via dog deworming, hygiene, avoiding consumption of contaminated water/vegetables in endemic areas.