Question
Hydatid cyst
Answer
A hydatid cyst is the larval (metacestode) stage of Echinococcus granulosus, developing in the intermediate host (usually sheep/cattle; humans are an accidental intermediate host), formed when an ingested egg hatches and the resulting oncosphere disseminates via the bloodstream, most commonly lodging in the liver or lungs, and slowly (over years) developing into a large, fluid-filled cyst.
Structure (from outside in):
- Pericyst — the outer, host-derived fibrous capsule, formed as a reactive/inflammatory response of the surrounding host tissue.
- Laminated (ectocyst) membrane — an acellular, layered membrane secreted by the parasite, whitish and elastic, permeable to nutrients but providing structural protection.
- Germinal (endocyst) layer — the innermost, thin, nucleated, metabolically active layer, from which brood capsules bud off internally; each brood capsule in turn produces numerous protoscolices (immature scolices, “hydatid sand” when free-floating in the cyst fluid), and daughter cysts can also form, either within the primary cyst (endogenous) or externally (exogenous, following pericyst rupture).
Cyst fluid: clear, colourless to pale yellow, highly antigenic — this antigenicity is clinically important because cyst rupture (spontaneous, traumatic, or during surgical/percutaneous manipulation) can release fluid into the peritoneal/pleural cavity or bloodstream, provoking a severe anaphylactic reaction, and can also disseminate protoscolices to seed secondary (multiple) hydatid cysts elsewhere.
Clinical significance: growth is typically slow and asymptomatic for years; symptoms arise from mass effect on the affected organ; imaging (ultrasound/CT/MRI) is the primary diagnostic tool, often showing characteristic internal daughter cysts/septations; serology (ELISA) supports diagnosis; treatment is surgical excision (with meticulous care to avoid intraoperative rupture/spillage) combined with perioperative albendazole, or the PAIR (Puncture-Aspiration-Injection-Reaspiration) technique with a scolicidal agent for selected cysts.