Question
Hydatid cyst
Answer
A hydatid cyst is the larval (metacestode) stage of Echinococcus granulosus, developing in the intermediate host, most commonly lodging in the liver or lungs, and slowly (over years) developing into a large, fluid-filled cyst.
Structure (from outside in): pericyst (outer, host-derived fibrous capsule); laminated (ectocyst) membrane (acellular, layered, parasite-secreted); germinal (endocyst) layer (innermost, nucleated, metabolically active layer, producing brood capsules, each containing numerous protoscolices — “hydatid sand” — and sometimes daughter cysts).
Cyst fluid: clear, highly antigenic — cyst rupture can provoke a severe anaphylactic reaction and disseminate protoscolices, seeding secondary hydatid cysts.
Diagnosis: imaging (ultrasound/CT/MRI) is the primary tool; serology (ELISA) supports diagnosis.
Treatment: surgical excision (avoiding intraoperative rupture/spillage) with perioperative albendazole, or the PAIR technique for selected cysts.