Hydatid cysts are endemic in Mediterranean and Middle Eastern countries. They are caused by the tapeworm parasite Echinococcus granulosus. An outer fibrous capsule is formed containing multiple small daughter cysts. These cysts are allergens which precipitate a type 1 hypersensitivity reaction.

Clinical features are as follows:

• Up to 90% of cysts occur in the liver and lungs
• Can be asymptomatic, or symptomatic if cysts > 5cm in diameter
• Morbidity caused by cyst bursting, infection and organ dysfunction (biliary, bronchial, renal and cerebrospinal fluid outflow obstruction)
• In biliary rupture, there may be the classical triad of; biliary colic, jaundice, and urticaria

Investigation

• imaging
  • ultrasound if often used first-line
  • CT is the best investigation to differentiate hydatid cysts from amoebic and pyogenic cysts
• serology
  • useful for primary diagnosis and for follow-up after treatment
  • wide variety of different antibody/antigen tests available

Surgery is the mainstay of treatment (the cyst walls must not be ruptured during removal and the contents sterilised first).