Hydatid disease is a zoonosis caused by the larval stage of the tapeworm Echinococcus. Human infection is known to be caused by E. granulosus and E.multilocularis.
The hydatid cyst has three layers:
- Pericyst: the outermost layer which is composed of modified host cells forming a dense fibrous coat.
- Middle laminated membrane which is acellular
- Inner germinal layer which produces the scolices and laminated membrane.
The middle and germinal layers form the true cyst wall or the endocyst, although the middle layer is occasionally referred to as the ectocyst.
The water lily sign is classically described in pulmonary hydatid disease on x-ray. However, "floating membranes" in a hydatid cyst can be seen on ultrasound and is referred to as "ultrasound water lily sign." According to the WHO classification of hepatic hydatid cysts, the water-lily sign is seen in stage CE3a, which represents a transitional stage between active and inactive disease. The mechanism of detachment of membrane is related to decreased intra-cystic pressure, degeneration, host response, trauma or following therapy.