Hydatid Disease (Echinococcosis)

 
Definition
 Hydatid disease, infection of man, sheep, and most other herbivorous and omnivorous mammals with larvae of the tapeworm Echinococcus.
 
Radiographic Appearance
In the Chest
 Radiographic features are one or more spherical or oval, well-defined, smooth masses of homogenous density usually in the middle or lower zones. Multiple cysts are seen in 1/3rd of patients and are bilateral in 20% of patients. There is a predilection for the lower lobes, the posterior segments and the right lung. CT scanning reveals fluid content within the cyst with a density close to that of water. Daughter cysts, when present, appear as curved septations. At CT, the wall thickness ranges from 2mm to 1cm. The rate of growth may be fairly be rapid, with doubling times of less than 6 months. A striking feature is that the cyst is really pliant and molds to adjacent structures, resulting in indentation, lobulation or flattening. Calcification is rare in lung hydatid, as compared to liver hydatid.
If the pericyst ruptures, air dissecting between the fibrotic lung forming the pericyst and the ectocyst of the parasite leads to a visible crescent of air between the two and is known as the meniscus sign or crescent sign. If the cyst itself ruptures, an air-fluid level results and daughter cysts may be seen floating in residual fluid. Sometimes, the cyst wall is seen crumpled up and floating in fluid which lies within the non-collapsed pericyst. This pathognomic appearance is described as water-lilly sign.. All these signs are well demonstrated on CT and MRI.

In the Liver
The ova burrow through the intestinal wall and are carried via the portal vein to the liver. There, they are caught in the hepatic sinusoids and cysts develop and grow leading to multiloculated cysts. The right lobe is more often involved and the outer layer can calcify. Uncomplicated cysts may be silent forever. the most severe complication is that of rupture into the peritoneal cavity or into the bile ducts. The hydatid cyst contains antigens which sensitize the host and rupture or leakage into the peritoneum can be fatal. the Liver

In the Brain
CT findings of a hydatid cyst include a large well-defined round cystic lesion, with a density similar to water or CSF, which usually shows absence of surrounding edema or contrast enhancement. Significant distortion of the brain parenchyma, ventricular displacement and / or hydrocephalus are common. Rarely the cyst may show rim enhancement, an enhancing nodule simulating a cystic astrocytoma or calcification in the wall of a degenerated cyst
 
Pathology

  • Due to infection with the helminth Ecchinococcus granulosa

  • Adult worm is found normally in the dog and sheep intestine

  • Man is an accidental intermediate host

  • Infection seen in Mediterranean areas, Australia and South America

  • Liver is the commonest organ involved

  • Cysts are unilocular, can be up to 20 cm in diameter and may be multiple

  • Daughter cysts may develop

  • 70% develop in the right lobe of the liver

  • Lung, brain and bone can also be infected

  • Pathologically hydatid liver cyst has three distinct layers:

    • Ectocyst - fibrous advential layer due to host response

    • Middle layer - laminated membrane of proteinaceous material

    • Endocyst - inner germinal layer from which the scolices may be detached


 
Treatment:

  • Pharmacological treatment is not curative

  • Used as an adjunct to surgery to kill spilled scolices

  • The drugs of choice are albendazole, mebendazole and praziquantel

  • If surgery is required a laparotomy is performed to exclude other cysts

  • The liver is packed off with hypertonic saline-soaked swabs

  • Cysts are then decompressed with trocar and cannula

  • Scolicidal agent (e.g. hypertonic saline or 0.5% silver nitrate) can be injected into cyst cavity 

  • Cavity is filled with saline and a suction drain inserted

  • Alternatively liver cysts can be excised


 
Images

http://www.surgical-tutor.org.uk


http://www.ijri.org

 

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