I. Life cycle of Echinonococcus granulosus:
The life cycle consists of a definite host and an intermediate host ( Fig. 2 ). Sheep and other ruminants are intermediate hosts, whereas dogs and other carnivores are definitive hosts. Humans get secondarily infected by ingestion of food or drinks contaminated with dog feces containing the eggs of the worm. Free embryos enter the portal circulation through the gut wall and gets lodged in the liver parenchyma. A few of these enter the systemic circulation and get lodged in the lungs and other organs.
Fig. 2: Life cycle of Echinococcus granulosus
References: www.cdc.gov.
II. Structure of the hydatid cyst:
The ingested worms in humans typically grow into cysts with clear fluid within. They are more commonly unilocular. A hydatid cyst consists of 3 layers: pericyst, ectocyst and endocyst.
Fig. 3: The three layers of Hydatid cyst: the pericyst,ectocyst and endocyst.
References: sciencedirect
III. Sites of involvement:
As the larval stages enter the body through the portal circulation, the liver is the most common site of involvement, followed by the lung and other sites.
Sonographic Features: A simple hydatid cyst appears as an anechoic cyst with or without echoes and hydatid sand ( Fig. 4 ). On patient movement, the hydatid sand is dispersed in the cyst and appears as falling snowflakes, which is called the “snowstorm sign.” The cyst wall appears as a double echogenic line.
Fig. 4: Type I : liver hydatid cyst. Sonography shows a well-defined
unilocular cyst (arrow) with posterior acoustic enhancement and a
double-wall sign.
References: Mohamed Taher Maamouri Hospital, Nabeul
A multivesicular cyst consists of multiple daughter cysts, which are seen as anechoic round structures in the mother cyst. Initially, they are noted in the periphery, whereas later, they fill the entire mother cyst, giving the appearance of a “spoked wheel” pattern ( Fig. 5 ). As the cyst matures, the endocyst starts detaching from the wall of the cyst and appears as an undulating, floating membrane. Complete detachment of the endocyst is seen as the “water lily sign”. Later, wall calcification of the cyst is seen, which progresses to the center of the lesion. In a few cases, the whole cyst appears calcified.
Fig. 5: Sonography shows a lesion with multiple closely placed daughter cysts (arrows) filling most of the lumen of the mother cyst.
References: Mohamed Taher Maamouri Hospital, Nabeul
Computed Tomographic Features: A hydatid cyst appears as a water attenuation cyst with a well-defined wall. Daughter cysts appear as round, peripherally placed cystic lesions in the mother cyst. Floating membranes appear thin and hypodense. The high-density fluid surrounding the daughter cysts appears as radiating spokes like a “rosette” pattern. Calcification is shown well on computed tomography.
Fig. 6: Postcontrast axial computed tomography scan shows unilocular cystic lesion
References: Mohamed Taher Maamouri Hospital, Nabeul
Fig. 7:
Fully calcified mass with serpiginous structures related to a hydatid cyst type V.
References: Mohamed Taher Maamouri Hospital, Nabeul
Magnetic Resonance Imaging Features: A hydatid cyst appears hypointense on T1-weighted images and hyperintense on T2-weighted images. Magnetic resonance imaging (MRI) accurately shows the hydatid matrix, daughter cysts, and pericyst.A detached endocyst appears as a floating membrane in the cyst and is described as the “snake sign”.
- Splenic Hydatid Cysts (Fig. 9)
Primary infection of the spleen is through the arterial route. Splenic hydatid disease may also arise with retrograde spread of parasites via the portal and splenic veins. Secondary splenic hydatid disease usually follows systemic dissemination or intraperitoneal spread after rupture of a hepatic hydatid cyst.
Fig. 9: Splenic hydatid cyst ruptured in the peritoneal cavity
References: Mohamed Taher Maamouri Hospital, Nabeul
The lung is the second most common site of parasite lodgment. Lung hydatid cysts tend to be larger because of compressibility of the lung. Round homogeneous opacities in the lung parenchyma are characteristic of simple uncomplicated cysts. The water lily sign or “Camelot sign” represents the separated endocyst, which is seen as a floating membrane within the pericyst, like a water lily. It is appreciated on a radiograph when the collapsed endocyst is calcified. The “crescent sign” represents a crescent-shaped air shadow caused by trapped air between the pericyst and the laminated membrane when a growing cyst erodes into a bronchiole. When a similar air crescent is noted at the lower edge of the cyst, it is know as the “inverted crescent sign.” Air between the endocyst and pericyst creates the “onion peel sign.” The “empty cyst sign” is seen on complete expulsion of the contents of the cyst after cyst rupture. Lung hydatid cysts rarely show daughter cysts and calcification.
Fig. 10: Lung hydatid cyst
References: Mohamed Taher Maamouri Hospital, Nabeul
Cardiac hydatid cysts are uncommon. The most common location is the myocardium, particularly the left ventricular free wall. Cardiac involvement can occur through invasion via the coronary circulation or through secondary rupture of a lung hydatid cyst into a pulmonary vein. The left ventricle is more commonly involved than the right ventricle.
Left-sided cysts tend to grow subepicardially and thus frequently perforate into the pericardium. On the contrary, right-sided cysts have a tendency to expand in the lumen of the chamber or subendocardially and hence are more prone to rupture into the ventricular cavity. On sonography, the appearance of a round, thin-walled, multiloculated mass is characteristic of an echinococcal cyst; however, some complicated intracardiac hydatid cysts may be mistaken for intracardiac masses.
Pericardial involvement is generally due to secondary rupture of a cardiac hydatid cyst. Rupture of a hydatid cyst into the pericardial cavity may lead to pericarditis with effusion, cardiac tamponade, and formation of secondary cysts.
Fig. 11: Mediastinal window of contrast-enhanced CT images show loculated pericadial cysts.
References: Mohamed Taher Maamouri Hospital, Nabeul
Fig. 12: Axial T2 weighted MRI images demonstrate high T2 signal, loculated lesions suggestive of a pericardial cysts of the antero-superior and inferior walls of the left ventricule
References: Mohamed Taher Maamouri Hospital, Nabeul
- Mesenteric and Retroperitoneal Hydatid Cysts ( Fig. 13 )
Mesenteric hydatid disease is usually secondary to spontaneous or iatrogenic rupture of a liver or splenic cyst. Any type of hydatid cyst can be seen. Unilocular type I hydatid cysts may be difficult to differentiate from lymphangioma or intestinal duplication cysts.
Fig. 13: SPLENIC, RETROPERITONEAL, HEPATIC AND MEDIASTINAL HYDATIC CYSTS
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Renal hydatid cysts are generally unilateral and involve the cortex. Any type of hydatid cyst can be seen in the kidney. At excretory urography, uncomplicated hydatid cysts may create a bulge in the outline of the kidney. Unilocular hydatid cysts can mimic simple renal cysts. Multilocular hydatid cysts can be misdiagnosed as renal cysts, cystic nephroma, or cystic variants of renal cell carcinoma.
Fig. 14: Renal hydatid cysts type III and V
References: Mohamed Taher Maamouri Hospital, Nabeul
Cysts Hydatid cysts are seen more commonly in the trunk and proximal limb muscles. Hydatid cysts in the soft tissues may have various radiologic appearances. Daughter cysts are not usually present in soft tissue and muscle hydatid cysts. Calcification is rare.
Fig. 15: Intra-muscular hydatid cyst
References: Mohamed Taher Maamouri Hospital, Nabeul
Bone hydatid cysts are most commonly seen in the spine and pelvis, followed by the femur, tibia, humerus, skull, and ribs. Pericyst formation is not seen, leading to faster expansion of the cyst in the bone. Eventually, it can breach the cortex and extend into the surrounding tissues. Radiologically, it appears as a cyst, which may be unilocular or multilocular. Spinal hydatid cysts can be confused with osteomylitis or bone neoplasms.
Fig. 16: HYDATIDOSIS OF THE ILIAC BONE
References: Mohamed Taher Maamouri Hospital, Nabeul
Fig. 17: Bone hydatidosis with epidural extension (arrow)
References: Mohamed Taher Maamouri Hospital, Nabeul
Fig. 18: Multilocular cystic formation scalloping on the sphenoidal bone
References: Mohamed Taher Maamouri Hospital, Nabeul
These cysts are commonly found in children. The most common locations are the cerebral hemispheres in the middle cerebral artery territory. Computed tomography and MRI show a well-defined oval or cystic mass with attenuation or signal intensity similar to that of cerebrospinal fluid. There is no perilesional edema, which differentiates these cysts from tumors or inflammatory lesions. The lesion generally does not enhance after intravenous administration of contrast material. A thin rim of peripheral enhancement is a rare occurrence in cases of perilesional inflammation. Calcification is extremely rare.
Fig. 19: Parietal multiloculated intra- axial process surrounded by edema (marked star) with density equal to the LCS ,exerting a mass effect on the median line (arrow)
References: Mohamed Taher Maamouri Hospital, Nabeul
These cysts are usually secondary in nature. Nearly 80% of all pelvic echinococcosis involves the pelvic peritoneum, ovary, and uterus. Pelvic hydatid cysts usually present as masses with pressure effects on adjacent organs. Sonographic features of pelvic hydatid cysts are similar to those of hepatic hydatid cysts. Computed tomography can better depict the extent of the mass in the pelvis.
Fig. 20: Pelvic hydatid cysts
References: Mohamed Taher Maamouri Hospital, Nabeul
- Complicated Hydatid Cysts
Since the liver is the most common organ affected, some specific complications have been described in cases of liver hydatid cysts. Common complications are transthoracic rupture in the pleural or pericardial space, peritoneal seeding, rupture in the biliary tree, portal vein involvement, and abdominal wall invasion. Other complications are cyst infection, cyst rupture, and hematogenous dissemination.
Fig. 21: Mechanical occlusion by peritoneal hydatidosis.
References: Mohamed Taher Maamouri Hospital, Nabeul
Fig. 22: Mediastinal window of contrast-enhanced CT image shows occlusion of the right pulmonary artery by a mass ( orange arrow), with fluid density. Parenchymal window of contrast-enhanced CT image shows lung hydatid cyst (black arrow).
References: Mohamed Taher Maamouri Hospital, Nabeul
Fig. 23: Open left liver hydatid cysts in the bile ducts with secondary Budd Chiari
References: Mohamed Taher Maamouri Hospital, Nabeul