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Imaging Spectrum of Hydatid Disease: Usual and Unusual Locations

Summary Hydatid disease is a parasitic infection caused by Echinococcus granulosus and Echinococcus multilocularis. It is common in endemic regions and can demonstrate a variety of imaging features that differ according to the affected organ and the stage of the disease. Liver and lungs are the most commonly affected organs. The classic features of hepatic hydatid disease are well known. However, diagnosing hydatid disease at unusual locations may be challenging because of myriad imaging features in each of these locations. Knowledge of the imaging spectrum in systemic hydatidoses in various organs is very valuable in improving the accuracy of radiological interpretation. The purpose of this article is to review the imaging features of hydatid disease at its varied locations.


Background Epidemiology
Hydatid disease is a widely prevalent parasitic infection caused by cestode species, Echinococcus granulosus (also known as cystic hydatid disease/echinococcosis) and Echinococcus multilocularis (also known as alveolar hydatid disease/echinococcosis). Human infection is common in sheep-rearing countries such as Australia and New Zealand, throughout England and Europe, the Middle East, Russia, Northern China, and Japan. In the Americas, the disease is prevalent in Alaska, Canada, Argentina, Uruguay and Chile. Alveolar hydatid disease (E. multilocularis) is a less widespread disease. The disease is mainly prevalent in northern and central Europe and parts of Canada [1].

Life cycle of Ecchinococcus and cyst structure
Dogs are the definitive host and sheep are the intermediate hosts [2]. Humans become infected after ingestion of parasitic eggs, either directly from contact with dogs, or more frequently by indirect ingestion of contaminated food or water [2]. The adult form of the cestode resides in the small intestine of the definitive host, releasing eggs containing infective oncospheres to the environment. These eggs are consumed by the intermediate host, following which a larval stage -metacestode, develops in various internal organs [3].
The metacestode stage usually manifests as a cystic structure (hydatid cyst). The wall of a hydatid cyst (HC) has three layers, an outer pericyst, which is the dense fibrous capsule formed by the host's inflammatory response, the middle laminated proteinaceous layer and the inner germinal layer, endocyst which is produced by the parasite itself. The layers tend to be thick in the liver, less developed in muscles, absent in bones, and sometimes visible in the brain [2].

Clinical presentation
Symptoms of cystic echinococcosis are site-specific with hepatic disease presenting with hepatomegaly and jaundice and pulmonary disease with chronic cough and chest pain. Complicated cysts may present with anaphylactic reaction.

Authors' Contribution:
On the other hand, alveolar hydatid disease is characterized by a long asymptomatic incubation period, presents later and has a chronic progressive course. The most common presenting symptoms of alveolar hydatid disease include cholestatic jaundice, epigastric pain, fatigue and weight loss [3].

Radiological diagnosis
Ultrasonography (USG), computed tomography (CT), magnetic resonance (MR) imaging can help to image hydatid disease. The modality that is most helpful depends on the stage of the disease (growth of the cyst) and the affected organ [2]. USG can clearly demonstrate hydatid sand-fine internal debris, floating membranes and daughter cysts. CT is best for detecting calcification and osseous involvement whereas MR imaging is helpful in detecting hydatid disease with neural involvement [4].
Imaging findings in cystic echinococcosis range from cystic lesions to lesions that appear solid [2]. The cyst may appear as a well-defined fluid collection. Frequently, there may be detachment of endocyst from the pericyst causing the appearance of a floating membrane. Curvilinear or ring-like calcification of the cyst can be seen during the natural evolution and can lead to complete calcification [5].

Serological diagnosis
Confirmation of hydatid disease is done by detecting specific serum antibodies using immunodiagnostic tests. The tests include enzyme linked immunosorbent assay (ELISA) using hydatid fluid (less specific) or purified antigens (Antigen B -cystic echinococcosis, Em2 -alveolar echinococcosis), immunoblot analysis, immunoelectrophoresis, detection of immunoglobulin G4, etc. However, disease in the bone, brain, eye and muscle induces less antibody response and is more difficult to detect by serological methods [3].

Types of hydatid disease and radiographic findings
Hydatid cyst may be classified into the following 4 types according to a widely accepted imaging classification [5]. Appearance of hydatid disease at different sites (cystic echinococcosis) follows the general pattern presented below. Specific features of the disease at each site are mentioned in the subsequent sections. Alveolar hydatidosis shows an infiltrative growth pattern with multicystic honeycomb appearance and calcifications Type 1: Simple Cyst with no Internal Architecture On ultrasound, Type I hydatid cysts appear as well-defined unilocular anechoic lesions. On CT, these appear as welldefined fluid attenuation lesions ( Figure 1). MRI features are those of a fluid-attenuation cystic lesion with a T1 isointense and T2 hypointense peripheral rim ("rim sign") surrounding the homogenous high signal cyst contents [6].

Type II: Cyst with daughter cysts and matrix
This type includes cysts with detached floating endocyst membranes or daughter cysts. The attenuation of the daughter cyst is hypodense/hypointense to maternal matrix on CT and MRI, respectively [4]. Multiple cysts when present are enclosed within a single capsule resulting in "wheel spoke" appearance [2].
At CT, type II HCs can be visualized in three stages depending on the age, number, and arrangement of the daughter cysts [7]. a. Type IIA: Contain round daughter cysts arranged at the periphery ( Figure 2). b. Type IIB: Contain larger, irregularly shaped daughter cysts that occupy almost the entire volume of the mother cyst ( Figure 3). c. Type IIC: Relatively high-attenuation round or oval masses with scattered calcifications and occasional daughter cysts ( Figure 4A). This represents degeneration of old cyst. d. Type III: Dead calcified cysts.
They manifest as round, hyper-attenuating areas on CT ( Figure 4B) and hypointense areas on MR imaging.

Type IV: Complicated hydatid cysts
Hydatid cyst complications include rupture and superinfection. Cyst rupture is mainly due to degeneration of parasitic membranes. Cysts may rupture into pleural and peritoneal cavities or into the biliary radicals [4] ( Figure 5). The incidence of hydatid disease at different sites with the various complications at each site is listed in Table 1 [3].

Sites of Involvement
The liver is the most frequently involved site in both isolated cases and in cases with cysts in other organs. The second most common site is the lung. Other sites of involvement include the spleen, kidney, adrenal glands, peritoneum, bladder, ovary, brain, spinal cord, lung, heart, bone and soft tissues [8].

Liver
Liver is the most commonly affected organ (68 to 75% of cases), with the right lobe being the most frequently involved portion. Echinococcal cysts appear as welldefined round fluid cystic lesions. Daughter cysts that indicate viability, give the lesion a multilocular appearance.
There may be detachment of endocyst from the pericyst, appearing as localized split with floating membranes inside the cavity [4]. During the natural course of the disease, complete calcification of the cyst may occur [4]. A classification was proposed by the World Health Organization Informal Working Group on Echinococcosis for the different types of E. Granulosus cysts seen on ultrasound examination of the liver (Table 2) [3]. Complications such as rupture into the biliary tree or transdiaphragmatic migration to involve the thoracic cavity can occur [4] ( Figure 5).
Alveolar echinococcosis is a rare parasitic disease caused by E.Multilocularis, liver being the most commonly involved site [9]. On USG, these lesions show a "hailstorm" pattern, characterized by multiple echogenic nodules with     Table 3. PNM staging of alveolar hydatid disease [3].
Review Article irregular, ill-defined margins. CT and MR features are those of multiple irregular, ill-defined lesions which are hypoattenuating on CT and hyperintense on T2-weighted MR imaging ( Figure 6). In advanced stages, irregular calcifications are found within the areas of central necrosis [7]. Hilar infiltration, if present, causes intrahepatic biliary radical dilatation. There may be atrophy of the affected liver segments if there is invasion of the portal or hepatic veins due to hypoperfusion [7]. The European Network for Concerted Surveillance of Alveolar Echinococcosis has proposed a classification denominated as PNM for describing the anatomical extent of the disease (Table 3) [3].
The following sections describe the appearance of cystic hydatid disease at different sites. Extra-hepatic site of primary alveolar hydatid disease is extremely rare. Secondary involvement/infiltration from primary hepatic disease is more common. The imaging features of alveolar hydatidosis common to all sites (lung, spleen and brain) include an infiltrative growth pattern with multicystic honeycomb appearance and clustered microcalcifactions or plaque-like calcifications [2].

Lung
Lungs are the most common site of involvement in children. Right lung is affected in 60%, both lungs in 20% of cases [10]. Pulmonary lesions can be primary or secondary to intrathoracic rupture of a hepatic hydatid cyst. It can also involve the pleural cavity, mediastinum and chest wall [11].
Pulmonary cysts may range between 1 and 20 cm in diameter [12]. Larger lesions cause mediastinal shift, can induce a pleural reaction or cause atelectasis of adjacent lung. Calcification of pulmonary cysts is rare [11].

A B
The MRI characteristics of a hydatid cyst may differ depending on the developmental phase (infected/viable/ dead). On magnetic resonance imaging (MRI), cysts show low signal intensity on T1-weighted images and high signal intensity on T2-weighted images [13].

Kidney
Involvement of the kidney is rare (approximately 3% of cases). When involved, the lesions are frequently solitary and located in the cortex. Imaging appearance is that of a well-defined uni-or multilocular cystic lesion which may show ring-like calcifications ( Figure 8). A characteristic feature would be the presence of daughter vesicles. The differential diagnoses of unilocular HC include simple cyst while the differentials of multilocular HC include multilocular renal cyst, cystic nephroma and also cystic-type renal cell carcinoma [2,14].

Bone hydatid cyst
Hydatid disease of bone is rare (prevalence ranges from 1.1% to 4%) [15]. On radiography, there can be osteolytic and inflammatory changes which may mimic osteomyelitis. Bone erosion and destruction may often cause confusion with malignant tumour. The primary role of CT and magnetic resonance imaging is in delineating the area of destruction and in recognition of extraosseous spread of the hydatid disease into the soft tissues [16,17] (Figure 9). Extraosseous HCs may show calcification, whereas intraosseous HCs rarely show calcification [2].

Spleen
Primary splenic hydatidosis is rare (less than 2%) with E.granulosus being the causative organism in the majority of cases. Most commonly, it develops secondary to intraperitoneal spread from ruptured liver hydatid cyst or due to systemic dissemination. They are usually solitary and show the classic imaging features of well-defined cystic lesions with daughter cysts or internal septae ( Figure 11). The differential diagnosis includes pseudocysts, abscess, epidermoid cysts and hematoma [19].

Peritoneum and Retroperitoneum
Peritoneal hydatid cyts are usually secondary to rupture of hepatic hydatid cysts into the peritoneal cavity. These are generally multiple and can arise anywhere within the peritoneal cavity, with imaging features similar to those located elsewhere (Figure 12). The differential diagnoses include mesenteric cysts and intestinal duplication cysts [10].

Pancreas
Primary pancreatic involvement is very rare (0.25% of the cases), and is often associated with hepatic hydatid disease. The most common location is in the pancreatic head. The cyst may appear radiologically similar to pancreatic pseudocyst. However, a thickened laminated wall with a thin calcified layer and concomitant hepatic lesions suggest a hydatid cyst. The differential diagnosis includes pseudocyst and cystic neoplasms such as mucinous and serous cystadenoma [20]. We had a case of multiple hepatic and intraperitoneal hydatid cysts with incidental finding of a cystic lesion in the head of the pancreas with laminated appearance of the cyst wall ( Figure 13).

Adrenal Gland
Hydatid cysts of the adrenal gland are also rare, and are usually secondary to disseminated hydatidoses. Hydatid cysts account for 6-7% of all adrenal cysts. Early lesions appear as simple cysts with daughter cysts with floating membranes and wall calcifications seen in later stages. The differential diagnosis of adrenal hydatid lesions includes endothelial cysts, pseudocysts, lymphangiomatous and

Pelvis
Primary hydatid cyst in the pelvis is extremely rare and occurs in approximately 0.7% of the patients with this disease. We had a case of primary pelvic hydatid disease involving the left seminal vesicle showing multiple daughter cysts within and calcification of the cyst wall (Figures 14, 15).
Ovarian involvement is rare and is generally secondary to peritoneal spread from ruptured hepatic hydatid. Unilocular hydatid cysts can mimic an ovarian cystadenoma. Presence of daughter cysts and internal septations results in a radiological appearance similar to complex ovarian cysts ( Figure 16) and even ovarian malignancy [22].

Bladder
Primary hydatid cyst of the bladder is rare. Urinary tract and bladder involvement can occur secondary to renal hydatid cysts, due to rupture of cysts into the collecting system [5].

Cardiac and Pericardial Hydatid Cyst
Hydatid cyst of the heart is very rare (0.02-2% of cases) and is usually secondary to hematogenous spread or rupture of a lung HC [23]. The most commonly affected cardiac chamber is the left ventricle (60% of cases). Other sites include interventricular septum (10-20%), right ventricle (10%), pericardium and less commonly atria [24].
On chest radiography, localised bulge of cardiac borders can be seen [8]. The cyst may be unilocular or multilocular. Transthoracic echocardiography, CT, and MR imaging are helpful in the evaluation of cardiac hydatid cysts [5] ( Figures 17, 18).

Central Nervous System
Cerebral Hydatid occurs in about 3% of cases of hydatid infestation [25]. They are usually solitary lesions, commonly in the middle cerebral artery territory [26]. We had CT and MRI show a well-defined, smooth, thin-walled, homogeneous cystic lesion with cyst fluid showing CSF isoattenuation and isointensity on CT and MRI, respectively. On unenhanced CT, the cyst wall is iso-hyperdense to brain tissue and on MRI it shows a T1&T2-hypointense rim. Wall calcification is rare [26]. Usually no rim enhancement or perilesional edema is evident unless superinfection is present. The differential diagnoses include cerebral abscess, cystic astrocytoma, arachnoid cyst and porencephalic cyst [27].
Most common spinal hydatid cysts are those that primarily affect the vertebral bodies as a result of portovertebral venous shunts [28]. Cysts show growth in the direction of least resistance; with time the parasite may destroy the cortex causing spread into surrounding soft tissues. CT and MRI show cystic fluid of CSF attenuation and signal intensity. Usually these lesions show no rim enhancement or wall calcification [26].

Treatment
There are several treatment methods for a patient with hydatid disease, including surgery, puncture aspiration injection reaspiration (PAIR), and chemotherapy. For asymptomatic patients, a "wait and observe" approach is likely more prudent. PAIR is a minimally invasive procedure, recommended for patients with unilocular hepatic cysts ³5 cm in diameter, for cysts with daughter cysts, for cysts with detached membranes, and for multiple cysts if accessible to puncture. Chemotherapy with benzimidazoles is recommended for patients with inoperable cysts [29].