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Echinococcus granulosus Revisited

Radiologic Patterns Seen in Pediatric and Adult Patients

¹ Department of Radiology, Leopold-Franzens Medical School and University Hospital Innsbruck, Anichstr. 35, 6020 Innsbruck, Austria.
² Department of Gastroenterology, Leopold-Franzens Medical School and University Hospital Innsbruck, Anichstr. 35, 6020 Innsbruck, Austria.

Received February 22, 2001; accepted after revision May 16, 2001.

 
Presented at the annual meeting of the Radiological Society of North America, Chicago, November 1999.

Address correspondence to B. V. Czermak.

Introduction

Top Introduction Pathophysiology Organ Manifestations Sonographic Follow-Up Conclusion References

 
Echinococcosis, also known as hydatid disease, is a parasitic infection in humans caused by the tapeworm Echinococcus in its larval stage. Of the two main forms of the infection, the unilocular cystic form caused by Echinococcus granulosus, which is considered here, is far more common than the rare multilocular alveolar form caused by Echinococcus multilocularis [1].

E. granulosus cysts are characterized by typical radiologic patterns and therefore are easily diagnosed. This pictorial essay gives an over-view of the various radiologic patterns produced by E. granulosus lesions as seen on sonography, CT, and MR imaging and discusses the involvement of various organs and tissues. We also describe typical sonographic changes seen in E. granulosus lesions during follow-up.

Pathophysiology

Top Introduction Pathophysiology Organ Manifestations Sonographic Follow-Up Conclusion References

 
E. granulosus is endemic in many sheep- and cattle-raising countries throughout the world. Definitive hosts are canines (usually dogs). Intermediate hosts are usually grazing animals (sheep or cattle) or humans. Humans may become intermediate hosts through contact with a definitive host or the ingestion of contaminated water or vegetables. When eggs of the adult tapeworm are ingested, the host's gastroenteric enzymes break down the external coating of the eggs and the embryos are freed. The embryos then migrate through the host's intestinal mucosa, enter the portal venules and lymphatics, and from there are carried to the liver, lungs, and the organs and tissues supplied by the systemic circulation. E. granulosus can affect any organ or tissue in the body, although the liver and the lungs are the most common sites of hydatid disease.

Once an E. granulosus embryo has lodged in an organ, it develops into a small cyst containing a minimal amount of fluid. The cyst increases by 2-3 cm every year, depending on the resistance of the host tissue. The cyst has three layers: the outer, pericyst; the middle, ectocyst; and the inner, germinal or germinative layer [1].

Organ Manifestations

Top Introduction Pathophysiology Organ Manifestations Sonographic Follow-Up Conclusion References

 
Abdominal Hydatid Disease
The liver is the most common site of E. granulosus infection; more than 75% of patients with hydatid disease have infected livers. Splenic involvement is rare, representing less than 2% of all manifestations of the infection in humans. The lesions may be single or multiple.

In early stages of the disease, the appearance of hydatid cysts may be uncharacteristic and mimic that of simple cysts (Fig. 1A). However, the double-line sign can often be seen on sonography in unruptured cysts (Fig. 1B); if present, it is of great value in establishing a positive diagnosis. T1-, T2-, and intermediate-weighted MR images display a low-intensity ring [2] that surrounds the cyst—the rim sign (Figs. 1C, 2A, and 2B).

View larger version (127K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —13-year-old girl with Echinococcus granulosus infection of liver. CT scan through right liver lobe shows one uncharacteristic, well-defined cyst, which has same range of attenuation as water. In early stages of infection, diagnosis may be difficult to establish.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —13-year-old girl with Echinococcus granulosus infection of liver. Axial sonogram through right liver lobe shows parallel stripes (arrowheads) that delineate space between parasite's ectocyst and host's pericyst—double-line sign. When seen, this sign helps to differentiate hydatid cysts from simple cysts, cystic tumors, pseudocysts, and metastases. In this patient, diagnosis was possible using sonography. Sonography is particularly useful for detection of double-line sign, cystic membranes, septa, and "hydatid sand."

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —13-year-old girl with Echinococcus granulosus infection of liver. Axial T2-weighted turbo spin-echo MR image (TR/TE, 2655/138) obtained 3 months after medical therapy shows degenerated cyst in right liver lobe. Detached membranes are seen within original cyst (black arrowheads) as "snake (or serpent) sign." Lesion is surrounded by low-intensity rim (white arrowheads).

View larger version (182K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —8-year-old girl with Echinococcus granulosus infection of liver. Axial intermediate-weighted spin-echo MR image (TR/TE, 2400/15) shows hydatid cyst in right liver lobe. Signal intensity is medium to low. Lesion is surrounded by low-intensity rim—typical rim sign (arrowheads) thought to represent pericyst.

View larger version (166K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —8-year-old girl with Echinococcus granulosus infection of liver. Axial T2-weighted spin-echo MR image (2.4/90) shows typical appearance of hydatid cyst with high and homogenous signal intensity. Lesion is surrounded by low-intensity rim (arrows).

In the next stages, endogenous daughter cysts and granddaughter cysts develop from the germinal layer by endoproliferation (Fig. 3A) and are the "daughter-cyst sign" (Figs. 3B, 4, and 5). The scolices (the larval stage of the parasite) detach from the germinal layer and fall to the bottom of the cyst as "hydatid sand" [1] (Fig. 6). Less frequently, exogenous daughter cysts may originate from herniation of a portion of the cyst wall (Fig. 7).

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —15-year-old girl with Echinococcus granulosus infection of liver. Oblique sonogram through right liver lobe shows endogenous formation of daughter cyst (arrow) by endoproliferation from germinal layer.

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —15-year-old girl with Echinococcus granulosus infection of liver. Oblique sonogram through right liver lobe shows daughter cyst (arrow) within mother cyst—"daughter-cyst sign."

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —15-year-old girl with Echinococcus granulosus infection of liver. Oblique sonogram obtained 2 months after medical therapy shows damaged cyst with detached membranes (arrowheads). Debris is seen at bottom of cyst (arrow).

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D. —15-year-old girl with Echinococcus granulosus infection of liver. Oblique sonogram obtained 6 months after medical therapy shows original hepatic cyst filled with twisted, detached membranes—sonographic spin (or whirl) sign. In addition, "hydatid sand" and debris are seen.

View larger version (185K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3E. —15-year-old girl with Echinococcus granulosus infection of liver. Axial sonogram obtained through right liver lobe 9 months after medical therapy shows hydatid cyst cavity (arrows) almost completely filled with echogenic material, giving lesion pseudosolid appearance. Diameter of cyst has decreased substantially, from 7 to 3 cm.

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —27-year-old man with Echinococcus granulosus infection of liver. CT scan shows typical large, multivesicular hydatid cyst in right lobe. Multiple daughter cysts are arranged mainly at periphery but also centrally (open arrows), occupying entire cyst cavity. Intervening septa are seen between daughter cysts. Small calcifications are seen in cyst wall (arrowhead).

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5. —38-year-old woman with Echinococcus granulosus infection of liver. Axial T2-weighted half-Fourier acquisition single-shot turbo spin-echo MR image (TR/TE, 4/64) shows multivesicular hydatid cyst in left liver lobe with daughter cysts and intervening septa. Daughter cysts may show high or low signal intensity, depending on factors such as composition and volume of fluid, infection, degeneration of parasitic material, and presence of scolices. Lesion is surrounded by low-intensity rim (arrowheads).

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6. —55-year-old man with Echinococcus granulosus infection of liver. CT scan shows two hydatid cysts in right liver lobe. Within larger one, free-floating daughter cyst (small arrowheads) is seen. Within daughter cyst, debris has sunk to bottom, resulting in formation of debris—cyst fluid level (large arrowheads). Within wall of mother cyst, curvilinear calcifications (arrows) can be seen.

View larger version (149K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7. —30-year-old man with Echinococcus granulosus infection of liver. CT scan shows multivesicular hydatid cyst containing one daughter cyst attached to membrane of mother cyst (arrows). Density of mother cyst is higher than that of daughter cyst because of advanced degenerative changes resulting in intracystic debris, "hydatid sand," and inflammatory cells. Difference of density gives characteristic CT picture that is diagnostic for presence of multivesicular E. granulosus cysts. Daughter-cyst densities vary between 0 and 15 H, in contrast to higher densities of mother cyst fluid (30-40 H). Exogenously proliferating cysts (arrowheads), which are only rarely observed, are seen in this patient.

When the fluid pressure in the hydatid cyst becomes too high, the fragile parasitic membranes may disrupt, and small dissections may occur through which parasitic fluid can enter the space between pericyst and ectocyst, causing detachment or collapse of the parasitic membranes. At this stage, undulated membranes that are pathognomonic for E. granulosus infections can be seen on sonography (Figs. 8A,8B,8C), CT (Fig. 8D), and MR imaging (Fig. 1C). Because of its snakelike appearance, this sign is called the "snake (or serpent) sign." Progressive collapse of parasitic membranes results in a twirled and twisted appearance of the membranes on sonography (Fig. 8A), CT, and MR imaging. This typical sign is called the spin (or whirl) sign. In advanced stages, peripheral calcifications are common (Fig. 9).

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A. —9-year-old boy with Echinococcus granulosus infection of liver, spleen, and lungs. Axial sonogram through right liver lobe shows hepatic cyst filled with twisted, detached membranes—sonographic spin (or whirl) sign.

View larger version (153K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B. —9-year-old boy with Echinococcus granulosus infection of liver, spleen, and lungs. Oblique sonogram through spleen shows detached membranes (arrows) within degenerated cyst—"snake (or serpent) sign," which indicates parasite is responding to medical therapy.

View larger version (150K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8C. —9-year-old boy with Echinococcus granulosus infection of liver, spleen, and lungs. Axial sonogram obtained with linear array shows detached membranes better than does oblique sonogram (B) obtained with curved array. Free-floating debris is seen between membranes.

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8D. —9-year-old boy with Echinococcus granulosus infection of liver, spleen, and lungs. CT scan obtained at level of spleen shows large unilocular cyst that has almost completely replaced normal splenic parenchyma. Detached membranes are seen within cyst (arrows).

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9. —80-year old man with Echinococcus granulosus infection of liver. CT scan shows large calcifications with irregular margins in right (arrows) and left (arrowheads) liver lobes, representing nonvital end-stage of disease. Calcification in left liver lobe shows laminated appearance, representing calcifications of collapsed parasitic membranes. Calcifications are best seen on CT. They usually involve pericyst and have curvilinear or ringlike appearance. With progressive degeneration, calcification may involve entire lesion and appear as dense, irregular mass.

Primary E. granulosus infections of the peritoneum are rare [3] (Fig. 10), as is renal involvement.

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10. —43-year-old man with Echinococcus granulosus infection of peritoneum. CT scan shows large mesentery hydatid cyst caused by peritoneal spread after rupture of hepatic cyst. Peritoneal cysts are usually metastases from hepatic, splenic, or mesenteric cysts that have ruptured into peritoneum.

Involvement of the Lungs and Heart
The lungs (Fig. 11A,11B) are the second most common site of E. granulosus infection in adults and the most common site of infection in children, having an overall infestation rate of 15-25% [4]. Typical radiologic signs are the meniscus sign and the water-lily sign (Fig. 8E). Under medical therapy, the spin (or whirl) sign can be seen (Fig. 8F). Unlike manifestations in other organs, calcification of cysts (Fig. 12) and the formation of daughter cysts are rarely seen in the lungs.

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A. —50-year-old man with Echinococcus granulosus infection of lungs in presence of involvement of liver. CT scan with lung parenchymal window setting shows multiple round and well-defined nodules. They are located in periphery and show contact with visceral pleura.

View larger version (91K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B. —50-year-old man with Echinococcus granulosus infection of lungs in presence of involvement of liver. CT scan with mediastinal window setting shows small cysts (arrowheads) within lesions. No calcifications are visible.

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8E. —9-year-old boy with Echinococcus granulosus infection of liver, spleen, and lungs. CT scan obtained at level of upper chest shows ruptured hydatid cyst. After rupture and discharge of cyst fluid into pleural cavity or into eroded bronchioles, endocyst collapses, sediments, and floats in remaining fluid at bottom of original cyst (arrowheads)—water-lily sign. Cyst in which no liquid remains resembles bulla in adults or staphylococcal pneumatocele in children.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8F. —9-year-old boy with Echinococcus granulosus infection of liver, spleen, and lungs. CT scan obtained at level of upper chest 12 months after medical therapy shows damaged cyst containing detached membranes (arrowheads) — spin (or whirl) sign.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12. —35-year-old woman with Echinococcus granulosus infection of lungs in absence of liver involvement. CT scan with mediastinal window setting shows multiple hydatid cysts in right lung. Curvilinear (arrowhead) and laminated calcifications (arrow), which are only rarely seen, are evident in this patient.

Rarely is involvement of the heart observed [5] (Fig. 13).

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13. —39-year-old woman with Echinococcus granulosus infection of heart in absence of liver involvement. CT scan shows well-defined hydatid cyst in myocardium (arrowheads) of left ventricle. Contrast-enhancing septa are seen within cyst. Extension of cysts into pericardium (arrows) is also seen.

Involvement of the Musculoskeletal System and Soft Tissue
Osseous lesions of E. granulosus are seen in 0.5-4% of patients. Most often, vascularized areas such as the vertebrae, long bone epiphyses, ilia (Fig. 14), skull, and ribs are affected. Radiologic patterns of echinococcosis in bones are rarely typical; therefore, diagnosis is difficult [6].

View larger version (86K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14. —30-year-old man with Echinococcus granulosus infection of bone in absence of liver involvement. CT scan with bone window setting shows large osteolytic area involving right ilium and sacrum. Lesion margins in ilium are sharp and slightly sclerotic (arrowheads), whereas margins in sacrum are blurred. Local behavior of hydatid cyst tissue is characterized by exogenous proliferation in irregular branching fashion along line of least resistance, especially inter-trabecular spaces.

Hydatid cysts involving soft tissue are seen in 0.5% to 4.7% of patients.

Involvement of the Brain
Hydatid cysts can be found anywhere in the brain, but the most common site is the region of the middle cerebral artery, especially the parietal lobe (Fig. 15). Brain manifestations of E. granulosus infections are seven times more common in children than in adults [7].

View larger version (155K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 15. —14-year-old man with Echinococcus granulosus infection of brain in absence of liver involvement. CT scan shows large and well-defined hydatid cyst of cerebrospinal fluid density in right cerebral hemisphere. Neither calcification nor perifocal edema is seen, which is typical for hydatid cysts involving brain. Within cyst, areas of higher attenuation are seen, representing debris (arrow). Multivesicular cysts are rarely seen in brain.

Sonographic Follow-Up

Top Introduction Pathophysiology Organ Manifestations Sonographic Follow-Up Conclusion References

 
Sonography is the method of choice for monitoring the efficacy of medical therapy in E. granulosus infections because hydatid cysts show typical sonographic changes during medical therapy. First, the laminated membrane detaches from the pericyst and forms endoluminal membranes (Fig. 3C) that gradually fold up. These membranes, together with debris composed of endocysts, fragmented daughter cysts, and dead scolices, gradually fill the cavity of the cyst with echogenic material (Fig. 3D). The cavity progressively fills with mostly uniform echogenic material, giving the lesion a pseudosolid appearance. The size of the cysts gradually decreases. Finally, the cysts lose all their typical sonographic features and can therefore be misinterpreted as tumors (Fig. 3E) or abscesses [8].

Conclusion

Top Introduction Pathophysiology Organ Manifestations Sonographic Follow-Up Conclusion References

 
Apart from uncharacteristic echo-free cystic manifestations, diagnosis of E. granulosus infection is possible in most cases from the onset, because of the typical radiologic patterns of daughter cysts, detached membranes, hydatid sand, and calcification. E. granulosus shows typical sonographic changes in the course of medical therapy.

References

Top Introduction Pathophysiology Organ Manifestations Sonographic Follow-Up Conclusion References

 

1. von Sinner WN. New diagnostic signs in hydatid disease; radiography, ultrasound, CT and MRI correlated to pathology. Eur J Radiol 1991;12:150 -159[Medline]
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3. Kotoulas G, Gouliamos A, Kalovidouris A, Vlahos L, Papavasiliou C. Computed tomographic localization of pelvic hydatid disease. Eur J Radiol 1990;11:38 -41[Medline]
4. Thumler J, Munoz A. Pulmonary and hepatic echinococcosis in children. Pediatr Radiol 1978;7:164 -171[Medline]
5. Gossios KJ, Kontoyiannis DS, Dascalogiannaki M, Gourtsoyiannis NC. Uncommon locations of hydatid disease: CT appearances. Eur Radiol 1997;7:1303 -1308[Medline]
6. Torricelli P, Martinelli C, Biagini R, Ruggieri P, De Cristofaro R. Radiographic and computed tomographic findings in hydatid disease of bone. Skeletal Radiol 1990;19:435 -439[Medline]
7. Danziger J, Bloch S. Tapeworm cyst infestations of the brain. Clin Radiol 1975;26:141 -148[Medline]
8. Bezzi M, Teggi A, De Rosa F, et al. Abdominal hydatid disease: US findings during medical treatment. Radiology 1987;162:91 -95[Abstract/Free Full Text]

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