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Echinococcus multilocularis Revisited

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

Received August 4, 2000; accepted after revision September 18, 2000.

 
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 Conclusion References

 
Echinococcosis is a parasitic infection of humans caused by the larval stage of the tapeworm Echinococcus. Of the two main forms of the infection, the unilocular cystic form caused by E. granulosus is far more common than the rare multilocular alveolar form caused by E. multilocularis [1].

E. granulosus cysts are characterized by typical radiologic patterns that are easily diagnosed, but diagnosis of E. multilocularis lesions is more difficult. The alveolar cysts grow by exogenous proliferation and behave like a malignant neoplasm. Therefore, E. multilocularis lesions can cause physicians to generate a long list of differential diagnoses, including malignant tumors. However, making the correct diagnosis is possible if imaging findings are correlated with appropriate clinical findings.

We give an overview of the various radiologic patterns produced by E. multilocularis lesions as seen on sonography, CT, and MR imaging and discuss the involvement of various organs and tissues and common complications.

Pathophysiology

Top Introduction Pathophysiology Organ Manifestations Conclusion References

 
E. multilocularis is endemic in much of the upper Midwest of the United States, Alaska, Canada, Japan, central Europe, and parts of Russia [2]. Definitive hosts are foxes and, less commonly, cats and dogs. Intermediate hosts are wild rodents. Humans are infested either by direct contact with definitive hosts or indirectly by intake of contaminated water or contaminated plants such as wild berries. E. multilocularis can affect any organ or tissue in the body, although the liver and the lungs are the most common sites of hydatid disease. Diagnosis can be established by imaging techniques, immunologic tests, or percutaneous biopsy.

E. multilocularis produces multilocular alveolar cysts (1-10 mm in diameter) that resemble alveoli and grow by exogenous proliferation with cysts progressively invading the host tissue by peripheral extension of the processes originating in the germinal layer. The larva causes invasive and destructive changes in the human host that often lead to complications.

Organ Manifestations

Top Introduction Pathophysiology Organ Manifestations Conclusion References

 
Involvement of Liver and Spleen
The liver is the most common site of E. multilocularis infection with more than 90% of patients having infected livers. Splenic involvement is rare, representing less than 5% of all manifestations of the infection in humans. The lesions may be single or multiple. Seventy percent of these lesions develop in the right liver lobe and 60% in the hilar region [3].

The clinical course of alveolar hydatid disease resembles that of a slowly developing tumor. In the liver, the oncosphere (the first larval stage of E. multilocularis) under-goes a metamorphosis into the metacestodal stage. Small, low-attenuation cysts with diameters of less than 1 cm (Figs. 1A and 1B) represent metacestodal vesicles [4]. These fluid-filled vesicles of the asexually proliferating metacestodes are composed of an inner germinal layer, a syncytial tegument, and an outer acellular "laminated" layer. Growing E. multilocularis lesions show a great tendency to form central liquefactive necrosis, which may be surrounded by vital metacestodal vesicles. Necrosis is caused by vascular involvement associated with ischemia [5].

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —40-year-old woman with Echinococcus multilocularis infection of liver. Oblique sonogram obtained through right liver lobe shows several small hypoechoic areas with indistinct margins (large arrowheads) within echogenic region with indistinct border (small arrowheads). Pattern is caused by metacestodal vesicles within stroma.

View larger version (143K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —40-year-old woman with Echinococcus multilocularis infection of liver. CT scan shows multiple ill-defined cystic lesions with different attenuation values scattered throughout liver. Lesions represent solitary and fused metacestodal vesicles and areas of liquefactive necrosis.

On sonography, these lesions usually show a "hailstorm pattern" (Fig. 2A). This pattern represents the histopathologically heterogeneous stroma containing microscopic metacestodal vesicles, areas of nonliquefactive necrosis, entrapped host tissue, and microcalcifications, which account for the stroma's relatively increased echogenicity [2, 4].

View larger version (143K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —7-year-old girl with Echinococcus multilocularis infection of liver. Axial sonogram obtained through right liver lobe shows multiple echogenic nodules with irregular and indistinct margins in right lobe—the "hailstorm pattern"—representing parasitic stroma.

CT (Fig. 2B) and MR imaging display multiple irregular, ill-defined lesions scattered throughout the involved liver tissue. This radiologic pattern resembles that of metastases or primary hepatic neoplasm [1].

View larger version (142K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —7-year-old girl with Echinococcus multilocularis infection of liver. CT scan shows multiple hypodense nonenhancing lesions with irregular and indistinct margins throughout liver.

Large areas of central necrosis are difficult to differentiate from abscesses. (Figs. 3A, 3B, and 4). However, there is poor or no enhancement after bolus administration of IV contrast medium, emphasizing poor vascularization of the parasitic lesion. Usually, no lymphadenopathy is observed [5].

View larger version (183K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —41-year-old woman with metastatic Echinococcus multilocularis infection. Oblique sonogram obtained through right liver lobe shows lesion with central liquefactive necrosis. Large hypoechoic region with some internal echoes can be seen. Hyperechoic border (arrowheads) of lesion is irregular and indistinct.

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —41-year-old woman with metastatic Echinococcus multilocularis infection. CT scan obtained at level of liver reveals large heterogeneous lesion in liver with irregular and indistinct margins showing same morphology as lesion in spleen. Small peritoneal nodules with punctate calcifications (arrowheads) can be recognized.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —68-year-old woman with Echinococcus multilocularis infection of liver. CT scan reveals heterogeneous, abscesslike lesion with irregular contours and indistinct margins. There is no contrast enhancement. No calcifications are visible.

In advanced stages, peripheral calcifications may be seen within the areas of central necrosis (Figs. 5A and 5B). Secondary pyogenic infection may occur at any time during the course of disease, resulting in abscess formation (Fig. 5C). Apart from the typical peripheral irregular calcifications, large homogeneous (Fig. 6), multiple punctiform (Fig. 7A), or scattered calcifications may be seen. Calcifications are found in 90% of all infected patients [1].

View larger version (169K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —38-year-old woman with Echinococcus multilocularis infection of liver in advanced stages. CT scan shows large hypodense mass with central necrosis and peripheral calcifications that are coarse, irregular, and rimlike.

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —38-year-old woman with Echinococcus multilocularis infection of liver in advanced stages. Axial T2-weighted MR image (turbo spin-echo; TR/TE, 5000/165) reveals central necrosis with high signal intensity. Peripheral calcifications show low signal intensity (arrows).

View larger version (150K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C. —38-year-old woman with Echinococcus multilocularis infection of liver in advanced stages. Axial CT scan obtained 10 years later than A shows air—fluid level and abscess formation within parasitic lesion. In this patient, abscess formation was sudden.

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6. —66-old-man with Echinococcus multilocularis infection of liver. CT scan reveals large calcification with irregular margins in right liver lobe, representing nonvital final stage of disease.

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A. —40-year-old man with Echinococcus multilocularis infection of liver. Axial T1-weighted MR image (turbo spin-echo; TR/TE, 8/4) shows dilated bile ducts (arrows) in left liver lobe and lobar atrophy. Multiple hypointense lesions scattered throughout liver represent calcifications (arrowheads).

Hilar infiltration is observed in approximately 50% of all patients, resulting in dilatation of the intrahepatic bile ducts (Figs. 7A and 7B) and invasion of the portal vein (Figs. 8 and 9, the portal branches, and the hepatic veins. These conditions lead to hypoperfusion and subsequent atrophy of the affected liver segments (Figs. 7A and 10). This atrophy is further aggravated by marked fibroinflammatory reactions of the liver tissue [3].

View larger version (101K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B. —40-year-old man with Echinococcus multilocularis infection of liver. T2-weighted MR image obtained using rapid acquisition with relaxation enhancement sequence reveals dilatation of bile ducts (arrows) in left liver lobe.

View larger version (182K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8. —73-year-old man with Echinococcus multilocularis infection of liver. Oblique sonogram obtained through liver hilum shows infiltration of portal vein by parasitic tissue (arrow).

View larger version (146K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9. —50-year-old woman with Echinococcus multilocularis infection of liver and retroperitoneum. CT scan reveals infiltration of liver hilum and retroperitoneum by parasitic tissue. Note infiltration of left portal vein (arrows) and inferior vena cava (large arrowhead) resulting in obstruction of these vessels. Small calcifications (small arrowheads) can be seen.

View larger version (166K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10. —49-year-old man with Echinococcus multilocularis infection of liver. CT scan shows right lobar atrophy caused by invasion of hilum by parasitic tissue. Scattered areas with calcifications, dilated bile ducts, and areas of liquefactive necrosis can be seen.

Involvement of the Lungs
Lung involvement is rare in E. multilocularis infections [6]. Patients with pulmonary lesions may remain asymptomatic for a long time, and thus the lesions are often discovered incidentally. The disease spreads either by hematogenous dissemination (Fig. 11) or by direct invasion of parasitic tissue from adjacent structures (Fig. 12A).

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11. —46-year-old man with Echinococcus multilocularis infection of lungs. CT scan with mediastinal window reveals irregular, ill-defined mass that is located peripherally and shows contact with visceral pleura. Within lesion, small cysts (arrows) can be seen. Different densities are visible; margins are indistinct. Pleural effusion and reactive pleural thickening are also evident.

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A. —60-year-old man with metastatic Echinococcus multilocularis disease. CT scan obtained at level of chest shows parasitic mass in lower median mediastinum caused by direct penetration of hepatic lesion through right hemidiaphragm (arrowheads). As result of infiltration and obstruction of inferior vena cava by parasitic tissue, venous blood flow is collateralized via azygos and hemiazygos veins. These veins thus appear markedly enlarged (arrows).

Multiple bilateral pulmonary lesions are difficult to distinguish from other pulmonary diseases such as pulmonary metastatic disease, E. granulosus infection, Wegener's granulomatosis, or other granulomatous lung diseases.

Involvement of the Musculoskeletal System and Soft Tissue
Clinical symptoms include thoracolumbar pain, abscess and fistula formation, and painless palpable nodules. In most patients, bone lesions develop as a result of progressive invasion of the adjacent echinococcal tissue into the bone. Therefore, the sternum and the vertebrae (Fig. 12C) are the most common sites of osseous lesions. Lesions caused by hematogenous dissemination have been described in the ribs, vertebrae, and gluteal region [7].

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12C. —60-year-old man with metastatic Echinococcus multilocularis disease. CT scan obtained at level of lower abdomen shows infiltration of third lumbar vertebra (arrowheads) by retroperitoneal lesion.

Soft-tissue lesions are displayed clearly on MR imaging and CT. Multiple small, clustered cystic structures can be identified (Fig. 13). Contrast enhancement is probably due to an inflammatory reaction of adjacent soft tissue [7].

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13. —54-year-old woman with metastatic Echinococcus multilocularis disease. CT scan shows involvement of liver (straight arrows), spleen (arrowheads), left adrenal gland (curved arrow), and soft tissue (open arrows). In contrast to lesions within abdominal organs, marked contrast enhancement is seen within soft-tissue lesions.

Involvement of the Peritoneal and Retroperitoneal Space
Involvement of the peritoneum occurs because of secondary infection caused by either cyst rupture or direct invasion from adjacent structures. Patients present with abdominal distention, meteorism, and constipation.

On CT, the omentum appears thickened by multiple thin-walled hypodense cysts resembling pseudomyxoma peritonei (Fig. 3C). Lesions of the same morphology can be found in the minor pelvis mimicking drop-metastases or primary urogenital tumors (Fig. 3D). However, ascites is often missing even in cases of extensive involvement of the peritoneal cavity [8]. Pancreas, adrenal glands (Fig. 13), and kidneys (Fig. 14) may be affected by direct spread of adjacent echinococcal tissue or by hematogenous metastases. Retroperitoneal lesions may infiltrate veins and enclose arteries (Figs. 12B,12C,12D).

View larger version (143K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —41-year-old woman with metastatic Echinococcus multilocularis infection. CT scan obtained at level of lower abdomen shows diffuse cystic infiltration of omentum by parasitic tissue resulting in thick, sheetlike mass containing multiple small cysts (arrows). There are small curvilinear calcifications in some cysts (arrowheads).

View larger version (169K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3D. —41-year-old woman with metastatic Echinococcus multilocularis infection. CT scan obtained at level of pelvis shows parasitic tissue extending into pelvis and encasing uterus and adnexa. Small calcifications (arrowheads) can be seen.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14. —35-year-old man with metastatic Echinococcus multilocularis disease. CT scan obtained at level of kidneys shows large retroperitoneal parasitic lesion infiltrating right kidney, inferior vena cava, and iliopsoas muscle. No contrast enhancement is visible at periphery of lesion calcifications (small arrowheads). Patient had previously undergone hemihepatectomy of right liver lobe for E. multilocularis infection. Bile ducts in left liver lobe are dilated (large arrowheads).

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B. —60-year-old man with metastatic Echinococcus multilocularis disease. CT scan obtained at level of kidneys reveals large retroperitoneal lesion with central necrosis. Lesion has infiltrated and obstructed inferior vena cava (arrowhead), enclosed aorta, and infiltrated right lower kidney (arrow).

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12D. —60-year-old man with metastatic Echinococcus multilocularis disease. CT scan obtained at level of pelvis reveals echinococcal tissue located distal to retroperitoneal lesion (B and C) and growing within inferior vena cava, using vein as sheath. Note small central punctate calcification (arrowhead).

Conclusion

Top Introduction Pathophysiology Organ Manifestations Conclusion References

 
E. multilocularis lesions are difficult to diagnose. The frequent presence of clusters of microcalcifications (appearing in 90% of the patients) is of great value in establishing a positive diagnosis, especially if the patient has few clinical symptoms and no history suggesting exposure to E. multilocularis, such as residing in a rural area. In cases of extensive peritoneal or retroperitoneal involvement, contrast enhancement and ascites usually are missing.

References

Top Introduction Pathophysiology Organ Manifestations Conclusion References

 

1. Claudon M, Bessieres M, Regent D, et al. Alveolar echinococcosis of the liver: MR findings. J Comput Assist Tomogr 1990;14:608 -614[Medline]
2. Didier D, Weiler S, Rohmer P, et al. Hepatic alveolar echinococcosis: correlative US and CT study. Radiology 1985;154:179 -186[Abstract/Free Full Text]
3. Rozanes I, Acunas B, Celik L, Minareci O, Gokmen E. CT in lobar atrophy of the liver caused by alveolar echinococcosis. J Comput Assist Tomogr 1992;16:216 -218[Medline]
4. Choji K, Fujita N, Chen M, et al. Alveolar hydatid disease of the liver: computed tomography and transabdominal ultrasound with histopathological correlation. Clin Radiol 1992;46:97 -103[Medline]
5. Akin O, Isiklar I. Hepatic alveolar echinococcosis: a case report. Acta Radiol 1999;40:326 -328[Medline]
6. Treugut H, Schulze K, Hubener KH, Andrasch R. Pulmonary involvement by Echinococcus alveolaris. Radiology 1980;137:37 -41
7. Merkle EM, Kramme E, Vogel J. Bone and soft tissue manifestations of alveolar echinococcosis. Skeletal Radiol 1997;26:289 -292[Medline]
8. Taneja K, Gothi R, Kumar K, Jain S, Mani RK. Peritoneal Echinococcus multilocularis infection: CT appearance. J Comput Assist Tomogr 1990;14:493 -494[Medline]

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