Tropical diseases

Hydatid disease - Echinococcosis

 

This is a very ancient disease, known to the Greeks and described throughout the historical literature of medicine. Hydatid cysts are perhaps more often se en in temperate climates than in the tropics, and particularly where there are sheep and dogs. In the most common variety, due to E. granulosus, the primary hosts are carnivorous, e.g. the dog, while herbivora, e.g. sheep, cattle and pigs (swine) serve as intermediate hosts. In the less common infection, with E. multilocularis, the wolf, fox, cat and dog are the principal hosts and reindeer, moose and rodents are the intermediate carriers. There are other species which are pathogenic for humans: E. vogeli, found in central and tropical South America, and E. oligarthrus, which is of questionable pathogenecity. Their clinical and imaging presentation is similar to that of E. granulosus. Humans are accidental intermediate hosts for all species. The parasite is a tiny tape worm only 3-6 mm in length, but it may occur in hundreds or thousands, particularly in the small intestine of dogs. Man becomes infected by close contact with dogs or by ingesting food, water or soil which has been contaminated by faeces. Children and agricultural workers are the most likely to be infected, but uncooked food may also transmit the parasite.

When the ova have been ingested by the intermediate host, the walls of the ova are destroyed and the larvae escape, to migrate through the mesenteric venules and lymphatics. The majority are filtered out by the liver, but some will pass through to the right side of the heart and then to the lungs. As a result, the liver and lungs are the organs most commonly infected, but larvae also may lodge anywhere in the body, including soft tissues and bone. When the larva is lodged in any tissue, it develops within seven days into a tiny cyst, (or, if E. multilocularis, multiple cysts) which steadily grows over a period of years at an average rate of about 1-3 cm annually. Cysts grow more quickly in lungs than in the liver, even more slowly in bones. The cysts may remain clinically silent for many years, depending very much on the anatomical site. Each hydatid cyst has two walls (the ecto-cyst and the germinal membrane, the endo-cyst). A third wall, the pericyst is formed as the cyst grows, causes an inflammatory re action and compresses the surrounding host tissue. Within the cyst are hydatid fluid and brood capsules (scolices), which can be seen radiographically and with ultrasound as "sand" at the bottom of the cyst.

a	Figure 14. Ultrasound of hydatid cysts. (Tunisia) a) Uncomplicated cyst. Stage I. (Liver) b) Uncomplicated cyst with hydatid sand. Stage I-Il. (Liver) c) Multivesicular cyst in right kidney. Stage Ill. d) Heterogenous hydatid cyst, a solid mass. Stage IV (Liver) e) The pathognomonic appearance of membranes within the cyst. (Liver) f) Heavily calcified old hydatid cyst. Stage V (Liver)
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The two major species of hydatid infection are E. granulosus (unilocular) and E. multilocularis (multilocular or alveolar). The latter is more serious and much less easily cured, but fortunately, less prevalent. For both types of infection, symptoms are usually the result of the pressure caused as the cysts enlarge, so that the clinical presentation may be cough, hemoptysis abdominal pain or mass, a large liver, jaundice, ascites, headache, bone pain or fracture. Many hydatid cysts are a chance finding on a chest radiograph or abdominal scan. Epidemiological surveys with ultrasound have been effective in high risk populations. Occasionally, internal rupture of the cyst, usually following trauma, may cause an acute illness.

Hydatid disease of the liver (E. granulosus)

Most hydatid cysts in the liver cause no symptoms for a long time. They are usually multiple and the majority are in the right lobe of the liver. The cysts may rupture spontaneously into the biliary tract, into the peritoneal cavity or upwards into the chest, including the pericardium. Cysts will not be demonstrated in the liver on plain radiographs until there is some peripheral calcification, which usually occurs after 5-10 years. With ultrasound, CT or MR, they are commonly demonstrated as simple, well-defined cysts in which the hydatid sand can usually be seen. However, the scanning appearances depend on the maturity and health of the cyst, so that five different stages may be recognized in the liver and elsewhere in the abdomen: the stages are similar when cysts are se en in chest radiographs (Fig. 14).

Stage I. A cyst with clear fluid. Ultrasound shows either a small punched-out space without clear walls or an anechoic cyst with back wall enhancement. The cyst fluid is clearly defined. The cyst walls vary in thickness, and segments of localised thickening are a helpful diagnostic sign of hydatid disease. Most cysts are round, unless situated at the periphery of the liver (or spleen) where the cyst may follow the parietal outline. The diameter of the cyst may be from 1 cm to 20 cm.

Stage Il. The fluid contents remain clearly defined but the cyst is less well rounded, perhaps because of lowered intracystic pressure. Sections of the cyst wall may have separated, either bulging just outside or into the cyst or floating freely within it. A floating membrane is another important diagnostic sign.

Stage IIl. The fluid is now divided by septa, which are often quite thick and result in oval or rounded spaces within the well- defined surrounding cyst. There will be enhanced back wall echoes, and there may be so many septa that there is a coarse net appearance. Whenever the cyst membranes are detached, the internal spaces are less well rounded.

Stage IV. A well-defined cyst is not seen. Instead, there is a roughly rounded mass with an irregular outline. There are three different ultrasound patterns:
(a) Hypoechoic, with some internal irregular echoes. This indicates an infected, multiseptate cyst.
(b) Hyperechoic. A solid "mass" without back wall shadowing.
(c ) An intermediate, mixed pattern, about half hypoechoic but with nodular hyperechoic clusters.
The stage IV patterns are not always easily recognised as being due to hydatid disease. It is important to search carefully for the linear or band pattern of the surrounding membranes, for the hyperechoic contour, perhaps with some acoustic shadowing, and for fluid in intra- or extracystic spaces. Another characteristic appearance is variation of the echoes in different areas within the mass. But perhaps most helpful in diagnosis is the finding of another hepatic, or extrahepatic cyst at a different stage. Hydatid cysts (of E. granulosus) are seldom solitary and all are not likely to be at the same stage of development.

Stage V. A thick-walled, unusually hyperechoic cyst, causing a well defined, cone-shaped acoustic shadow. Cysts vary so much in size that while small cysts may be imaged completely, very large cysts may only be seen in part, usually the arching image from the thick front wall. The full outline must be scanned.

In older, or damaged (E. granulosus) cysts, hyperechoic calcification may be seen within the cyst walls. When calcification is heavy, it may indicate severe damage or even death of the cyst. Separation of the internal germinal layer occurs within 48 hours of the appropriate medical treatment, but may also be seen as a result of injury, usually from indirect trauma to the liver or other organs. With scanning it is possible to recognize the cysts at an early stage, and show that there are multiple cysts at different stages of development in most patients. Although the accurate diagnosis of hydatid disease is easy when there are multiple typical cysts, it is sometimes difficult to differentiate a solitary cyst from a hepatoma. In some parts of the world hepatomas are common and also

a	Figure 15. CT of hydatid cyst. a) Typical multiseptate hydatid cyst in the liver. (Tunisia) b) Large hydatid liver cyst with membranes separated to give the "double wall" appearance. Stage Il. (China) c) Multiple hydatid cysts of the liver and spleen of different sizes. (Tunisia; courtesy of Professor M. Ben Cheikh) d) Old, thick-walled hydatid cyst in the liver. Stage V (Tunisia; courtesy of Professor M. Ben Cheikh) e) Multiple cysts in the peritoneum following rupture of a hepatic hydatid cyst. (China) f) Partially calcified hydatid cyst with detached membranes and increased density after oral drug therapy. (Tunisia)
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a	Figure 16.Hydatid cysts in the lungs. a) A large hydatid cyst in the right lower lobe, which has ruptured into a bronchus and now contains air and fluid, on which the membrane can be seen. There is a right pleural effusion. (Tunisia; courtesy of Professor J. Daghfous) b) A large fluid-filled hydatid cyst in the left lung, with air between the membranes: the "crescent sign" (arrows). (Tunisia; courtesy of Professor J. Daghfous) c) Two (perhaps more) hydatid cysts in the lungs of a 4-year-old boy. (Tunisia) d) Multiple hydatid cysts (eight or more) in the lungs and mediastinum. Same are full of fluid, others have ruptured into the bronchi. (Tunisia; courtesy of Professor J. Daghfous) e) A large hydatid cyst close to the right side of the mediastinum. (China)
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present as large irregular echogenic masses with well-defined margins. Other hepatomas are more homogeneous and may resemble a metastasis. When infected, a hydatid cyst cannot be distinguished from an abscess or a hematoma and the correct diagnosis cannot be made unless there are other recognisable hydatid cysts elsewhere, or well-defined septa can be recognised within the cyst.

There are some differences between the ultrasound and the CT findings, although the same 5 stages can be seen (Fig. 15). In particular, CT may not demonstrate the walls of the cyst so clearly, unless the surrounding liver is contrast-enhanced. Most cyst walls do not enhance, but a few may and show higher attenuation. CT will show clearly the daughter cysts within the main (mother) cyst. They are usually at the cyst periphery, smaller and with lower attenuation. When dislodged, the daughter cysts float freely within the mother cyst, changing position when the patient moves. This is a useful diagnostic sign and, if there is any doubt, it is worth scanning the patient in different positions to demonstrate this movement.

CT can clearly demonstrate partial separation of the cyst membranes, causing a "double wall", or show the ribbon appearance of a separated germinal membrane. When curled or shrunken, a membrane may be shown floating on the fluid contents of the cyst, described as the appearance of a water-lily on a pond. (This is an equally important sign when seen on an erect chest radiograph.) The floating membrane is another diagnostic finding, which is unlikely to be mimicked by any other condition.

On MRI the signal intensity of hydatid cysts is the same as any other similar cyst, except that the rim is of lower intensity on both T1 and T2 weighted images.

Hydatid disease in the lungs

A chest radiograph is always required when a hydatid cyst is suspected in the liver or peritoneal cavity (Figs. 16, 17). Most lung cysts are clinically silent and present as well-defined circular densities in any part of the lung or mediastinum. Pulmonary hydatid cysts are very seldom calcified. As already noted, some cysts have an internal fluid level, and the membranes and daughter cysts may be seen floating within the cyst, one of the few pathognomonic signs in radiology. If the outer membrane is broken, there may be a thin layer of air between the inner and outer walls. If all the fluid has been coughed up, the cyst may resemble a thin-walled

a	Figure 17. a) CT scan of multiple pulmonary hydatid cysts. (Tunisia) b) CT scan of a partly collapsed mediastinal hydatid cyst. The folded membranes are clearly shown. (Tunisia; courtesy of Professor M. Ben Cheikh)
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tuberculous cavity. When secondarily infected, the appearance is the same as any lung abscess. When entirely filled with fluid, or pus, the cyst can resemble a solid tumour. However, hydatid cysts are seldom solitary, nor are they usually at the same stage of development. Nevertheless, there may be so many cysts in the lungs that they resemble metastases. Rupture of the cyst into the pericardium or pleural cavity can be a dramatic event, and the patient may be severely ill (Fig. 17). Care should be used to avoid spill age when aspirating a pulmonary cyst.

Skeletal hydatid disease (E. granulosus)

The recognition of hydatid disease in any bone requires a high level of suspicion, unless the patient is known to have cysts elsewhere or comes from an endemic area. Confusion with any other osteomyelitis is the usual error, but a tumour may also be suspected (Fig. 18).

In the long bones, hydatid cysts first appear as ill-defined, thin-walled radiolucent areas, usually near the epiphyseal end of the bone. They are

a	Figure 18. a) Posterior mediastinal hydatid cyst, affecting the vertebral body and transverse process. (Tunisia) b) Destruction of the right side of the pelvis and the right femur by hydatid disease. (Tunisia) c) CT scan of a hydatid cyst in the gluteal muscles. (Tunisia; courtesy of Professor M. Ben Cheikh)
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often multi-Ioculated, without a sharp line of demarcation. Later, well defined cystic areas will develop, the shape affected by the position within the bone. CT is helpful because it can provide better definition, especially when the cyst is in the vertebrae. The cyst may penetrate through the cortex and periosteum, producing a localised abscess. If here is a periosteal reaction, this is unlikely to be due to the hydatid disease alone and almost always indicates superimposed infection. Hydatid cysts may involve any of the long bones, the pelvis, skull, or spine. Particularly in the pelvis it is difficult to differentiate the multilocular alveolar cysts from bone tumours (see below). With any variety of hydatid disease, the destruction may become very extensive. It may be mistaken for myeloma, metastatic disease, primary bone tumours, fibrous dysplasia, other bone cysts, as well as tuberculosis or other osteomyelitis. A chest radiograph and a liver scan are essential. Aspiration is not without minor risk of a mild allergic reaction if any of the cyst contents are spilled, but can be safely performed.

Figure 19. Cerebral hydatid cysts. CT scan. (Tunisia) (Previous surgery with relapse)

Hydatid disease in other locations (E. granulosus)

Hydatid cysts can develop in the spleen, pancreas, kidneys, or the peritoneal cavity, as well as in other parts of the body. In the brain they may present clinically in the same way as any other space-occupying lesion (Fig. 19). The walls of some cysts will calcify and be clearly visible on plain radiography, a very helpful diagnostic sign. However, CT or MRI are essential to establish the location and number of cysts within the skull because many cannot be seen on plain radiographs. Hydatid cysts are avascular and do not blush on cerebral angiography.

Alveolar hydatid disease (E. multilocularis)

The alveolar type of disease (E. multilocularis) is less common than E. granulosus and is most often seen in colder climates, e.g. North America, Central and North Eastern Europe, Russia, Siberia, Australasia. Although it also causes a chronic granulomatous reaction, there are many smaller cysts (like a bunch of grapes) and the whole process is more destructive and much less easily cured than E. granulosus. Clinically, E. multilocularis infections differ from E. granulosus only because of local multiplicity and a more malignant course. Alveolar hydatidosis is usually chronic, afebrile, and almost always fatal if aggressive therapy

Figure 20. Alveolar (E. multilocularis) hydatid cyst of the liver, with fine punctate calcification and multiple small cysts. (China)

is not available. In some ways the clinical progress may resemble that caused by E. granulosus, for example, by rupture through the diaphragm from the liver with resulting pleural and pulmonary disease. But in most cases the imaging appearances of E. multilocularis infections are different and the clinical course more persistent, almost malignant in failure to respond, in recurrence and in outcome.

Alveolar hydatid disease produces a mass-like infiltrating les ion without well-defined borders in almost all organs or tissues, including bone.

Alveolar hydatid disease of the liver
In the liver (or kidney) it may be possible to demonstrate multiple small calcifications in the hydatid mass on plain radiography: they are usually clustered and characteristic (Fig. 20). But scanning is much more reliable. On CT in the early stages, there are low density masses, without any cystic appearance, and there is no contrast enhancement. Any calcification is nodular or amorphous, quite unlike the ring or linear calcification of E. granulosus.

MRI of early alveolar echinococcosis is a little less effective than CT in establishing the exact diagnosis, because such small masses, less than 2 cm, may not be clearly demonstrated and microcalcification will not be seen. However, the larger masses caused by the multiple, clustered cysts and surrounding fibrosis are easily recognised with a low signal ratio on T1 and T2 weighted images. (Occasionally there may be a higher signal on T2-images.) Indeed, the T2 weighted MR-images usually show the outline and extension into the liver and surrounding tissues more accurately than is seen with CT. When there is central necrosis, the general pattern, of cysts surrounded by an irregular boundary with a low signal intensity, is easily recognised by MRI.

Similarly, the ultrasound appearance of alveolar hydatid disease of the liver is that of a non-specific pseudotumour, a mass of variable size and shape. Ultrasound may show cystic areas and internal calcification, quite unlike the peripheral calcification of E. granulosus. The pressure of the echinococcal mass may cause dilatation of the biliary tract (better seen with ultrasound and CT than MRI) or narrowing of the portal or hepatic veins, even of the inferior vena cava.
Differentiation of alveolar hydatid disease from malignancy in the liver (or indeed in kidney, bone or elsewhere) can be very difficult and biopsy is necessary to confirm the diagnosis, especially when the punctate calcification is seen. Unlike E. granulosus infections, there are seldom multiple sites involved.

Alveolar hydatid infection of bone is a very difficult diagnostic problem. The natural course of the disease is very unpredictable and may vary from rapid local spread, to chronic, slowly invasive destruction. Sometimes diagnostic imaging is a better way to watch progress and assess the whole situation than clinical impression.

Interventional procedures in hydatid disease

Intervention may be requested in either form of hydatid disease, but particularly for the more common E. granulosus, when biopsy or cyst aspiration is required or systemic therapy has failed. Therapeutic success has followed ultrasound guided aspiration, utilising various drugs (e.g. hypertonic sodium chloride, alcohol) injected after removal of all or part of the cyst contents. Some advocate irrigation of the cyst before injection, others find this unnecessary. In the past, aspiration was regarded with anxiety, mainly from fear that spillage would cause severe anaphylactic shock or metastatic spread. In the recent series reported, there have not yet been any complications. Modem equipment and techniques seem to have made it safe, except for an occasional, moderate, brief allergic re action with fever, which is easily controlled medically. There is no contraindication to repeated aspirations. The success of aspiration and the results do depend on the location of the cysts, e.g. intraperitoneal cysts respond more quickly than hepatic. Progress can be assessed with ultrasound, showing decrease in size, change in shape, less posterior wall enhancement and increased echogenicity (or increased CT density).

 

Philip E.S. Palmer, with Stanley P. Bohrer, Carlos Bruguera, Xing-Rong Chen, Mahmoud R. EImeligi, Hassen A. Gharbi, S.B. Lagundoye, M. W. Wachira