Hydatid disease persists in some parts of the world. In parts of South America the incidence varies between 13 and 76 new cases for every 100000 of population each year. In the United States of America, Great Britain, and northern Europe, the disease is rare. The incidence is intermediate in eastern Australia, where the disease remains endemic in Victoria and New South Wales. The frequency with which people travel makes it necessary for surgeons in most parts of the world to know something about the pathology, natural history, and treatment of clinical hydatid disease.

The most common form of hydatid disease is caused by Echinococcus granulosus. In humans, this is a localized cystic disease, found most commonly in the liver (about 70 per cent) or lung (about 25 per cent).

E. multilocularis infection is uncommon. It is most likely to cause clinical disease in central Europe, Siberia, Turkey, Alaska, and Northern Canada. This infection occurs in an invasive, alveolar form, and is most found in the liver. The lesions do not have the well demarcated, cystic structure of E. granulosus infection. It may spread by way of the blood vessels. In short, it behaves like a malignancy, and its prognosis is worse than infection with E. granulosus.

E. vogleri and E. oligarthrus, behave like E. multilocularis in humans. They are rare, and are found only in South America. Infection by these species is not considered further in this chapter.

The hydatid is a dimorphic parasite (Fig. 1) 2675. It exists exclusively as a short flat worm (cestode) in the primary host, which is usually a member of the dog family. The cestode lives in the small bowel of the primary host, is some 4 to 6mm in length and has 3 to 5 segments below the head. The terminal segment of the worm is the gravid segment, which contains between 300 and 500 ova, and which is periodically shed in the dog's faeces. The ova are resistant to drying, and may remain viable for weeks after being passed. A grazing animal swallows the ova. The outer covering of each ovum is digested, allowing a hexacanth embryo to hatch in the upper bowel of the ruminant, to penetrate the bowel wall, and to gain access to the portal circulation. Although most embryos are trapped in the liver, some pass through to the lungs or to other parts of the body. Embryos may also make their way into the lacteals and pass directly into the systemic circulation by way of the cisterna chyli. The life-cycle is completed when the dog eats contaminated offal.

Humans usually contract the disease in childhood or adolescence when ova are swallowed after contact with an infected dog. Man is an accidental intermediate host, representing a dead end for the parasite, since dogs seldom have access to human viscera.

The primary host for E. multilocularis is usually a fox, and the intermediate host a microtine rodent such as the vole or the lemming.

Preventive campaigns depend on interrupting this life-cycle. While there is a tendency for families to segregate those who are known to be infected, humans are not infective to other humans.

The mature cyst of E. granulosus characteristically contains three layers, regardless of the organ in which it grows. These layers comprise the adventitia, a layer of scar tissue and compressed host organ tissue; the laminated membrane, a gelatinous acellular layer manufactured by the parasite, and loosely adherent to the adventitia, making a natural cleavage plane through which the infective part of the parasite can be removed; and the germinal epithelium, from which the brood capsules bud, which contain protoscolices or potential worm heads.

The fluid within the cyst may contain minute daughter cysts and viable fragments of germinal epithelium. Spillage of fluid into a body cavity can allow further hydatids to develop.

The intermediate phase of E. multilocularis infection is not characterized by an adventitial layer confining the parasite, nor is there a major cystic cavity within the mass of parasitic tissue. Instead, the parasite infiltrates like a malignancy, and forms a honeycomb of microcysts. Because there is no natural cleavage plane between the parasite and the host organ, the principles of surgical treatment of E. multilocularis infection involve principles similar to those used in cancer surgery.

Infestation usually occurs in childhood; growth is slow. As hydatid cysts grow, they tend to make their way towards regions of low pressure. In the liver, for example, they make their way from the depths of the liver towards the peritoneal surface, particularly beneath the diaphragm. Cysts may also make their way into nearby bile ducts and, less frequently, into the hepatic veins. In the lung, cysts tend to present themselves on the pleural surface of the lung, and to rupture into the bronchi. Compression of surrounding tissue occurs, but so slowly that even large cysts are usually asymptomatic.

As the cyst grows, the attachment between the laminated membrane and the adventitia becomes more tenuous, and laminated membrane tends to break away from the adventitia. This shearing stimulates the parasite to form daughter cysts (Fig. 1) 2675. These are most commonly contained within the cavity of the main cyst, as endogenous daughter cysts. Exogenous daughter cysts form outside the cavity of the main cyst, when the adventitia splits as it stretches.

Because a hydatid cyst grows slowly, the patient may be completely unaware of the parasite, even when it reaches a large size. A radiograph obtained for another purpose may show a densely calcified cyst in an elderly person who has never been aware of hydatid infection. Calcification does not mean that the cyst is dead: even a heavily calcified cyst may contain viable elements.

Symptoms tend to occur in association with complications. As the cyst dies, leakage of fluid produces pericystitis, which causes pain and low-grade fever. When bacterial infection occurs, the patient presents with the clinical picture of an abscess. A growing hydatid may erode the tubular structures within the parenchyma of the host organ. Extrusion of hydatid membrane may occur into the bile ducts, the bronchi, or blood vessels. Obstructive jaundice, coughing up of hydatid fluid and membrane (resembling grape skins), and multiple pulmonary emboli can all occur. Leakage of bile into a hydatid cyst usually results in death of the main cyst, although not necessarily of daughter cysts; this may be accompanied by severe pain. Hydatids can cause allergic phenomena in the form of urticaria, pruritus, asthma, and even anaphylaxis.

Physical findings are often unimpressive. Hydatid cysts in the liver usually grow in the dome of the right liver in segments VII and VIII, and push the liver downward. The most common physical finding, therefore, is diffuse hepatomegaly rather than a mass in the liver. Pulmonary signs may depend on the degree of pressure exerted on the bronchi, on consolidation surrounding the cyst, and on rupture of the cyst into the bronchi.

A large battery of tests for hydatid disease is available, but those most commonly used are the complement fixation test, indirect fluorescent antibody test, enzyme-linked immunosorbent assay, radio-allergosorbent test and specific immunoelectrophoretogram. Laboratories are usually equipped to perform at least two of these tests. The complement fixation test is useful, because it becomes negative after cure of a hydatid cyst, and can therefore be used to assess the efficacy of treatment. It is, not, however, particularly sensitive and there are many false-negative results. The immunoelectrophoretogram test is the most specific so far devised.

Simple posteroanterior or anteroposterior radiographs are sometimes helpful when they show calcification in the wall of a hydatid cyst. Heavy plaques of calcification are seen in the walls of dead cysts, although live daughter cysts may survive within the main cyst.

Ultrasound and CT scans can establish the diagnosis of hydatid disease when daughter cysts are identified within a mother cyst (Fig. 3) 2677. Both ultrasound and CT are helpful in determining the extent of the disease, as well as defining the relationships between the cyst and adjacent structures, such as the inferior vena cava when the cyst occurs in the liver.

Large and recurrent hydatid cysts require additional investigation. Endoscopic retrograde cholangiopancreatography or bronchography may be helpful when previous surgery has distorted local anatomy or if the surgeon suspects a communication between a cyst and the biliary or bronchial tree. Radio-isotope scanning has no place in diagnosis. Magnetic resonance imaging produces clear and detailed pictures, but its place in diagnosis is not yet established.

Although the simplest treatment is evacuation and drainage, it is not suitable unless the cyst is infected with bacteria. The old operation of marsupialization, in which the open cyst is sutured to the parietal peritoneum, so that a drainage track is formed at this site, has also been abandoned, since the associated morbidity rate is high.

The standard procedure for treating most cysts in soft tissue of any organ is evacuation of the cyst and obliteration of the residual cavity. Adequate exposure is important, since evacuation of the cyst must be carried out without spillage of its contents (Fig. 4) 2678.

To treat liver cysts, a long, curved, transverse abdominal incision with a vertical extension carried to the right of the xiphisternum allows upward retraction of the rib cage, giving excellent access to the peritoneal cavity. It is rarely necessary to open the chest, even when the cyst occurs in the right dome of the liver. A search for previously undetected cysts should be undertaken, and the retroperitoneum should be examined, since hepatic hydatid cysts sometimes extrude laminated membrane and germinal epithelium into the bare area. Infective material may track downward as far as the pelvis in the extraperitoneal plane. Cysts may be found in the lesser omentum around the pancreas, in the spleen, and in the kidneys in association with hepatic hydatids. Intraoperative ultrasound is most helpful in determining the extent of disease and in detecting exogenous daughter cysts that might otherwise be missed.

Once the disease has been adequately assessed, the area around the cyst should be packed with sponges soaked with 3 per cent saline solution, which appears to be less irritant to tissues than alternative scolicides. Once the packing is complete, it is traditional to aspirate as much of the contents of the cyst as possible, and to replace the aspirate with a scolicide. Silver nitrate (0.5 per cent), hypertonic saline (3 per cent or 20 per cent), 2 per cent formalin, and cetramide have been used, but there is little evidence of their effectiveness in the clinical setting. Each of these would have to reach an adequate concentration within the cyst; it is impossible, however, to predict the degree of dilution caused by residual cyst contents. In addition, accidental injection of formalin into the biliary tree has been associated with a progressive sclerosing cholangitis. It is rarely possible to evacuate a cyst completely: if there are many daughter cysts and if the laminated membrane is detached from the wall of the main cyst, it may be impossible to remove more than a few millilitres of fluid.

Once some degree of decompression and control has been gained, stay sutures are inserted into the presenting adventitia. Drainage of a large cyst will require provision of three suction tubes with an internal diameter of at least 25mm, because the gelatinous laminated membrane will block conventional tubing. A useful device to assist in maintaining control is an evacuation cone. The suction cone devised by Aarons has a grooved inferior rim connected to a side arm, which is connected to suction, sealing the cone to the surface of the cyst. An incision is then made through the centre of the cone, and the large bore evacuation tubing is used to clear away the material that pours out, and to suck away fluid and laminated membrane from the inside of the cyst. Evacuation takes place in the plane between laminated membrane and adventitia. Use of the suction tubing as the source of vacuum is the most effective way to remove the laminated membrane as it falls away from the adventitia.

Once the cyst has been evacuated, the incision in the adventitia can be enlarged, allowing the depths of the cyst to be inspected. Unsuspected daughter cysts may be found in the depths of the main cavity of a large hepatic cyst. These should be extracted with the suction tubing. The inner lining of the adventitia is then swabbed with 3 per cent saline solution, and the cyst is partly filled with 3 per cent saline solution to kill any residual viable elements.

The saline solution is aspirated, and all redundant adventitia is removed. In the liver, this excision is taken back to the surrounding viable tissue, and the liver edge is oversewn to secure haemostasis. This procedure usually leaves a saucer-shaped cavity in the liver, which must be inspected for bile leaks. The most effective way to detect bile is to pack the residual cavity with a clean, dry sponge and to compress the adjacent liver between the hands. After a minute the sponge is removed and inspected for bile staining. Bile leaks are closed. Cholangiography should be carried out, and the common duct explored if there is evidence of laminated membrane within it.

When removing the laminated membrane from a pulmonary cyst, it is sometimes possible to make an incision through the adventitia, and to force out the intact laminated membrane by increasing the respiratory pressure. The laminated membrane may rupture: the whole operative area therefore needs to be surrounded with gauze packs. There is nearly always a bronchial leak through the thin adventitia of a pulmonary cyst, which must be identified and closed.

In the lung, closure of dead space by suture may be possible. In the liver, various techniques of closure have been reported, but the most effective is omentoplasty. The greater omentum is mobilized from the transverse colon, maintaining the blood supply from both the right and left gastroepiploic arteries if possible. The omentum is laid into the cavity. Bile leakage is common for several days after removal of a hepatic hydatid cyst, but generally stops spontaneously.

A bronchobiliary fistula is best managed through a thoracotomy. The involved lung is removed and the cyst in the liver is evacuated in the manner already described.

Calcified hepatic cysts measuring up to about 12cm and situated well away from the inferior vena cava can be treated by cystopericystectomy. A dissection plane is developed between the adventitia and the liver: this plane is not clearly defined, and is crossed by bile ducts and vessels. The advantage of the operation is that calcified tissue is removed, and the dead-space is easier to close. However, more blood is lost during cystopericystectomy than is usual during the standard operation of evacuation and omentoplasty. Complete excision of cysts in other areas, such as the retroperitoneum, can be achieved with the same technique.

Hepatic, pulmonary, or renal resection may be necessary for the treatment of complex cysts with multiple exogenous daughter cysts and for recurrences. Resection is the best surgical procedure for E. multilocularis cysts, because there is no central cavity to be emptied, nor is there a clear margin for pericystectomy.

Cysts superinfected with bacteria are considered to be abscesses and are treated as such by drainage and antibiotic therapy. Infection in a calcified cyst often persists until all the calcified material has been removed. Persistent biliary fistulae may be found in heavily calcified cysts. An asymptomatic, heavily calcified cyst in an elderly person should be left alone, since the potential morbidity of surgical removal outweighs the benefit.

The mortality rate associated with surgical treatment should be lower than 5 per cent. A cumulative recurrence rate of 22 per cent has been reported, with all recurrences occurring within the first 3 years. Recurrence is much more common among patients who have evidence of spread of the disease at the time of the primary operation. Recurrence can also result from spillage of cyst fluid during operation.

The thiabendazoles can be used for the treatment of echinococcosis: albendazole appears to be particularly effective in some cases. The drug is usually given in a dose of 10 mg/kg.day in two divided doses for 28 days. Treatment is then suspended for 14 days, and liver function tests and full blood counts are checked. An ultrasonic or radiological measurement of cyst size is carried out. After 14 days without drug treatment, the cycle is repeated and continued until cyst measurements are stable for two successive examinations.

There is still dispute about the value of treatment with albendazole for patients first presenting with hydatid disease. In Australia, the drug is available only for clinical trials, and its use is restricted to patients with recurrent disease, with multiple body cavities affected, with surgically inaccessible disease, or with serious associated medical conditions. Albendazole is particularly effective treatment for recurrent E. granulosus infection detected on routine examination after primary hydatid surgery. It may also be effective against E. multilocularis.

There will be, no doubt, further developments in hydatid chemotherapy. Surgery remains the first line of treatment, but albendazole has a place in the management of some complex problems.

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