The foregoing, which can be regarded as the natural history of hydatid cysts in the intermediate host, is often modified by rupture, which may cause biliary obstruction, infection, dissemination and anaphylaxis, all of which are more important than the mass effect of the enlarging intact lesion. Type I lesions are more frequent than type II lesions. The incidence of type III lesions is unknown since these usually have no clinical importance and are rarely recorded, but personal experience (Lewall) suggests they are the most common type. The progression of hydatids from simple to complex is shown schematically in the column on the left side of (Fig. 3.26).

Fig. 3.26 Schematic representation of the natural history and complications of hydatid cysts. Cysts which have undergone contained rupture may die, but they do not become infected. The pericyst may calcify at any stage of maturation of the lesion and this does not imply that the lesion is dead. However, calcification of the endocyst and its contents is proof that the parasite has died. (Courtesy of Dr. Lewall and Clinical Radiology, 1998).

It should be noted that Gharbi (1981), in his excellent report on 121 hydatid cysts of the liver seen in Tunisia, described 5 different types of ultrasound scans. This classification based on the sonographic analysis of the morphology and structure of liver hydatids, is still in use in many parts of the world but we prefer the classification described in this chapter which is simpler and based on pathophysiology rather than a particular scanning modality. Gharbi's classification is as follows for comparison purposes: type I cyst: pure fluid collection; type II cyst: fluid collection with a split wall; type III cyst: fluid collection with septa: type IV cyst: heterogeneous echo patterns; type V cyst: reflecting thick walls.

Degeneration by aging, trauma, toxins, chemotherapy, and lack of nutrients may lead to damage of the cyst wall with an increased risk of rupture. If this layer is damaged, infection nearly always occurs and the host may suffer hyperpyrexia and be severely ill; rarely, a fatal anaphylaxis may occur. If such a cyst is infected, it may transform into an abscess. After some time, the parasitic membranes are dissolved or destroyed. At this stage it is difficult to differentiate from other infected cysts or abscesses (e.g. amebic or bacterial). Sometimes the pathognomonic hydatid hooklets are still seen in infected cysts, when all other evidence of parasitic material has been destroyed. When ruptured, the older mother cyst nearly always dies, but daughter cysts often survive. Perhaps 10-30% of cysts die and disintegrate without treatment.

The increasing size of a cyst may lead to degeneration, causing the production of brood capsules or daughter cysts. In older and larger cysts, not only may indigenous daughter cysts develop from the germinal layer but some investigators contend that exogenous daughter cysts may develop also from herniation of a portion of the cyst wall into adjacent normal tissues. These are thought to separate entirely from the mother cyst to become secondary hydatid cysts, often in the mesentery and pelvis. Convincing histopathological proof for this contention is not available.

The severity of Echinococcus infection is governed by three basic factors: (1) the species; (2) the organ or body tissues in which the larva develops; and (3) whether or not secondary cysts develop. The E. granulosus unilocular cyst has only one cyst or several totally isolated cysts, each of which is enclosed in its own solid laminated membrane, which acts as a barrier preventing the living germinal epithelium of the endocyst from invading adjacent normal tissue. The unilocular cyst induces formation of an enveloping fibrous capsule or pericyst. The E. vogeli polycystic or multilocular cysts, however, have many cysts embedded within a common adventitious membrane. The E. multilocularis alveolar cysts show an aggressive growth pattern within the liver with metastatic lesions elsewhere. A series of proliferating vesicles infiltrate into the surrounding host tissues, resembling malignancy. This type of hydatid disease will be discussed later.

In many patients with E. granulosus infection, there are numerous cysts, depending on the number of ova ingested and the number that survive: two or more cysts may be present in the same or different organs. Estimates of multiple cysts vary widely: 5-25% in some series, 30-60% in others. On average, lesions are over twice as likely to be solitary as multiple and multiorgan involvement occurs in 10-15% of patients. Multiple cysts are more common in the liver than in the lungs and elsewhere (except in metastatic hydatidosis), but, even when multiple, are distinct from the alveolar "malignant" cysts of E. multilocularis. Despite the frequency of multiple cysts, it should be remembered that a solitary cyst in any location within the body might well be a hydatid cyst (it may truly be the only cyst present or it may not be possible to recognize others clinically or by diagnostic imaging).

As already noted, the liver is the most common site of infection in echinococcosis, being involved in 50-70% of all cases. The lungs are infected in approximately 15-30% of all patients and are followed by the kidneys (4%), muscle (4%), spleen (3%), central nervous system (3%), soft tissues (3%), bone (2%), peritoneum and mesentery (2%), and heart, mediastinum, pleura, breast, thyroid, salivary glands, omentum, pancreas, orbit and spinal canal (<1-2%).

In one large series of 1,350 hydatid patients, 250 had cysts in the lung, with 16 cases being bilateral. In children, however, there is some evidence from Australia that pulmonary hydatid disease may be as frequent as liver involvement there. Of 90 patients in a children's hospital in Sydney, 45 had pulmonary and 46 had hepatic cysts; of 72 pediatric patients from Melbourne, 36 had pulmonary cysts, 18 hepatic and 9 combined pulmonary and hepatic.