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Chapter 3

HYDATID DISEASE (ECHINOCOCCOSIS)

With Dr. Walther von Sinner and Dr. David Lewall

Hydatid disease was known to Hippocrates and Galen; it was well recognized by many physicians in the 19th century. It is not a truly tropical disease, since it is one of the few parasitic diseases that occur with far greater frequency in the temperate and subtropical zones of the world. Although it is most commonly seen in countries where sheep are raised, man and numerous other animals are frequently infected. Even the moose and the house mouse are intermediate hosts for this ubiquitous parasite. It is found throughout much of the world and must be considered in the differential diagnosis of a cyst or mass in virtually any patient who is residing in, or has traveled through, an endemic area. The prevalence is unknown because of underreporting and the probability that the majority of infections are undiagnosed. Many asymptomatic hydatid cysts have been first discovered by population surveys using ultrasound, as in Tunis and northern Kenya. In one series, 60% of hydatid cysts in the liver were clinically unsuspected: others are found by chance during a radiographic examination for some unrelated reason. The diagnosis can often be confirmed by direct cyst aspiration and treatment can be initiated. The disease is often managed more effectively in countries where it is known than in the United States and Europe, where it tends to be an exotic intruder. Among the many parasites infecting man, few are more serious in their consequences than the larval phase of hydatid disease.

There are two widely different principal varieties of the infection. The more widespread and better known cystic hydatid disease is caused by the larval form of the dog tapeworm Echinococcus granulosus and is characterized by the formation of one or more expanding unilocular cysts. The less common alveolar form is caused by Echinococcus multilocularis and produces destructive invasive lesions more closely resembling a malignancy than a parasitic infection. Because of the great disparity in the pathological, clinical, and radiological manifestations of these two varieties of hydatid disease, they will be discussed separately. The much less common Echinococcus vogeli, and the very rare Echinococcus oligarthus, have been responsible for occasional cases of polycystic hydatidosis: the first human infections with these hydatids were not recognized until 20 years ago.

Cystic hydatid disease is unusual in that rupture and its sequelae (biliary obstruction, infection, dissemination, and anaphylaxis) are more important than the mass effect of the cyst, despite the enormous size sometimes attained by the lesion. The natural progression from simple to complex cyst, and then to senescence and death, as well as rupture and its sequelae can be imaged noninvasively by ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI), and in some instances by plain radiographs. The gross appearance of hydatid cysts, even after rupture, is often appreciated better by the radiologist than by the pathologist, who usually receives only fragments of tissue (although even a fragment of the wall of a cyst has a characteristic gross and microscopic appearance and is diagnostic of hydatid disease).

Of the 20 or more classifications of cystic hydatidosis that have been proposed, most are based on imaging rather than on pathology of the lesion; many are not helpful in making outcome comparisons, nor are they ideal for planning surgical or medical treatment. A major problem is that, as imaging has improved over the past three decades, classifications have changed. The use of percutaneous drainage in addition to established surgical and medical therapy has increased the need for a consistent system of classification. In this chapter the pathophysiology of hydatid disease is reviewed and a pathology-based classification (Lewall, 1998) with imaging correlation (von Sinner and Lewall) is presented.

Synonyms

Hydatid disease. Echinococcosis. Echinococciasis. Tapeworm disease. Hydatidosis.
Sp: Hidatidosis. Equinococia. Fr: L'echinococcose. Maladie hydatique. Ger: Beckenknochenechinokokkus. Echinokokkose. Blasenwurm-krankheit. Echinokokken-krankheit.

Definition

Hydatid disease is infection by larvae of the genus Echinococcus (Table 3.1). Four species can infect humans, but there are two main varieties, E. granulosus and E. multilocularis, with a widespread distribution. The uncommon E. vogeli, and the rare E. oligarthus, are found only in Central and South America.

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Copyright: Palmer and Reeder