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ALVEOLAR HYDATID DISEASE (E. multilocularis)

Alveolar hydatid disease is an infection caused by larval stages of the tapeworm, Echinococcus multilocularis. These larvae invade the infected organ, which in at least 90% of patients is the liver, and cause great destruction, resembling an infiltrating malignancy.

Geographic Distribution

This less common alveolar form of hydatid disease is seen chiefly in south central Europe in the area of the Alps, especially in France, Switzerland, Germany and Austria, and throughout much of Russia and Siberia from the White Sea eastwards to the Bering Strait (Fig. 3.1). Other endemic areas in Asia extend from Turkey eastwards through Iran, Afghanistan, India, China, Mongolia, and northern Japan. In North America the parasite is present in subarctic Alaska and Canada, with recent expansion into several north central states of the United States. It has also been reported rarely from Australia, New Zealand, Argentina, Uruguay, and England.

Epidemiology and Pathology

Dogs, foxes, wolves, and cats are the usual definitive hosts for E. multilocularis, while field mice and other small rodents are the natural intermediate hosts (Figs. 3.2 and 3.156). In the Arctic regions, especially among Eskimos, sled dogs are the usual source of human infection. In agricultural regions in central Europe and elsewhere, man may become infected by eating fruits or vegetables contaminated by the feces of foxes or other canines (Fig. 3.156), by handling contaminated soil, or by intimate association with infected dogs or cats.

Fig. 3.156 Mode of transmission of E. multilocularis in agricultural regions in Central Europe and elsewhere. See also Fig. 3.2 for alternate modes of transmission of the parasite.

For many years there was a controversy as to whether alveolar hydatid disease represented a separate entity. It was first described in 1855 as an "ulcerating multilocular hydatid of the liver". The larva of E. multilocularis is multiloculated rather than unilocular. Its growth is invasive and destructive, in part because the human host cannot develop an intact enveloping pericyst. This larva therefore behaves like a malignant neoplasm and is very difficult to eradicate surgically. The liver is the primary organ of involvement in over 90% of human infections. Larval growth is exogenous with spread by budding, and the affected tissue is honeycombed by the alveolar hydatid (Figs. 3.5 and 3.157). Once the cystic mass has developed to a certain size, it undergoes central necrosis, forming an abscess-like cavity while the more peripheral cysts continue to multiply. The infection may spread by direct extension to adjacent organs and lymph nodes or hematogenously to produce metastatic foci in the lungs or distant organs.

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Fig. 3.157 Alveolar or E. multilocularis hydatid disease of the liver. (A) Experimental infection in the liver of a muskrat. The multicystic nature of the parasite is well demonstrated in this formalin- fixed specimen. (B) E. multilocularis cyst in a gerbil liver. (C) E. multilocularis infection in a vole liver. Note the many cysts of the parasite. Numerous brood capsules and invaginated scolices are present. The small dark granules are calcareous bodies. (D) H & E stained section from the liver of an Eskimo who died of E. multilocularis infection. The parasites appear as irregularly lined cystic spaces (left and center) with hepatic and inflammatory cells on the right. (A, C and D courtesy of R. L. Rausche).

Although the disease runs a chronic, afebrile course in most patients, the outcome is usually fatal. The patient will have hepatomegaly, with clinical signs of an enlarging mass within the liver, and portal hypertension, splenomegaly, jaundice, and ascites (Fig. 3.158). It is usually very difficult to distinguish clinically between alveolar hydatid disease and carcinoma of the liver. The diagnosis is established by histologic examination of biopsy specimens, although radiographic examination may be quite helpful when the peculiar calcifications seen in the majority of cases of alveolar hydatidosis are present.

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Fig. 3.158 (A and B) Terminal alveolar hydatid disease in a cachectic woman from India. The abdomen is markedly protuberant from massive hepatomegaly and ascites. Generalized wasting of the patient is obvious. E. multilocularis is rarely seen in India, whereas E. granulosus infection is not uncommon. (Courtesy of Dr. A. Chandrahasan Johnson, Tulsa, Oklahoma).

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