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The larval stage of the cestode (tapeworm) Echinococcus multilocularis causes alveolar echinococcosis, one of the less common forms of human echinococcosis (hydatidosis, or hydatid disease). Echinococcus multilocularis occurs in the northern hemisphere, including central Europe and the northern parts of Europe, Asia, and North America.
The adult E. multilocularis (1.2 to 3.7 mm) resides in the small bowel of the definitive hosts, which are foxes and, to a lesser extent, dogs, cats, coyotes, and wolves. Gravid proglottids (bisexual reproductive segments) release eggs that are passed in the feces. After ingestion by a suitable intermediate host (a small rodent), the egg hatches in the small bowel and releases an oncosphere that penetrates the intestinal wall and migrates through the circulatory system into the liver, where larval growth remains indefinitely in the proliferative stage, resulting in invasion of the surrounding tissues.