The adult parasite is a tiny tapeworm, 3-6 mm in length, hundreds or thousands of which may be found in the small intestine of a canine (Fig. 3.4A).The worm is composed of four segments (Fig. 3.4B,C): the head or scolex is pyriform with four suckers and a rostellum containing a double row of hooklets, enabling the worm to attach itself to the intestinal mucosa of the canine. Behind the short neck are three proglottids, the terminal and broadest of which is a gravid egg-containing segment. The shed gravid proglottids and/or eggs are excreted in the feces of the primary host and contaminate the ground, including pastureland. They are subsequently ingested by intermediate hosts such as sheep, cattle, horses, pigs, goats, and camels. Monkeys, kangaroos, tapirs, moose, squirrels, and the house mouse are less common intermediate hosts.

Fig. 3.4 The adult stage of Echinococcus granulosus. (A) Massive infection with thousands of tiny tapeworms in the small intestine of a dog. (B) The hooks on the scolex (right) appear as a dark ring. Numerous eggs are present in the terminal proglottid (left). (C) Line drawing of an adult E. granulosus showing the pyriform scolex or head (right), which contains four suckers and a rostellum with a double row of hooklets by which the worm attaches to the intestinal mucosa of the canine. Behind the short neck are three proglottids, the terminal and broadest of which is a gravid, egg-containing segment. AFIP 53-20053-1.