Nine species of Echinococcus have been recognized to date: Echinococcus granulosus sensu stricto (G1 to G3), Echinococcus equinus (G4), Echinococcus ortleppi (G5), Echinococcus canadensis (G6 to G10), Echinococcus multilocularis, Echinococcus vogeli, Echinococcus oligarthrus, Echinococcus felidis, and Echinococcus shiquicus. All but E felidis and E shiquicus are able to infect humans with their larval stages. The definitive hosts are canids (eg, dogs, coyotes, wolves, dingoes, jackals), except for E oligarthrus, which has been isolated only in wild cats. Humans become accidental intermediate hosts when the eggs from the feces of dogs, wolves, or other canids are ingested.
PATHOGENESIS AND EPIDEMIOLOGY
Human cystic echinococcosis caused by the larval stage of E granulosus sensu stricto (G1 to G3), E equinus (G4), E ortleppi (G5), and E canadensis (G6 to G10) is the most frequent form of disease. The adult worm of E granulosus is found in the intestine of dogs, wolves, and other canids. The worm measures only about 0.5 cm in length. It has a scolex with hooks; a neck region; and 1 immature, 1 mature, and 1 gravid proglottid. The dog usually harbors hundreds or thousands of adult tapeworms. The eggs, which are morphologically similar to those of Taenia species, are excreted in the feces. When an intermediate host, such as sheep, ingests the eggs, the embryo hatches from the egg, penetrates the intestinal mucosa, and enters lymphatics or blood vessels. The host defense mechanisms destroy many embryos, but those surviving develop into expanding cystic structures called hydatid cysts (Fig. 333-1). The rapidity of cyst growth is quite variable and partially dependent on the tissue localization, but an increase in diameter of 1 cm or more per year is not uncommon, with faster growth in children. Spherical brood capsules arise from the inner germinal membrane of the cyst wall. Protoscolices, the precursors to the scolices of the adult worms, develop from germinal membrane and the inner surface of the brood capsules and accumulate within the cyst as “hydatid sand” (Fig. 333-2). If the cyst, or a portion of it, is eaten by a suitable definitive host, adult tapeworms develop in the small intestine. Hydatid cysts are capable of developing in nearly any tissue, including the central nervous system and bone; however, 90% of them develop in either the liver or the lung, most frequently in the liver.
A hydatid cyst capsule removed surgically from the lung. (Used with permission from S. Santivañez, 2016.)
Hydatid sand. A: Scolices invaginated into cyst membrane (140×). B: Evaginated scolex with hooklets; stalk is present, by which the scolex is continuous with the germinal epithelium (140×).
Human infection with hydatid cysts is most common in sheep- and cattle-raising areas ...