Five species of Echinococcus have
been recognized to date. Echinococcus granulosus, E multilocularis, and
more rarely E vogeli and E oligarthrus infect
humans with their larval stages. The definitive hosts are canids,
except for E oligarthrus, which includes wild felids
as intermediate hosts. Humans become accidental intermediate hosts when
the eggs from the feces of dogs, wolves, or other canids are ingested.
A fifth species, E. shiquicus, has been recently
recognized, but it is not known whether it may infect humans.1,2
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;
one immature, one mature, and one gravid proglottid. The dog usually
harbors hundreds or thousands of adult tapeworms.1,3 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.
338-1). The rapidity of cyst growth
is quite variable and partially dependent upon 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.
338-2). 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
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 such as the countries bordering the Mediterranean, Australia,
New Zealand, and the Andean region in South America, particularly
Peru and Argentina.1,5 In the United States, most
infections are found among immigrants from endemic areas. However, there
have been foci of infection among Basque shepherds in California,
Mormon ranchers in Utah, and Native Americans in Arizona ...