Chapter 131: Zoonotic Cestodes

HYDATID DISEASE (ECHINOCOCCOSIS)

Hydatid disease (echinococcosis) is the infection of humans by the larval stages of taeniid cestodes of the genus Echinococcus. Humans are incidental intermediate hosts for several Echinococcus species, which cause various forms of “hydatid disease,” including cystic echinococcosis (CE), alveolar echinococcosis (AE), and neotropical echinococcosis [which includes polycystic (PE) and unicystic echinococcosis (UE)]. Four species/complexes of Echinococcus are currently recognized, of which three cause distinctive forms of human disease: Echinococcus granulosus sensu lato with cystic echinococcosis (CE), Echinococcus multilocularis with alveolar echinococcosis (AE), and Echinococcus vogeli with polycystic echinococcosis (PE). The fourth species, E. oligarthrus, has only rarely been identified in humans and is associated with unicystic echinococcosis (UE).¹ Diverse subpopulations of E. granulosus, distinguished by genetic, morphologic, and biologic characteristics, have long been recognized; the taxonomic significance of these differences remain unresolved and controversial.¹ However, recent demonstration of consistent genetic differences has prompted calls for splitting this species. A cause of high morbidity and with capacity for considerable pathology, Echinococcus spp. are collectively some of the most important zoonotic cestodes from a public health standpoint.^1,2

Life Cycle

The life cycles of Echinococcus species involve carnivores as final hosts and herbivores or omnivores as intermediate hosts (Fig. 131-1). In their adult stages, Echinococcus species are “true tapeworms” belonging to Eucestoda (family Taeniidae) measuring 3–6 mm long (E. granulosus sensu lato) or 1.7–3.0 cm long (E. multilocularis) at maturity and possessing generally three to five proglottids with a large, oblong terminal proglottid. The scolex possesses a rostellum with two concentric rings of hooks and four suckers, and is attached to the strobila (“body”) with a narrow, tapering neck.³ All members of Echinococcus use carnivorous mammals as definitive hosts, where they typically localize in the lower duodenum and jejunum of the definitive host. All known human-infecting Echinococcus have definitive hosts that are members of the Canidae family, with the exception of the very rare E. oligarthrus, which uses various tropical members of Felidae. Eggs containing infective oncospheres are expelled in large numbers in feces. These eggs are immediately infectious to intermediate hosts, not requiring a period of development in the environment, and are rather hardy. Eggs have been shown to be viable for up to a year in cool, humid conditions.³

Following ingestion of eggs by susceptible intermediate hosts, oncospheres emerge from the egg in the small intestine and embed in the small intestinal wall where they enter host circulation. Oncospheres develop into metacestodes; the encapsulation of the metacestode by the intermediate host ...