Helminth-Cestode: Echinococcus granulosus and Echinococcus mutilocularis

Characteristics of Echinococcus and life cycle
Echinococcus spp are cestode parasites belonging to the family Taeniidae. All taeniid parasites
have complex life cycles that include a carnivorous definitive host and a second mammalian
intermediate host in which the larval or metacestode develops. It is the larval stage that is
pathogenic to humans and other mammals. The adult stage is a small tapeworm 2–7 mm in length
that inhabits the small intestine of carnivores such as dogs or foxes. Echinococcus granulosus
generally infects dogs (figure 1) whilst E. multilocularis the usual definitive host are foxes, but
the parasite will also infect dogs (figure 2). Molecular evidence suggests that E. granulosus is
really a complex of several species with different intermediate host preferences and variable
pathogenicity to man (table 1, figure 1). E. granulosus sheep strain (E. granulosus sensu stricto)
is the most important human pathogen. Echinococcus adults characteristically produce taeniid
eggs, approximately 40um in diameter. These eggs are morphological indistinguishable from eggs
produced by tapeworms of the genus Taenia. The eggs are passed in the faeces of the definitive
host and are infective to the intermediate host. The larval stage in intermediate hosts develops as
metacestode usually in the visceral organs (see figure 1). Humans are aberrant intermediate hosts
and become infected by ingestion of Echinococcus spp. Eggs. When humans are infected with the
E. granulosus complex, cystic echinococcosis (CE) develops, which is a space occupying fluid
filled cyst. Alveolar echinococcosis (AE) develops when humans are infected with E.
multilocularis larvae. This results in a more infiltrative lesion, initially in the liver, which can
metastasize in the late stage and is usually fatal if untreated. Echinococcosis can be directly
transmitted from contact with infected dogs or foxes or indirectly through a variety of ways
including contaminated food or water.<<<