Within the genus Echinococcus, there are four species recognized: Echinococcus granulosus, Echinococcus multilocularis, Echinococcus vogeli, and Echinococcus oligarthrus. The larval cestodes of all four species can develop in the human host and can cause various forms of hydatid disease. A fifth species, Echinococcus shiquicus, a tapeworm of Tibetan foxes, has recently been described, but there have been no infections reported in humans. Recent mitochondrial DNA studies have identified Echinococcus felidis as a distinct species. The adult worm resides in the African lion and the larval form is believed to occur in wild ungulates. To date, no cases have been reported in humans. This chapter focuses on disease caused by cystic echinococcus (CE) caused by E. granulosus and alveolar echinococcus (AE) caused by E. multilocularis.
E. granulosus is made up of a number of biologically and genetically distinct entities that have been referred to as strains or subspecies. Classic CE is caused by the adult worm, E. granulosus, that resides in the jejunum of dogs and other canines (definitive hosts) and produces eggs that are passed in the stool. Eggs ingested by cows, sheep, moose, caribou, or humans (intermediate hosts) liberate an embryo in the duodenum, which passes through the intestinal mucosa to enter the portal circulation. Over 85% are filtered by the liver and lungs, where they lodge and develop into hydatid cysts.
AE disease results from infection by E. multilocularis. Transmission to humans is usually through accidental ingestion of parasite eggs shed by dogs that had previously eaten an infected rodent. In humans, the metacestode (larval) form develops in the liver, proliferating indefinitely by exogenous budding, and invades the surrounding tissue, mimicking a malignancy.