Echinococcus granulosus

This species has a worldwide distribution, found in both domestic and wild settings. Humans are infected especially in countries with extensive pastoral activities. The highest frequencies of E. granulosus are found in Eurasia, Africa, Australia, and South America. (Arambulo, 1997; Eckert, et al., 2000; Lightowlers, et al., 2000)

Adult E. granulosus are found in the intestines of canines such as dogs, wolves or dingos. It can also be found in lions. Echinococcus granulosus larvae are found primarily in the lungs and liver, but may be found in any organ of the intermediate host. These hosts include sheep, moose, wallabies, camels, warthogs, and reindeer. Humans are most often infected with hydatid cysts in sheep-raising communitees. Accidental ingestion of E. granulosus eggs results from humans touching dogs that come in contact with livestock. (Roberts and Janovy, 2000)

The mature adult measures 3 to 9 mm long and consists of only 3 proglottids (the immature, mature and gravid), a scolex with four suckers, and a double crown of 28 to 50 hooks on the rostellum, at the tip of the scolex. Echinococcus granulosus may be found as a hydatid cyst in any organ of an intermediate host. The size of a cyst ranges from 4.1 to 7.2 cm and takes about 5 months to develop. Eggs of E. granulosus are indistinguishable from other taeniid eggs, typically 32-36 by 25-30 micrometers. (Kearn, 1998; Wardle, et al., 1974)

The gravid proglottid detaches from the adult and ruptures, releasing eggs into the feces of the definitive host. The intermediate host ingests eggs in contaminated food or water. The egg hatches to an oncosphere which develops into a unilocular hyatid cyst. Daughter cysts bud from the inner walls of the cyst via asexual reproduction, giving rise to numerous juveniles called protoscolices. Canids eat the intermediate host and hence acquire the protoscolices which develop to adults. (Ohio State University, 2001; Roberts and Janovy, 2000)

The gravid proglottid detaches from the adult and ruptures, thereby releasing eggs into the feces of the definitive host. The intermediate host ingests eggs in contaminated food or water. The egg hatches to an oncosphere which develops into a unilocular hyatid cyst. Daughter cysts bud from the inner walls of the cyst via asexual reproduction, giving rise to numerous juveniles called protoscolices. Canids eat the intermediate host and hence acquire the protoscolices which develop to adults. (Ohio State University, 2001; Roberts and Janovy, 2000)

Oncospheres in the intermediate host's intestine penetrate the mucosa using its six little hooks. They enter a portal venule and may end up in any organ, most commonly in the lung or liver and develops slowly into a unilocular hydatid cyst. When the cyst is eaten by the definitive host, the protoscolices evaginate and attach to the villi in the small intestine. It takes about 56 days to become an adult. (Kearn, 1998; Roberts and Janovy, 2000)

Cestodes in general have sensory organs in the scolex, which are attached to longitudinal nerves extending down the body. The nerves are attached to organs and the cestodes can detect tactile stimulation. (Brusca and Brusca, 2003)

The adult Echinococcus granulosus is parasitic on canids, the definitive hosts. Nutrients are absorbed across the worm's tegument. Many mammals may be intermediate hosts, especially herbivores. (Roberts and Janovy, 2000)

These animals are probably not preyed on directly but are ingested. Egg and larval mortality are high due to the parasite not reaching appropriate hosts.

Echinococcus granulosus parasitizes canines such as dogs, wolves or dingos. It can also be found in lions. Intermediate hosts include sheep, moose, wallabies, camels, warthogs, and reindeer.

In humans, Echinococcus granulosus is the most common agent of cystic hyatid disease. Cysts may be asymptomatic for years, but pressure of the cyst on surrounding tissues or bones may lead to blindness, collapse of infected bones, or even sudden death if the cyst is in the heart. If a cyst ruptures the host may get anaphylactic shock, a hypersensitive reaction to a flood of foreign material in the body that results in death. Treatment for this disease often involves removal of the cyst via major surgery. (Lightowlers et al, 2000) (Lightowlers, et al., 2000)