**8.1 The outer (pericyst) layer**

*Overview on Echinococcosis*

Genus: Echinococcus (Rud, 1801) Species: granulosus (Batsch, 1786)

The adult phases of the *E. granulosus* lives in the mucous layer of the definitive host's small intestine, and the eggs are highly resistant to harsh environmental conditions for several months or even a year depending on environmental conditions [15]. Therefore, it remains a source of infection to the intermediate hosts during drinking contaminated water and food, including humans that may also be infected by contact with infected dogs, especially in children, whereas eggs adhere to dog

The eggs reach the stomach of the intermediate host and then decompose the chitinous cortex by digestive juices and release the embryo (oncospheres) of the sixth-hooks, and the oncospheres penetrate the intestine and reach the liver, lungs, and other organs including the brain and muscles to develop into hydatid cysts at

When the definitive host feeds on infected organs of the intermediate host, the parasite will reach its small intestine, where the primary heads grow into adult worms within 7–4 weeks, and each worm produces thousands of eggs per day, start-

*The adult* E. granulosus *(sensu lato) (2–7 mm long) resides in the small intestine of the definitive host. Gravid proglottids release eggs that are passed in the feces and are immediately infectious. After ingestion by a suitable intermediate host, eggs hatch in the small intestine and release six-hooked oncospheres that penetrate the intestinal wall and migrate through the circulatory system into various organs, especially the liver and lungs. In these organs, the oncosphere develops into a thick-walled hydatid cyst that enlarges gradually, producing protoscolices and daughter cysts that fill the cyst interior. The definitive host becomes infected by ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices evaginate, attach to the intestinal mucosa , and develop into adult stages in 32–80 days. Humans are aberrant intermediate hosts and become infected by ingesting eggs . Oncospheres are released in the intestine , and hydatid cysts develop in a variety of organs . If cysts rupture, the liberated protoscolices may create secondary cysts in other sites* 

**7. The life cycle of the parasite**

hair around the anus [16, 17].

the end of about 5 months [18].

ing the cycle again [15] (**Figure 3**).

*within the body (secondary echinococcosis) [19].*

**50**

**Figure 3.**

The outer layer is also called adventitia or ectocyst that encases the hydatid cyst, and this layer is produced by the host cells (modified dense fibrous protective tissue) as the host's response to the infection. There is a close interaction between the host tissue and the parasite, and this layer plays an important role in the development and survival of the cyst. Any degradation of the outer layer leads to the degeneration or explosion of the hydatid cyst; the diameters of a pericyst layer vary depending on the host organ where the hydatid is present, but in general, the diameters are about a few millimeters [22].
