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

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 hair around the anus [16, 17].

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 the end of about 5 months [18].

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

#### **Figure 3.**

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

**51**

**Figure 4.**

*Cross section in the hydatid cyst [20].*

Echinococcus granulosus

age, approximately 15–1 cm3

**8.1 The outer (pericyst) layer**

**8.2 Laminated (medial) layer**

**8.3 Germinal (inner) layer**

*DOI: http://dx.doi.org/10.5772/intechopen.90708*

diameters are about a few millimeters [22].

**8. Structure of hydatid cyst (larval stage)**

[21].

The hydatid cysts (**Figure 4**) of *E. granulosus* are often spherical or semispherical if not compressed by adjacent organs. The size of the hydatid cysts varies with

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

It is a solid, noncellular chitinous layer, white in color, consisting of microfibers fibers and dense granules rich in amino carbohydrates observed under electron microscopy [23]. It plays a role in protecting the parasite from the immune response of host and providing the suitable environment for its continued growth in addition to its role in reducing the effect of drugs used in the treatment of the disease [6].

This is the inner layer of hydatid cyst, a cellular living layer containing nuclei and associated with the lamellar layer by fingerprints formed by the germinal layer [20, 24]. It acts to protect components of the cyst and controls the osmotic pressure of the cyst wall [25]. The buds are formed from the germinal layer and grow toward the cavity of the cyst, and after the buds become vacuolated and stalked, the process of forming buds from the inner layer of cells begins from
