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Abstract
Human echinococcosis is a disease that results from parasitism by the larval stage of four Echinococcus species of which Echinococcus granulosus (E.granulosus) causing cystic hydatid disease and E.multilocularis causing alveolar hydatid disease are the most important. Minor species are E.vogeli causing polycystic hydatid disease and E.oligarthrus (Goldsmith et al., 2005).
Cystic echnicoccosis (CE), termed “hydatid disease” or “hydatidosis” is caused by infection with the larval stage (metacestode) of the dog tapeworm E.granulosus. It is a major zoonosis of worldwide distribution and is especially prevalent in sheep raising countries (McManus et al., 2003). The disease is characterized by long – term growth of metacestode cysts in humans and domestic animals. It is not only important as a public health problem in areas where the disease is endemic, but also responsible for significant economic loss in livestock (Li et al., 2003).
CE is considered as an emerging disease in various regions, e.g. the Middle East, Central Asia, and Northern and Eastern Africa (Eckert et al., 2001).
Now, it is well recognized that E.granulosus exhibits extensive intraspecific (strain) variation that may impact the diagnosis, epidemiology, pathology and control of hydatid disease. It has also important implications for the design and development of vaccines, as well as diagnostic reagents and drug treatment (Zhang et al., 2003).
Ten genetically distinct strains (G1 – G10) of E.granulosus have been described. They exhibit a considerable diversity of morphology, development, host range, infectivity to humans, pathogenicity and antigenicity (Breyer et al., 2004). The most previously characterized isolates from humans, could be consigned to E.granulosus G1 (common sheep strain) and E.granulosus G6 (camel strain). Human infection with E. granulosus G7 (pig strain) is unknown, although a closely related genotype was described from humans and named G9 (Scott et al., 1997). The sheep strain (G1) with known pathogenecity in humans (Bowles and McManus, 1993c) was identified in sheep, goats, cattle, camels and humans. While the camel strain (G6) with apparently low infectivity to humans, was found only in camels and goats (Wachira et al., 1993).
Precise diagnosis of cystic echniococcosis cannot be established by using the available aids to the clinician such as roentgenography, radioisotopic scanning, ultrasonography and computerized tomography. However, they can determine the location, size and appearance (i.e. fluid filled or calcified) of a mass lesion (Rickard and Lightowlers, 1986). Therefore, the frequent difficulty in obtaining a definitive diagnosis by these means is the reason why immunological methods have played an