الفهرس 
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Abstract
The present study was carried out to investigate the composition and structure of parasitic communities of Oreochromus niloticus with respect to levels of water quality in three ecosystems. A total of 360 O.niolticus were examined from three water resources; Lake Idku, Edfina; Natural Nile resource and Lake Mariout during the period from May 2017 to April 2018. All fish samples were examined for external and internal parasites and water samples were chemically and heavy metals analyzed. The results demonstrated that out of 360 examined fish specimens of O.niloticus. , 199 were found to be infested with a total infestation rate of 55.27 %. Lake Idku recorded a high parasitic burden of (72.5%) followed by Edfina region (56.66 %) then Lake Mariout with the lowest level of parasitic infestation (36.66%). The results demonstrated the detection of the following parasites from the gills, fins, skin and intestine of the examined fish: Trichodina spp, Chilodonella hexasticha , Apiosoma spp , Ambiphrya spp , Myxobolus spp , Lamproglena monodii, Macrogyrodactylus spp, Gyrodactlus spp, Acanthosentis tilapiae and Metacercaria of Clinostomum spp. Ciliates protozoan parasites predominated than other parasites where they recorded the highest incidence of 83.4% followed by Monogentic trematodes, Crustacea and Acanthocephala by 21.6%, 19.09% and 7.53%, respectively while the infestation rate of Digenea was the lowest (2%). Concerning water analysis, Edfina, Lake Idku and Lake Mariout showed an elevation in water quality parameters than permissible limits, but Lake Mariout consider to be more polluted due to elevation of Sulfides level than the permissible limits.
It was found that toxic pollution reduces the diversity of heteroxenous parasites, whereas parasites with direct Life-cycles (monoxenous) are less affected. So that this study can support the hypothesis that parasites communities may use as a good indicators of environmental stress. On the other hand, may enable parasitologists to consider and treat parasites not only as creatures that threaten the health of their hosts but also as responsive organisms with applied bioindication value in an environmental sense.