الفهرس 
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Abstract
The present study aimed to analyze and compare the gills and intestinal fauna of the silver catfish, Bagrus bajad (Forsskal, 1775) from two freshwater habitats in the Damietta Branch of the River Nile. This study was carried out for four successive seasons, from January to December 2018. Also, Precise schematic drawings and measurements were done and documented for each parasite species. A good deal of attention was paid to clarifying the relationship between physiochemical features as well as between biotic factors of the host (host sex and condition factor) and infection variables (prevalence, mean intensity and abundance) of helminths of B. bajad. Furthermore, the present study also aimed to estimate nine heavy metals in water, fish gills and intestine and parasites. The histopathological impacts of helminths fauna and their possible contribution to the silver catfish host were examined. Three digenean helminth parasite species (Acanthostomum absconditum, Acanthostomum spiniceps and Haplorchoides cahirinus) were encountered in the intestine of catfish, B. bajad dwelling both habitats, midstream and downstream. The monogenean Quadriacanthus bagrae was only detected from the gills of the silver catfish dwelling in the midstream habitat whereas the acanthcephalan parasite Rhadinorhynchus nasher (new species) and nematode parasite Amplicaecum sp. respectively were only detected in the intestine of bagrid fishes inhabiting the downstream habitat. from the midstream habitat accommodated obviously higher number of worms (6180, 65.01%) than their conspecifics dwelling in the downstream habitat (3326, 34.99%). Seasonal population dynamics of helminth parasites showed that all digenean parasites, A. absconditum, A. spiniceps and H. cahirinus attained their highest numbers (279.33, 85,00 and 246.33, respectively), and highest abundance (11.41, 3.35 and 10.39, respectively) in the midstream during spring. Similar spring were highest for the mean intensity (21.01 and 11.9, respectively) of digenean parasites, A. absconditum and A. spiniceps in the midstream. While the digenean parasite, H. cahirinus attained its highest mean intensity (33.17) at the downstream during winter. On the other hand, the digenean parasites, A. spiniceps and H. cahirinus attained their highest prevalence (38.14% and 43.66%, respectively) during Autumn and spring respectively in the midstream, while digenean parasite, A. absconditum attained its highest prevalence (70.77%) in the downstream during winter. In the midstream habitat, monogenean parasite Q. bagrae attained its highest numbers, prevalence, mean intensity and abundance (190.33, 50.72%, 15,66 and 7.71, respectively) during autumn. While in the downstream, canthocephalan parasite, R. nasher attained its highest numbers, prevalence and mean intensity (10.0, 12.96%, and 3.24, respectively) during summer, while its highest abundance (0.86) was during spring. On the other hand, nematode parasite, Amplicaecum sp. attained its highest numbers, mean intensity and abundance (7.0, 3.28 and 0.28, respectively) during autumn. While its highest prevalence (9.84%) was during summer. Water temperature showed weak relationships with the prevalence and mean intensity values of all studied helminth species. On the other hand, TDS and EC showed a negative significant correlation with the mean intensity values of all digenean and monogenean parasites at midstream habitat. At the midstream habitat, dissolved oxygen showed a positive significant correlation with the prevalence and mean intensity values of A. spinicpes, H. cahirinus and Q. bagrae. Water temperature, TDS and EC showed a positive significant correlation with the prevalence of the acanthocephalan R. nasher at the downstream habitat. Digenean parasites of the Silver Catfish, Bagrus bajad accumulated higher amounts of all studied heavy metals than gills, intestine and water samples in the selected study sites except for chromium that recorded its highest concentration in the gills. Also, with few exceptions, the concentration of all heavy metals in tissues (gills, intestine and digenean worms) was found to be influenced by their concentration in water. In the midstream habitat, the all digenean parasites and the monogenean Q. bagrae recorded higher mean prevalence on female compared on male fish of B. bajad. The results of this study showed that mainly condition factor of fish of class I was highest infected with number of parasites then followed by class II then finally class III showed the lowest infection with parasites. In histological preparations considerable blood supply and lymphocytic infiltration were recognized in the vicinity of the digenean parasite attachment. Abundant goblet cells were also relevant. Observations on close contact between the anteriormost region of the Q. bagrae and the tips of gill lamellae demonstrated that the distal extremities of gill lamellae are swollen and fused, with the drainage of a considerable amount of blood in the vicinity of the host-parasite interface. Strict control enforced by laws regulation should be applied over polluted industrial wastewater, agricultural drainage water and sewage water debouches into the River Nile. All these sources of water input affect the physicochemical characteristics of water, biological components, fish production and human health. It is recommended to focus on further studies, on the mechanisms of heavy metal uptake by aquatic organisms, especially parasitic ones, to find new ways to eliminate these pollutants from the environment. It is important to make ultrastructural descriptions with molecular identification for the newly identified acanthocephalan species, Rhadinorhynchus nasher in order to evaluate the molecular phylogenetic analyses.
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