الفهرس 
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Abstract
Chemicals are primarily used in aquaculture in the treatment and prophylaxis of disease problems, which are the largest single cause of economic loss. A total number of one hundred and twenty apparently healthy O. niloticus weighed 50± 5 g were obtained from fish farm at Al-Manzala, Al-Dakhalia Governorate, Egypt, and transported in large plastic water containers to the laboratory of fish disease and management, Mansoura Veterinary laboratory of Animal Health Research Institute of the Agricultural Research Center. The fish were stocked in fully prepared glass aquaria filled with approximately 80 liters of dechlorinated tap water for two weeks as acclimatization period before starting the experiment during which they fed on a basal experimental diet containing 30% protein and offered to fish 2 times daily at 3% of their body weight provided in equal rations at 09:00 Am and 13:00 Pm. Fish were divided into four groups (30 fish each), fish were randomly assigned in two replicates into 4 groups of 15 fish / aquarium tank. The experimental period lasted 21 days: group 1 (G1): Control group, fish were reared in dechlorinated tap water free from any chemicals and fed a basal diet (30% crude protein). group 2 (G2): Fish were fed the basal diet supplemented with Oxytetracycline at (80 mg ̸ kg/biomass/day). group 3 (G3): Fish were exposed to 1 ̸ 4 LC50 (0.1 ppm) of malathion 5% (powder form) in water and fed on a basal diet (30% crude protein). group 4 (G4): Fish were exposed to Formalin 37% and Malachite green in the ratio (100: 0.2 ppm). Blood, serum, tissue and water samples were collected from each group for haematological, biochemical, antioxidant activities, lipid peroxidation, DNA damage detection, molecular investigation, chemicals residue analysis, physicochemical properties of water and histopathological examination. Our finding revealed the following : 1- Hematological profile revealed significant decrease in RBCs, Hb and PCV % values in all chemicals treated groups with lowest value in MAL and FMG then OTC group. Also, MCHC decreased in all chemicals exposed group than the non- exposed control group but not significant after the two periods. MCV exhibited an increase in OTC (G2), MAL (G3) and FMG (G4) exposed group when compared with control (G1) group after 11 and 21 days. 2- Total leukocytic count (TLC) and lymphocytic counts were in similar trend, they were significantly increased in chemical treated groups compared to control. Particularly, they were highly increased in MAL treated group. Also, heterophils were significantly decreased in OTC treated group while increased in MAL and FMG treated groups compared to control. 3- Serum biochemical parameters of Nile tilapia revealed that there was adverse effect of different chemicals on these parameters including (TP, albumin, globulin, ALT, AST and ALP) Serum aminotransferase enzymes revealed high activities in fish exposed to MAL, OTC or FMG. Also, creatinine and urea level revealed higher value in chemicals treated group compared to control. In opposite total protein and its derivatives displayed decreased value. 4- Oxidant and antioxidant parameters revealed that MDA level was significantly higher in all chemicals treated groups compared to the control group. Besides, antioxidant enzymes activities in liver homogenate revealed that CAT, GSH, GPx and SOD levels were significantly decreased in the all chemicals treated groups compared to the control group. 5- Immunoglobulin (IgM (was significantly decreased in chemicals treated groups compared to control. In the same trend, serum lysozyme activity was significantly decreased in chemicals treated groups compared to control. 6- Significant increase in liver DNA damage in MAL treated group followed by FMG treated group and then OTC treated group as compared by control group after 11 and 21 days which indicated by increases in tail length, tail DNA% and tail moment. 7- MHCIIA, IgM heavy chain and TLR7 genes were significantly downregulated in head kidney in all groups with lowest expression in G3 followed by G4 as compared to the control group G1, while TNF-a gene was significant upregulated in the liver tissue of all chemicals treated groups with highest expression in G3 followed by G4, as compared to the control group (G1) after 11 and 21 days. 8- Chemicals residue were determined in liver tissue and water sample after 11 and 21 days, where in liver tissue OTC found to be lower than the MRLs and not detected after 21 days MAL and formalin found to be higher than the MRLs. While in water MAL only detected and found to be higher than MRLs. 9- The physico-chemical characteristics of the water for all the experimental periods remained constant and found to be within the limits of EOS- (1589/2005). 10- Histopathological examination revealed histopathologic changes of chemicals exposed liver tissue where sever large area of hemorrhage, congested blood sinusoids, degeneration of pancreatic and hepatic cells, necrosis and degeneration occurred in pancreatic acini in addition to severe vacuolation in hepatocytes. While chemicals exposed kidney tissue revealed increase in melanomacrophage center, mild increase in lymphopoietic tissue over hemopoietic tissue with presence of hemosiderosis, area of necrosis in the parenchyma, severe congestion of the blood vessel and degenerative changes in the parenchymatous tissue.