الفهرس 
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Abstract
Bacterial diseases affecting marine fishes are numerous and they cause high economic losses in marine culture sector in Egypt, therefore; this study was conducted for two main goals; Isolation and identification of the Photobacterium damselae ssp. piscicida affecting cultured seabream (Sparus aurata L.) as well as studying the relationship between the water quality, heavy metal concentrations, some pesticides pollutants in water and the incidence of Photobacterium damselae ssp. piscicida diseases. The determination of LD50 of the Photobacterium damselae ssp. piscicida and the chronic experiment in addition of trials for immunization were carried out. 1) In our study, a total of 120 moribund and surviving of seabream (Sparus aurata L.) fish were sampled between January and December 2017 from two farms chosen to represent the Egypt’s Northwest region (Wadi Mariout Lake in Alexandria province) and southeast region (Mathalath El-Diba in Domitte province) located along the Mediterranean Sea.. Fish were transported a live to the laboratory of the department of poultry and fish diseases, Faculty of Veterinary Medicine, Alexandria university. The freshly dead fish specimens were subjected to full clinical, postmortem (PM) lesions, parasitic and mycological as well as bacteriological examinations. 2) After culture on media and biochemical reactions, (110 isolates) from all naturally examined fish species were obtained from Seabream. Then, biochemical and serotyping of these isolates which relate to Photobacterium species. After that, the percentage of the Photobacteria damselae serotypes for sea bream was identified. 3) The incidence of bacterial isolates in the internal organs of all naturally examined fish species (Gills, Liver, Spleen, Heart and Kidney) has been studied and the results revealed that : a) The total number of bacterial isolates from gills was (34 isolates), liver was (28 isolates), (14 isolates) from kidney, (22 isolates) from spleen and (12 isolates) from heart and this means that the highest percentage of isolates were from gills (30.91from bacterial isolates. b) It was found that highest percentage of Photobacterium damselae ssp. piscicida infection occurred with higher incidence in naturally infected seabream in liver (37.04 %) then gills (33.33 %) and (14.81 %), (7.41%) and (7.41 %) in spleen, heart and kidney respectively. c) Antimicrobial susceptibility tests against different Ph. damselae ssp. found that were resistant to Ampicillin, Gentamicin, Rifampicin and Ciprofloxacin. Also, they were intermediate susceptible to Oxytetracycline and Enrofloxacin. Conversely, these strains were sensitive to Sulphamethoxazole and Trimethoprim, Florophenicol and Vibriostat O129. 4) During the bacteriological examinations, Water samples were taken for determination of water quality, heavy metal and Pesticides concentrations in water samples of Seabream were also determined using spectrophotometric methods and then we made comparison with permissible levels in water for marine culture and also permissible levels in fish tissues used for human food and the results revealed that : a) Water quality : The non-ionized, nitrite, nitrate, alkalinity, hardness and hydrogen sulphide were higher than the acceptable range. Water temperature was fixed all over the present work 27.3 °C. DO levels was 5.41mg L-1 and these results are lowered than the acceptable range. b) Heavy metal concentrations in water : Results showed Cadmium (Cd); Copper (Cu); Zinc (Zn); Mercury (Hg) and Lead (Pb) levels were lower than the permissible limits in all water samples during the present work. On the other hand, the iron (Fe) levels were higher than the permissible limits in all water samples during the present work. c) Results of average of Pesticides concentrations in water : Results showed an According to Table (5) and Figuer 2 the 2, 4’ DDE; 4, 4’ DDD and p-p’ were detected in irrigation water and drainage water. On the other hand, the 4, 4’ DDD and p-p’ was not detected in pond water. 5) Experimental infection : A total number of (70) seabream Sparus aurata L. (20 gm. ± 4) obtained from earthen pond culture in same area of Wadi-mariout were transported to the laboratory of Poultry and Fish diseases department at faculty of Veterinary medicine - Alexandria university in plastic bags supplied with oxygen (2/3) and were left for 2 weeks for acclimation. a) For calculation of the LD50 Photobacterium damselae ssp. piscicida with different dilutions beginning from 1.2 × 10-1 to 1.2 × 10-6 was I/P injected in 6 groups of fish (10 fish / group) while the last group was also I/P injected with 0.2 ml sterile saline solution 0.9 % and used as control group, then mortalities, clinical signs and PM lesions were recorded after the injection by one week. Then samples for histopathological examination were taken. Re-isolation of the injected bacteria was determined for verification of death. b) Chronic experiment : by IM injection of 0.2 ml of 1/10 of the LD50 (1.2 × 10-4 CFU/mL) in 20 fish (in 2 equal groups) and also IM injection of 0.2 ml of sterile saline solution 0.9 % in other 20 fish (in 2 equal groups; control +ve and Control –ve), then mortalities, clinical signs and PM lesions were recorded for one month after injection and also samples taken for histopathological examination . Results revealed that : 1) The LD50 was (1.2 × 10-4 CFU/mL) and there were a generalized signs of septicemia appear as: hemorrhagic spots on external body surface, on fins especially tail fin and on gill cover while in some fish have hemorrhagic swollen protruded anus. Moreover, some fish have nervous manifestations. 2) The PM lesions appear as severe congestion in gills, liver, kidneys and intestine while the most characteristic sign was copious amount of bloody stained ascetic fluids drops from the fish upon opening. 3) The re-isolation of the Photobacterium damselae ssp. piscicida causing these signs and PM lesions found to have the same culture and biochemical characteristics of the injected bacterium and this confirm the cause of mortalities, signs and PM lesions. 4) No mortalities, signs or PM lesions were determined during the chronic experiment and this confirms that Photobacterium damselae ssp. piscicida has acute and sub-acute forms and not has a chronic form of infection. 5) Results of sea bream immunization and challenged with Photobacterium damselae ssp. Piscicida : The experimental Ph. damselae ssp. piscicida vaccines conferred no protection to the respective vaccinated fish groups. In contrast, an RPS value of 16% was calculated for the Ph. damselae ssp. piscicida vaccines, respectively. 6) Results of histopathological changes of Photobacteriosis (Photobacterium damselae ssp. piscicida) : The histopathological changes due to infection with Ph.damselae ssp. piscicida in sea bream revealed that, bacterial clusters in the telangiectasias of secondary lamellae of the gill, pericardial cavity filled with erythrocytes and bacterial clusters Granulomatous lesions consisted of clusters of bacteria, macrophages and histiocytes in sea bream. Spleen, noted also the clusters of bacteria in the vessel lumen and focal necrosis in the spleen associated with numerous colonies of Ph. damselae subsp. piscicida. Liver and kidney granulomas were evident in muscle and Multi-focal coagulative necrosis was present in posterior kidney tubules.