الفهرس 
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Abstract
Enteric redmouth disease (Yersiniosis) caused by Yersinia ruckeri (Y. ruckeri) possess a serious problem affecting marine and freshwater fishes due to severe economic losses.In our study, a total of one hundred and fifty (150) earthen pond cultured Oreochromis niloticus (O. niloticus) were collected from private fish farms at Kafr-el Sheikh governorate throughout an outbreak during winter season whereas there was marked abrupt change in water temperature ranged in 16-17 0C. Examined fish were found to have typical signs and PM lesions of enteric redmouth disease. These fish were subjected to the followings:-1.Full clinical examination (clinical signs and PM lesions).2.Bacteriological examination (Isolation and Identification of the causative agent), whereas bacterial inocula were taken from internal organs (Kidney, spleen, heart and liver) and subcultured on tryptic soya agar and Selective Yersinia agar media at 20-25 0C for 24-48 hrs. Bacterial isolates were then subjected to full traditional biochemical tests and API 20E and 20 NE for further identification. The identified bacterial isolates were confirmed using conventional PCR for detection of the virulence gene (Yrp1 gene) of the identified Y. ruckeri.3.Samples were taken from internal organs to determine the histopathological alterations occurred.4.The prevalence of the identified Y. ruckeri isolates in the naturally examined fish was also determined.5.Antibiogram test was applied for the identified Y. ruckeri isolates using different antimicrobial discs.- The results of naturally examined O. niloticus revealed that:-1.Naturally examined O. niloticus has typical signs of Yersiniosis as darkening of skin color, corneal opacity, exophthalmic eye, hemorrhagic anal opening, erythematic appearance of lips with reddening appearance and petechial hemorrhages on the body. While the PM lesions have petechial hemorrhages on liver with engorged inflamed gall bladder and congestion of the heart with gastroenteritis.2.Cultural, morphological and biochemical characters of the isolated bacteria have typical characters of Y. ruckeri.3.The isolated Y. ruckeri was confirmed with conventional PCR, whereas the PCR product had 100 % typical identity and Consistent size with Yrp1 gene of Y. ruckeri standard ATCC reference strain.4.The histopathological findings of naturally examined fish characterized by congestion in the gill arch, congestion and vacuolation of the hepatic cells, mild edema and focal hyaline degeneration of musculature, activation of melanomacrophage centers (MCCs) and focal hyperplasia in the lymph follicles, mucinous degeneration and focal epithelial desquamation in the epithelial lining with mononuclear leucocytic infiltration in the lamina propria of the intestine and vacuolation of some renal tubular epithelium.5.It was found that a total of 115 out of 150 fish were found to be clinically infected with an average prevalence of (77%) and the prevalence of Y. ruckeri isolates was higher from liver of naturally examined fish (45.84 %), followed by kidney (33.33%).6.It was found that the bacterial isolates were markedly sensitive to Ciprofloxacin and Sulphamethoxazole-Trimethoprim combination, moderately sensitive to Enrofloxacin, erythromycin and Doxycycline and less sensitive to other antibiotics. - In order to determine the virulence of the identified Y. ruckeri, a pathogenicity test was applied using a total of one hundred and ten (110) apparently healthy O. niloticus with average body weight (50 ± 20 g).a) For calculation of the LD50:- 0.2 ml of the identified Y. ruckeri (106 cells / ml) with different dilutions from 10-1 to 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 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 positive and Control negative), 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 (10-2.5) and there were generalized signs of septicemia appear as: erythema and petechial hemorrhages on the abdomen, protruded hemorrhagic vent opening, darkening of skin color and corneal opacity with orbital hemorrhage.2.The PM lesions appear as hemorrhages in gills and heart, moderate congestion of the spleen and kidneys, and petechial hemorrhages on liver.3.The re-isolation of the bacterium 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.Clinical signs of chronic infection started in the 1st week of infection and continued until the end of the experiment (4th week). The clinical signs appeared as sluggish movement and loss of water reflexes with no external or PM lesions.5.Histopathological results in chronic experiment showing hyperplasia in the secondary lamellae, nuclear pyknosis and vacuolation in the hepatocytes with marked activation of melanomacrophage centers and mild tubular nephrosis with focal hyperplasia in the hematopoietic tissue.