الفهرس 
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Abstract
Streptococcosis is the disease which has a considerable attention in recent years among fishes.
In this investigation 720 fishes of various species and life stages 240 from each Nile tilapia (O.niloticus), Nile catfish (C.gariepinus) and Gray mullet (M.cephalus) which were collected from different localities in Egypt in different seasons of the year, as well as 10 moorhen (Gallinula cholrapus) were subjected to clinical and bacteriological examination to determine the prevalence of streptococcus infection.
Morphological and biochemical identification of the isolated streptococcus species were applied. Beside 416 apparently healthy live O.niloticus with average Body weight 50±2 were used for studying the pathogenicity of the isolated streptococci, effect of environment (temperature, and salinity) on the pathogenicity of Str.faecalis and trials of • treatment of experimentally infected fish by different chemicals and antibiotics according to the results of sensitivity test against Strfaecalis.
Forty five healthy one day old duckling were used to determine the pathogenicity of streptococcus species isolated from fish to duckling (aquatic birds). Clinical, postmortem and histopathological examination of experimentally infected fish and duckling as well as effect of Strfiecalis on growth performance of 72 fingerlings O.niloticus were another
objectives.
The results of survey revealed that, the percentage of streptococcus infection among examined fishes (21.8%), In C.gariepinus (28.8%) in mullet (20%) and in Tilapia (16.7%). The percentage of infection was highest in Summer (10.3%) followed by (6.7%), (3.1%) and (1.8%) in Spring, Autumn and
The percentage of infection was the highest in private fish farms especially in Sahl El-Hosainia (28.6%) than Abbasa fish farm (22.5%) and El-Tall El-Kabeer fish farm (15%), wild fish showed the lowest percentage of infection (12.2%).
The highest percentage of infection occur in adult fish (32.5%) followed by (13.8%) and (5%) in pre adult and juvenile fish respectively.
Clinically infected fish showed dark coloration of the body, hemorrhage on several parts of the body, mouth, base of fins and opercula, abdominal dropsy and exophthalmia and grayish ulcers. with emaciation.
The bacteriological examination revealed isolation of 171 isolates from heart blood, kidneys, spleen, liver, ascetic fluid, skin, gills, fins and intestine of examined fishes. The isolates were identified as Strfaecalis, Strfaecium, Str.agalactia, Str.pvogen, Str.pneumoni, Str.bovis, Str.durans and Struberis) in prevalence 27.5%, 45.6%, 9%, 7%, 1.8%. 1.8%, 40.9% and 2.9 respectivel. Beside 17 isolates of Strfaecalis were obtained from moorhen.
The results of biological tests revealed that, only 3 isolates of Strfaecalis, 2 isolates of Strfaecium and one isolate of Str.pnetimoniae were pathogenic to 0.niloticus with varying degree of virulence, and only one isolate of Strfaecalis was high virulent especially. When fish were stressed by varying degree of temperature and salinity. Streptococcus infection could be passed through all routes of inoculation to 0.nilotiens. Str. faecalis and Strfaecium have adverse effect on growth performance of O.niloticus under suitable environmental conditions. Str. faecalis and Strfaecium from fish were pathogenic to duckling with morbidity 100% • and mortalities 80% and 60% respectively.Clinical signs on experimentally infected fish were dark coloration
and septicemia, abscesses under the skin or emaciation with vertebral deformity, while in duckling the signs were sleeping with white diarrhea, emaciation of eye lids, conjunctivitis and arthritis.
Histopathological alterations were recorded in both experimentally infected fish.
The results of treatment revealed that, the pathogenic streptococcus species were sensitive to Ciprofloxacin, Gentamycin and Doxicyclin and resistant to Naledixic acid in vitro. Chloramphinicol is the most effect treatment for the experimentally infected fish.