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The current investigation was undertaken to determine the potential causes of summer mass mortalities among farmed Nile tilapia (Oreochromis niloticus). Pure bacterial colonies were isolated from moribund O. niloticus from 13 different fish farms suffered from high mortalities ranged from (50-80%), during the period from April to October 2018. Fish showed external hemorrhagic spots, skin darkening, abdominal distension and exophthalmia. Internally, congestion and enlargement of internal organs with serous or hemorrhagic fluid was the most obvious picture. The phenotypic and biochemical characterization using API20E identified the bacterial isolates as (A. veronii, A. hydrophila, A. caviae and A. sobria; Aeromonads), (Ps. Fluorescence; Pseudomonas spp), (E. sakazakii and E. cloacae; Enterobacter spp), (C. freundii; Citrobacter spp), (S. odorifera, S. liquefaciens, and S. marcescens; Serratia spp), (S. lutiensis, S. equine; Streptococcus spp), Lactococcus lactis and Proteus vulgaris, with the most prevalence to aeromonads. Most isolates were accurately identified by PCR and gene sequencing. Water physicochemical parameters were measured at the farm sites showed an increase in the pH and ammonia levels. In order to confirm the pathogenicity of the bacterial isolates, an experimental infection was conducted using different doses. The results revealed that A. veronii (HY2) at dose of 9×10 8 cells/ml was the most pathogenic with mortality rate 100 %. This study concludes that A. veronii, C. freundii, P. vulgaris and P. fluorescens are implicated in Nile tilapia summer mortalities, without neglecting the role of water quality in worsening the problem. Bacterial infection.

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Streptococcosis is a serious bacterial disease responsible for significant economic losses in wild and farmed fishes It is the second disease threat to O. niloticus. Several species of Streptococcus have been reported worldwide as etiological agents of the disease. Also, other related bacterial species such as Lactococcus garvieae, Vagococcus salmoninarum, and Enterococcous faecalis with varied degrees of pathogenicity have been implicated in streptococcal infection. Streptococcosisis characterized by hemorrhagic septicemia, pop eye, nervous manifestation, abnormal swimming behavior, and high mortalities. Control of Streptococcosis is principally achieved by implementing some preventive measures and treatment with antibiotics and to a lesser extent vaccination. Probiotics and immunostimulants can be used to enhance host immunity against the infection with some success. The majority of streptococcal species exhibited multiple antimicrobial resistance; thus, vaccination appears to be the most effective method of controlling in aquaculture. This present review summarizes some aspects of Streptococcosis such as history, epidemiology, diagnosis, and possible control measures in cultured O. niloticus and its status in Egypt Aquaculture Egypt Nile tilapia Streptococcosis Vaccine

The current study aimed to control the bacterial infection associated with summer mortalities in Nile tilapia. The sensitivity of the isolated bacteria, including Aeromonas Veronii (A. veronii) strain (HY1, HY2, HY3, HY4 and HY6), Citrobacter freundii (C. freundii), Proteus vulgaris (P. vulgaris) and pseudomonas flurosence (P. flurosence) for antibiotic was tested and antibiotic resistant genes were investigated. In order to verify the result of antibiotic sensitivity; experimental trial was performed to determine the effect of florfenicol and erythromycin against Nile tilapia (Oreochromis niloticus) infection with different pathogenic bacteria [4 strains of A. veronii (AHY2), AHY3), AHY4) and AHY6), C. freundii, P. flurosence and P. vulgaris]. The results revealed that all tested bacterial isolates were 100 % sensitive to florfenicol and erythromycin, 50% sensitive to gentamycin, 25 % sensitive to nalidixic acid and sulphamethoxazol-trimethoprim, while all bacterial isolates were 100% resistant to oxytetracycline and tetracycline lincomycin, ofloxacillin and penicillin, 87.5% resistant to ampicillin, 75% resistant to sulphamethoxazol-trimethoprim, 62.5% for nalidixic acid and 37.5% for gentamycin. The relative percent survival (RPS) of challenged medicated fish with erythromycin ranged from 45.0-91.7%. Where, the survival % in HY3 infected groups was the highest (91.7%) followed by C. freundii (87.5%), HY6 (83.3 %) and P. vulgaris (81.3%). Antibiogram profile Antimicrobial resistant gens Bacterial diseasesNile tilapia