Study Of Streptococcal Disease’s Pathogen(s) And Its Biological Characteristics In Red Drum Sciaenops Ocellatus In China

In August2011, an enzootic disease characterized by hemorrhage throughout body surface,enlarged spleen and kidney occurred in cage farmed red drum fish, in Dongtou, Zhejiang ofChina. The diseased fish weighed between100-300g，and the cumulative mortality within60days was higher than70%. Several bacterial isolates (strainWZMH110819) that exhibited thesame phenotypic traits and biochemical characteristics were isolated from the lesions ofdiseased fish. According to the results obtained from biochemical tests and sequence analysisof the16S rDNA gene, the bacterial isolates were identified as Streptococcus iniae. In thechallenge trials, the LD50value of the clinical bacterial isolate WZMH110819was9.65×106CFU per fish. Moreover, bath exposure or oral administration by Streptococcus iniae alsocaused a considerable number of deaths in fish. Antibiotic susceptibility tests showed thatstrain WZMH110819was sensitive to most of the antibiotics tested including ampicillin,ceftazidime, erythromycin, kanamycin and gentamicin in vitro.These drugs were found tohave a strong bactericidal effect (i.e their MIC≥MBC) against bacterial isolate (strainWZMH110819). Meanwhile the study on the growth properties of strain WZMH110819indicated that it grows better in the temperature, pH and salinity range between20-37°C,6.0-8.0, and0-20‰with their optimum values of30°C,7.0and0‰, respectively. This findinghas provided a helpful basis in controlling and prevention of this disease in red drum. It wouldalso provide a theoretical basis in developing a vaccine in order to reduce the negative reddrum affects on the species.