Abstract
In August, 2011, an enzootic disease characterized by hemorrhage throughout body surface, enlarged spleen and kidney occurred in cage farmed red drum fish, in Dongtou, Zhejiang of China. The diseased fish weighed between 100 to 200 g, and the cumulative mortality within 60 days was higher than 70%. Several bacterial isolates that exhibited the same phenotypic traits and biochemical characteristics were isolated from the lesions of diseased fish. According to the results obtained from biochemical tests and sequence analysis of the 16S rDNA, the disease’s pathogen (strain WZMH110819) was identified as Streptococcus iniae. In the challenge trials, the LD50 value of the clinical bacterial isolate WZMH110819 was 9.65×106 CFU per fish. Moreover, bath exposure or oral administration by Streptococcus iniae also caused a considerable number of deaths in fish. Antibiotic susceptibility tests showed that strain WZMH110819 was sensitive to most of the antibiotics including ampicillin, erythromycin and gentamicin in vitro. This finding has provided a basis for the control and prevention of further outbreaks of this enzootic disease in red drum farms. Key words: Streptococcus iniae, Sciaenops ocellatus, biochemical characterization, 16S rDNA analysis.
Highlights
Streptococcal infections in fish have become an increasingly important health problem in modern intensive aquaculture (Eldar et al, 1995a)
S. iniae was firstly isolated from the subcutaneous abscesses in Amazon freshwater dolphins Inia geoffrensis (L.) in aquariums (Pier and Madin, 1976), and large epidemic diseases in fish associated with this pathogen occurred
Sequence analysis showed that bacterial strain WZMH110819 showed 100% similarity with the reference S. iniae strain ATCC 29178T, but showed 98% similarity with S. parasuis and 97%
Summary
Streptococcal infections in fish have become an increasingly important health problem in modern intensive aquaculture (Eldar et al, 1995a). The main pathogenic species responsible for these streptococcal infections include Streptococcus iniae, Streptococcus agalactiae, Streptococcus parauberis and Streptococcus dysgalactiae (Eldar et al, 1995a; Domenech et al, 1996; Bercovier et al, 1997; Nomoto et al, 2004). The red drum (Sciaenops ocellatus) is one of the most important economic fish species and it was introduced in America and Asia (Weinstein et al, 1997; Lau et al., 2003; Koh et al, 2004; Lau et al, 2006).
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