Massive mortality associated with Streptococcus iniae infection in cage-cultured red drum (Sciaenops ocellatus) in Eastern China

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 LD 50 value of the clinical bacterial isolate WZMH110819 was 9.65×10 6 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.