Quantitative detection of streptococcosis infection in dead samples of Nile Tilapia (Oreochromis niloticus)

Aims The aims of the study were to evaluate whether epidemic strains of streptococcosis infected tilapia can be isolated and identified from dead fish for epidemiological investigation. Methods and Results Firstly, tilapias were inoculated with a lethal dose (1 x 10(8) CFU per fish) of Streptococcus agalactiae and brain tissues were harvested for bacteriological examination and qPCR assay 3, 12, 24 and 48 h postdeath. Streptococcus agalactiae was the only dominant bacterium cultivated on the brain heart infusion (BHI) plate and the bacterial load was about 10(7) CFU per mg. Secondly, tilapia were killed via ice water shock and immersed either in an aquarium containing 2 center dot 27 x 10(4) CFU per ml S. agalactiae or in a pond with streptococcosis outbreak. Streptococcus agalactiae failed to grow on the BHI plate but were identified (<6 x 10(2) CFU per mg) via qPCR assay. Finally, an epidemiological investigation of streptococcosis was conducted in the main tilapia breeding areas of South China. A total of 387 tilapia samples were collected including 24 suspected healthy, 35 moribund and 328 dead fish. The achieved detection rates were 0, 100 and 94 center dot 82% via bacteriological examination, and 0, 100 and 98 center dot 78% via qPCR assay respectively. The concentration of S. agalactiae in brain tissues ranged between 10(5) and 10(7) CFU per mg. Conclusions Streptococcus agalactiae can survive for 48 h in the brain of dead fish. Dead tilapia can be a useful alternative for epidemiological investigation when the diagnostic analysis of moribund fish is unavailable or impractical. Significance and Impact of the Study This detection method expands the sampling range, reduces the difficulty of sample collection and improves efficiency. Consequently, this method provides an alternative for epidemiological investigation of tilapia streptococcosis.