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

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.

Firstly, tilapias were inoculated with a lethal dose (1 × 10⁸  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 10⁷  CFU per mg. Secondly, tilapia were killed via ice water shock and immersed either in an aquarium containing 2·27 × 10⁴  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 × 10²  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 94·82% via bacteriological examination, and 0, 100 and 98·78% via qPCR assay respectively. The concentration of S. agalactiae in brain tissues ranged between 10⁵ and 10⁷  CFU per mg.

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.

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.