First identification of pathogenic Edwardsiella anguillarum, Nocardia asteroides, and Streptococcus agalactiae in tilapia (Oreochromis niloticus) farmed in Mexico.

Three different bacteria strains of the Nocardia, Edwardsiella, and Streptococcus genera were isolated from tilapia (Oreochromis niloticus) cultivated in Mexican farms with reports of granulomatous disease and mortality. The isolates were obtained from the tilapia spleen and characterized by biochemical tests, real-time PCR, and 16S rRNA sequencing. Comparisons with the GenBank Database indicated a 99.63 % and 99.87 % sequence identity to the 16S rRNA of the isolated samples with Edwardsiella anguillarum and Nocardia asteroides, respectively, and 100 % to Streptococcus agalactiae. The pathogenicity of these bacteria was confirmed in a bioassay challenge by intraperitoneally injecting healthy adults of tilapia. The infected tilapia displayed characteristics of pathological processes such as lethargy, erratic swimming, darkening of skin, and death. High mortalities were observed the first ten days post-inoculation, but no granulomatous lesions were detected during the 15-day infection period. However, the same inoculated bacteria were recovered from the spleen of dead animals or survivors at the end of the experiment and identified by PCR amplification and sequencing. These findings indicate that tilapia in Mexican aquaculture is susceptible to diseases described in other countries, and the freshwater aquaculture industry is at risk.