Evaluating a nested-PCR assay for detecting Streptococcus agalactiae in red tilapia (Oreochromis sp.) tissue

Streptococcosis caused by Streptococcus agalactiae is an important disease affecting the world-wide fish industry; developing sensitive diagnostic tools for its rapid identification has thus become an urgent need. A nested PCR technique was implemented in this study for detecting S. agalactiae in frozen and paraffin-wax-embedded tilapia tissue. S. agalactiae was successfully detected in several samples taken from naturally-infected adult fish whereas larvae and fry tissue all proved negative. Such observations were consistent with previous studies by our group where the presence of S. agalactiae was detected by histopathology and immunohistochemistry in tissue taken from adult animals but not from tilapia having less than 20 g body weight. The results suggested that the microorganism's vertical transmission may not occur in the farm conditions pertaining to this study, thereby making S. agalactiae-free breeding farms feasible.

► We implemented a nested PCR technique for diagnosing S. agalactiae in tilapia tissues.
► The nested PCR amplified S. agalactiae DNA in naturally infected tilapia over 20 g.
► The nested PCR can be used in frozen and in paraffin-embedded fish tissues.
► The nested PCR has high specificity and sensibility to detect S. agalactiae.