Abstract:
Tilapia (Oreochromis niloticus) is presently a major aquaculture commodity in Thailand. Resulting from farming intensification, tilapia aquaculture has encountered with disease outbreaks. Streptococcosis, a disease caused by streptococci bacteria, has been reported in many countries and has economic consequences on mass mortality in all stages of tilapia farming. The purpose of this study is to identify streptococcal pathogens in farmed tilapia by using the conventional microbiological and molecular techniques. Streptococcosis vaccine development and its application were studied in tilapia farms. Diseased fish from overall culture areas reporting outbreaks between 2003 to 2010 were examined for pathological and microbiological characterization. Clinically infected fish showed septicemic condition, including generalized hemorrhage, exophthalmia with ocular opacity, abdominal distension, skin abscesses and erratically swimming. Bacterial isolates recovered from the kidney and brain tissue of the diseased tilapia were identified using API system and PCR assay. PCR amplification of the 16S rRNA gene with species-specific primers employed to 139 clinical isolates revealed that 131 isolates (94.24%) were S. agalactiae and 8 clinical isolates (5.75%) were S. iniae. The sequencing analysis of 16S rRNA gene and sodA gene suggested high sequence similarity (> 97%) with the corresponding portion of fish pathogen genome, S. agalactiae (16S rRNA gene: GenBank accession no. GQ169772-74, GQ338316-18; sodA gene: GenBank accession no. HM004089-94) or S. iniae (16S rRNA gene: GenBank accession no. GQ169769-71, GQ338313-15; sodA gene: GenBank accession no. HM004083-88). Formalin killed cell (FKC) and extracellular product (ECP) were used as a vaccine against S. agalactiae infections. The vaccination of farmed fish (200 gm body weight) resulted in specific agglutinating antibodies for at least 10 weeks post-vaccination. The antibodies conferred protection against a single intraperitoneal challenge of 1.5 x 10⁸ cell S. agalactiae / fish. Mortality of non-vaccinated fish reached 95% (19/20), whereas mortality of the vaccinated fish was 21.5% (4/19). The efficacy of FKC vaccine and FKC added with ECP vaccine was relatively not different. In addition, no evidence of negative impacts on health performance was observed in the vaccinated tilapia.
