Comprehensive Investigation of Streptococcosis Outbreaks in Cultured Nile Tilapia,Oreochromis niloticus,and Red Tilapia,Oreochromis sp., of Thailand

Streptococcosis causes economic losses due to mass mortality at all culturing stages in Nile tilapia, Oreochromis niloticus, and red tilapia, Oreochromis sp., farming throughout Thailand. Diseased tilapia collected from outbreak areas during 2003–2012 were examined using histopathological, biochemical, and molecular tools. Infected fish showed clinical signs of septicemia, and bacteria were found in visceral organs. All gram‐positive cocci isolates were negative to catalase and oxidase, and exhibited β‐hemolysis; however, they possessed various biochemical profiles. PCR amplification of the 16S rRNA gene was used for 165 samples, and resulted in identification of 143 (86.67%) with Streptococcus agalactiae and 14 (8.48%) with Streptococcus iniae, and 8 (4.85%) with mixed infection. High similarity (≥98%) of 16S rRNA gene sequences to the reference strain S. agalactiae (accession no. EF092913) and S. iniae ATCC29178 type strain was observed in the typing of S. agalactiae and S. iniae from Thai farmed tilapia. This investigation documented that at least two species of streptococcal bacteria, S. agalactiae and S. iniae, were involved in tilapia streptococcal infection in Thailand. The molecular recognition of the etiologic agents showed that S. agalactiae was the dominant species that cause disease in all culture areas, whereas S. iniae were discovered only in cases from the northeastern and central regions.