Naraid Suanyuk, Nirut Sukkasem, Nopparat Tanmark, Terutoyo Yoshida, Toshiaki Itami, Ronald L. Thune, Chutima Tantikitti, and Kidchakan Supamattaya

pp. 341 - 348

Abstract

Streptococcal infections are becoming an increasing problem in aquaculture and have been reported worldwide in a
variety of fish species. Here we describe the isolation and characterization of Streptococcus iniae from Asian sea bass (Lates
calcarifer) and red tilapia (Oreochromis sp.) cultured in southern Thailand. Conventional and rapid identification systems,
as well as the polymerase chain reaction (PCR), were used to determine that the isolate was S. iniae. The virulence of this
S. iniae was higher in Asian sea bass than in red tilapia, as shown by the 10 day-LD₅₀ in Asian sea bass and red tilapia, being
1.08x10⁴
 and 1.14x10⁷
 CFU, respectively. Histopathological changes in Asian sea bass are more severe than those observed
in red tilapia. The changes can be found in several organs including liver, pancreas, heart, eye and brain. Histopathological
findings included cellular necrosis, infiltration of lymphocytes and granuloma formation in the infected organs.