Atchariya Suwannasang, Machalin Dangwetngam, Akkarawit Issaro, Wutiporn Phromkunthong, and Naraid Suanyuk

pp. 499 - 506

Abstract

Streptococcus agalactiae (Group B Streptococcus, GBS) infections cause serious worldwide damage to fish farming.
Here, we evaluated the pathological manifestations and immune responses of Nile tilapia (Oreochromis niloticus) infected
with GBS serotype Ia (GBS-Ia) and GBS serotypes III (GBS-III). Experimental infection of fish with GBS by intraperitoneal
injection indicated the severity of GBS-Ia infection as shown by the LD₅₀ of GBS-Ia and GBS-III, being 1.58×10⁶
 CFU/fish
and 2.10×10⁸
 CFU/fish, respectively. The onset of disease and clinical observation revealed notable differences between the
GBS-Ia and GBS-III infections. Histological findings showed severe changes of the liver, pancreas, kidney and brain of fish
infected with GBS. In addition, significantly different haemato-immunological parameters were observed between GBS-Ia and
GBS-III infected fish. Differences in the pathological manifestations and immune responses of GBS-Ia and GBS-III infections
in tilapia are of practical use for fish pathologists who monitor tilapia mortality resulting from GBS infections.