Chapter 35 - Streptococcosis

Abstract

Streptococcosis outbreaks are induced by the genera Streptococcus and Lactococcus, in wild fish and in cultured fish and in intensive culture system in seawater, brackish water, and freshwater. Streptococcus is gradually recognized as a severe disease of aquatic animals. Infections with Streptococcus spp. have been identified from cultivated fish in Taiwan, Japan, Middle East, Spain, Italy, South Africa, Australia, and the United States. Four major pathogenic species of Streptococcaceae, a family of major Gram-positive bacteria, have been isolated from diseased fish: Streptococcus iniae, S. agalactiae, S. dysgalactiae, and Lactococcus garvieae. The disease is characterized by generalized septicemia, exophthalmia, corneal opacity, serositis, meningoencephalitis, and lesions in skin, leading to heavy mortality. Bacterial isolation from diseased fish is done using brain heart infusion agar and blood agar incubated for 18–24h at 25°C. To identify Streptococcus spp. and Lactococcus spp., Lancefield's serological grouping and hemolytic pattern in blood agar have been used and are classified into α, β, and γ (nonhemolysis) hemolysis types and Lancefield groups B, C, and nontypable, respectively. The polymerase chain reaction assay uses primers based on unique regions of the S. iniae 16S ribosomal RNA (rRNA) gene, the species-specific internal transcribed spacer (ITS) region between 16S and 23S rRNA genes of S. agalactiae. The species-specific ITS region between 16S and 23S rRNA gene in S. dysgalactiae, and unique regions of the 16S rRNA gene of L. garvieae allow for the species identification and yield the 432bp, 300bp, 280bp, 259bp, and 1100bp amplicons, respectively. The commercial formalin-inactivated injectable vaccine against S. iniae, S. agalactiae, S. dysgalactiae, and Lactococcus garvieae was licensed for use in sea bass, tilapia, yellowtail, or amberjack cultured in Europe, Japan, Korea, and Taiwan.