Streptococcus phocae, an emerging pathogen for salmonid culture.

Area of Science:

• Aquatic Animal Health
• Bacteriology
• Fish Pathology

Background:

• Recurrent summer disease outbreaks in Chilean Atlantic salmon (Salmo salar) cage culture since 1999.
• Affected fish exhibited clinical signs including exophthalmia, hemorrhages, and pathological changes in internal organs.
• Previous investigations suggested potential Gemella genus involvement based on biochemical tests.

Purpose of the Study:

• To identify the causal agent of disease outbreaks in farmed Atlantic salmon in Chile.
• To characterize the identified bacterial pathogen and assess its relationship to known streptococcosis agents.
• To evaluate the antigenic properties of the isolates for potential vaccine development.

Main Methods:

• Isolation and characterization of bacterial pathogens from diseased salmon.
• Biochemical identification using the rapid ID 32 Strep system.
• Molecular identification via 16S rRNA gene sequencing and Restriction Fragment Length Polymorphism (RFLP) analysis.
• Serological studies to determine antigenic relationships.

Main Results:

• Gram-positive cocci were consistently found in affected fish.
• Biochemical tests initially suggested Gemella, but 16S rRNA sequencing identified the pathogen as Streptococcus phocae.
• All salmon isolates were antigenically homogeneous but distinct from seal-derived strains.
• The bacterium is associated with significant cumulative mortality (up to 25%) in salmonids.

Conclusions:

• Streptococcus phocae is identified as the emerging causal agent of significant disease outbreaks in Chilean salmonid aquaculture.
• The findings necessitate the inclusion of S. phocae in the understanding of warm water streptococcosis in farmed salmon.
• Antigenic homogeneity suggests potential for developing targeted preventive measures, such as vaccines.