Experimental streptococcal meningo-encephalitis in cultured fish

Area of Science:

• Aquatic animal health
• Bacteriology
• Fish pathology

Background:

• In 1984, a significant fish disease emerged in Israeli aquaculture, affecting hybrid tilapia and trout.
• The disease presented with high morbidity and mortality rates, specifically 30% in tilapia and 50% in trout.

Purpose of the Study:

• To identify the causative agent of the novel fish disease affecting tilapia and trout.
• To experimentally reproduce the disease and determine the pathogenicity of isolated bacterial species.

Main Methods:

• Isolation and identification of bacterial species from diseased fish.
• Experimental infection of tilapia and trout with isolated bacterial strains.
• Determination of Lethal Dose 50 (LD50) values for isolated strains and assessment of virulence after in vivo passages.

Main Results:

• Two new streptococcal species, Streptococcus shiloi and Streptococcus difficile, were isolated from diseased tilapia and trout.
• Experimental reproduction confirmed S. shiloi and S. difficile as the causative agents of meningitis and meningo-encephalitis.
• In vitro LD50 values were 10(7)-10(8) cfu, increasing to 10(2)-10(5) cfu after in vivo passages, indicating enhanced virulence.

Conclusions:

• Streptococcus shiloi and Streptococcus difficile are identified as novel pathogens responsible for a severe disease in cultured tilapia and trout.
• The study highlights the potential for new bacterial species to emerge and cause significant losses in aquaculture.
• No extrachromosomal elements were found to correlate with the virulence of the isolated strains.