Study on Streptococcus agalactiae infection in Javanese medaka (Oryzias javanicus Bleeker, 1854) model

Area of Science:

• Aquatic animal health
• Fish disease models
• Bacterial pathogenesis

Background:

• Streptococcus agalactiae causes significant losses in aquaculture.
• Javanese medaka (Oryzias latipes) are increasingly used as a model organism for toxicological and disease studies.
• Understanding disease development in fish models is crucial for developing effective control strategies.

Purpose of the Study:

• To determine the median lethal dose (LD50) of Streptococcus agalactiae in Javanese medaka.
• To describe the clinico-pathological changes and disease progression following S. agalactiae infection.
• To evaluate Javanese medaka as a model for studying streptococcosis.

Main Methods:

• Javanese medaka were infected with S. agalactiae via intraperitoneal injection and immersion routes across a range of concentrations (10^3 to 10^8 CFU/mL).
• Median lethal dose (LD50) and clinico-pathological changes were assessed up to 240 hours post-infection (hpi).
• Disease development was monitored for 96 hpi at specific infection doses for each route.

Main Results:

• The LD50 was lower for intraperitoneal injection (4.5 × 10^2 CFU/mL) compared to immersion (3.5 × 10^3 CFU/mL).
• Clinical signs included lethargy, erratic swimming, corneal opacity, and exophthalmia.
• Histopathology revealed generalized congestion in organs, particularly liver and brain, and tubular necrosis in kidneys.
• Bacterial load (CFU/g) in fish tissues correlated significantly with mortality and lesion severity.

Conclusions:

• Javanese medaka effectively model Streptococcus agalactiae infection, exhibiting characteristic clinical and pathological signs.
• Both intraperitoneal injection and immersion routes are suitable for inducing streptococcosis in this model.
• This study validates the Javanese medaka as a valuable tool for investigating streptococcosis and related fish diseases.