The density threshold of Streptococcus agalactiae causing Streptococcosis in tilapia (Oreochromis sp.) cultured in freshwater ponds

Abstract

Tilapia (Oreochromis sp.) is one of the key aquaculture species in Vietnam. In freshwater environments, tilapia is most affected by S. agalactiae, but information on S. agalactiae density threshold causes disease in tilapia has been limited. The purpose of this study was to determine the S. agalactiae density threshold causing disease Streptococcosis on tilapia and some related environmental factors such as DO, temperature, pH, NH₃, NO_2^-N. The results showed that S. agalactiae causes tilapia had typical pathological manifestations when the density of S. agalactiae in fish liver and pond water fluctuated in the range of 1.1x10⁷-2.9x10⁸ cfu/g and 1.0x10³-1.2x10⁴ cfu/ml, respectively. Fish infected with S. agalactia showed no pathological symptoms when the S. agalactiae density in fish liver and water ranged from 1.3x10³-9.6x10⁵ cfu/g and 2.3x10¹-8.3x10¹ cfu/ml, respectively. The density of S. agalactiae in water, temperature, NH₃, and pH affected the density of S. agalactiae in fish. The highest probability of disease occurrence was 98.3% when the density of S. agalactiae in water was ≥10³ cfu/ml, followed by 71.4% when the temperature was ≥31^oC, 25.9% when NH₃>0.1 mg/l, 14.01% in case NO₂-N>0.05 mg/l, and the lowest was 0.07% when DO<4 mg/l.