Epizootiological aspects of Streptococcus iniae affecting tilapia in Texas

Abstract

Some epizootiological aspects of a bacterial septicemia of hybrid tilapia ( Tilapia nilotica X T. aurea) caused by a biotype of Streptococcus iniae were investigated. Bacterial growth in Brain Heart Infusion broth was detected at 10, 25, 35 and 45 °C, between pH 5.5–8.5, and at salinities of up to 30 ppt. After suspension of the bacteria in distilled water, viable cocci were recovered for up to 5 days at 5 °C and < 24 h at 25 °C. The 96 and 168 h LD 50 in 30–90 g fish was found to be 4.9 × 10 5 and 3.18 × 10 5 colony forming units (injected intraperitoneally), respectively. Artificial infection was achieved through oral administration as well as by dipping fish in water containing the bacterium. Greater rates of mortality were observed in artificially infected tilapia maintained at 20 °C than those kept at 15, 25, 30 or 35 °C; and in water of pH 9 compared with fish maintained in more acidic water. Sodium azide based selective media were not successful at recovering any viable cells from earthen growout-ponds that had contained infected fish. Hybrid striped bass were found susceptible whilst red drum and channel catfish were resistant to artificial infection.