EPIZOOTIOLOGICAL ASPECTS OF STREPTOCOCCUS-INIAE AFFECTING TILAPIA IN TEXAS

Some epizootiological aspects of a bacterial septicemia of hybrid tila pia (Tilapia nilotica x T. aurea) caused by a biotype of Streptococcus iniae were investigated. Bacterial growth in Brain Heart Infusion bro th was detected at 10, 25, 35 and 45 degrees C, between pH 5.5-8.5, an d at salinities of up to 30 ppt. After suspension of the bacteria in d istilled water, viable cocci were recovered for up to 5 days at 5 degr ees C and < 24 h at 25 degrees C. The 96 and 168 h LD50 in 30-90 g fis h was found to be 4.9 x 10(5) and 3.18 x 10(5) colony forming units (i njected intraperitoneally), respectively. Artificial infection was ach ieved 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 degrees C than those ke pt at 15, 25, 30 or 35 degrees C; and in water of pH 9 compared with f ish maintained in more acidic water. Sodium azide based selective medi a were not successful at recovering any viable cells from earthen grow out-ponds that had contained infected fish. Hybrid striped bass were f ound susceptible whilst red drum and channel catfish were resistant to artificial infection. (C) 1997 Published by Elsevier Science B.V.