Growth response and acquired resistance of Nile tilapia, Oreochromis niloticus (L.) that survived Streptococcus iniae infection

Abstract

This study determined the growth performance and acquired resistance of Nile tilapia, Oreochromis niloticus (L.) that survived Streptococcus iniae infection. Tilapia were challenged with three doses of S. iniae (8.8 × 103, 8.8 × 104 and 8.8 × 105 CFU fish−1 for low, medium and high challenges respectively). Groups of non-injected and tryptic soy broth-injected fish were maintained as controls. Significantly (P<0.05) higher mortality (45.0%) occurred in the high challenge treatment than in the low challenge treatment group (29.6%). The medium challenge group had mortality (36.3%) that did not differ significantly from the high or low treatment. Few fish died in the non-injected and broth-injected treatments (3.4% and 0.8% respectively). The tilapia that survived S. iniae infection used to assess growth performance were selected from survivors without gross clinical signs of disease. These fish were randomly stocked at a rate of 30 fish into each 57 L aquarium in triplicate and fed to apparent satiation for 8 weeks. No significant differences were detected in weight gain, feed intake, feed efficiency ratio or survival between S. iniae-survived tilapia and the control treatments following the 8-week growth performance trial. Following the 8-week feeding study, tilapia were challenged with 1 × 106 CFU fish−1 of S. iniae to assess acquired immunity. Mean cumulative mortality was significantly higher (P<0.05) in the control treatments (41.7% for the non-injected and 43.3% for the broth-injected fish) than in the low, medium and high challenge treatments (7.4%, 3.3% and 8.3% respectively). Serum protein was significantly (P<0.05) elevated in the S. iniae-survived tilapia that were subsequently challenged when compared with controls challenged for the first time. Agglutinating antibody titre was significantly higher in the fish in the medium and high challenge treatments, compared with the control fish challenged for the first time. The results suggest tilapia that survive S. iniae challenge without showing overt disease signs performed as well as non-infected tilapia. Further, the S. iniae-survived tilapia challenged following the 8-week growth performance trial gained acquired resistance to homologous S. iniae challenge.