Submitted to: Aquaculture Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/30/2006
Publication Date: 7/25/2006
Citation: Shoemaker, C.A., Lim, C.E., Aksoy, M., Welker, T.L., Klesius, P.H. 2006. Growth response and acquired resistance of Nile tilapia, Oreochromis niloticus (L.) that survived Streptococcus iniae infection. Aquaculture Research. 37: 1238-1245. Interpretive Summary: Loss due to infectious disease is consistently reported by United States aquatic animal health producers as one of their top problems. Loss not only occurs in the form of mortality (i.e., direct loss) but also do to decreased feed efficiency and growth performance. This study examined the performance of tilapia that survived a Streptococcus iniae infection. Streptococcus iniae is a Gram-positive bacterium and is the number one disease agent of farmed tilapia and hybrid striped bass in the U.S. responsible for about $10 million in losses annually. Our study found that tilapia that survived S. iniae infection (i.e., not showing signs of disease) consumed, utilized feed and grew as well as tilapia that had not been infected. The survivors were also shown to have acquired resistance to re-infection with S. iniae (i.e., the fish were immune to second challenge). The immunity to second infection was due to the ability of the tilapia’s immune system to remember and respond by producing antibodies against S. iniae.
Technical 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 x 103, 8.8 x 104 and 8.8 x 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 x 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 and subsequently challenged fish as compared to the controls challenged for the first time. Agglutinating antibody titer was significantly higher in the fish in the medium and high challenge treatments, compared to 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.