Submitted to: Journal of the World Aquaculture Society
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/5/2011
Publication Date: 6/11/2012
Citation: Welker, T.L., Lim, C.E., Aksoy, M., Klesius, P.H. 2012. Use of diet crossover to determine the effects of ß-glucan supplementation on immunity and growth of Nile Tilapia. Journal of the World Aquaculture Society. 43:335-348. Interpretive Summary: Disease is a pervasive problem for fish reared in intensive aquaculture. The use of antibiotics and other chemical compounds to prevent disease or treat infected fish has been discouraged due to possible negative effects on consumers and the environment and creation of antibiotic resistance in fish and non-fish pathogens. Alternative, safe immunostimulants, such as ß-glucan, are desirable and preferred to these chemical treatments, but their efficacy in preventing bacterial diseases in aquatic organisms appears to be dependent upon the feeding regimen (dietary concentration and feeding duration). Previous research has shown that long or very short feeding durations to be ineffective in preventing streptococcal disease in Nile tilapia. As the next step in determining an effective ß-glucan feeding regimen for tilapia, we examined the effect of diet crossover, where fish were fed alternating periods of unsupplemented (control) and ß-glucan supplemented diets. Juvenile Nile tilapia were fed either a basal, control diet or a diet supplemented with 1 g/kg ß-glucan for 4 weeks. At the end of this period, fish receiving ß-glucan were continued on the same diet or switched to the basal diet for 2 weeks. Growth was significantly improved in tilapia fed ß -glucan diets compared to the unsupplemented diets. When tilapia fed the ß -glucan diet were switched back to the control diet, growth was similar to that of fish continually fed the control diet. Feeding tilapia ß-glucan and then switching to the unsupplemented diet improved the respiratory burst of neutrophils, but other immune responses (spontaneous hemolytic complement, bactericidal, and lysozyme activities) were unaffected. Although resistance to streptococcal disease was not significantly affected, survival to S. iniae infection was more than 20%higher in fish fed the ß -glucan diet followed by 2 weeks of feeding the unsupplemented diet and then changed back to the ß -glucan supplemented diet post-challenge for 3 weeks than for fish fed the control or ß-glucan diets continually.
Technical Abstract: Juvenile Nile tilapia Oreochromis niloticus were fed either a basal, control diet (n=6 aquaria) or a diet supplemented with 1 g/kg ß-glucan (n=24 aquaria) for 4 weeks. At the end of this period, fish receiving ß-glucan were continued on the same diet (n=12 aquaria) or switched to the basal diet (n=12 aquaria) for 2 weeks. Growth performance was significantly improved in tilapia on ß -glucan diets compared to the control after 4 weeks. After 6 weeks, tilapia continuously fed the ß -glucan supplemented diets (B – B) had better growth than those fed the control diet uninterrupted (C – C) or switched from the ß -glucan diet to the control (B – C) after 4 weeks (C = control, B = ß –glucan). Feeding tilapia ß-glucan for 4 weeks and then switching to the basal diet caused a significant increase in the respiratory burst of polymorphonuclear lymphocytes, but spontaneous hemolytic complement, bactericidal, and lysozyme activities were unaffected. Tilapia were then challenged with Streptococcus iniae by intraperitoneal injection. The two groups were split again (n=6 aquaria) post-challenge and continued on the same diet or switched to the alternative diet for another 3 weeks. The six initial control aquaria were maintained on the basal diet for the duration of the study. Feeding regimens were: C – C – C, B – B – B, B – C – B, B – B – C, and B – C – C (4 weeks – 2 weeks – 3-weeks post-challenge). Survival to S. iniae infection was more than 20%higher in fish fed the ß -glucan diet followed by a 2 week basal diet recovery period and then changed back to the ß -glucan supplemented diet post-challenge (B – C – B) than for fish fed C – C – C or B – B – B. However, the differences were not significant.