Use of Diet Crossover to Determine the Effects of β-glucan Supplementation on Immunity and Growth of Nile Tilapia, Oreochromis niloticus

Juvenile Nile tilapia were fed either a basal (control) diet (n = 6 aquaria) or a diet supplemented with 1 g/kg beta-glucan (n = 24 aquaria) for 4 wk. At the end of this period, fish receiving beta-glucan were continued on the same diet (n = 12 aquaria) or switched to the control diet (n = 12 aquaria) for 2 wk. After 6 wk, tilapia continuously fed the beta-glucan supplemented diets had improved weight gain and feed efficiency than those fed the control diet uninterrupted or switched from the beta-glucan diet to the control after 4 wk. Feeding tilapia beta-glucan for 4 wk and then switching to the basal diet for 2 wk caused a significant increase in the respiratory burst of polymorphonuclear lymphocytes (17.77 X 103 units/1000 white blood cells [WBC]) compared to catfish fed the control diet (13.50 X 103 units/1000 WBC) or the beta-glucan diet continuously (13.57 X 103 units/1000 WBC), but other immune parameters were unaffected. Tilapia were then challenged with Streptococcus iniae. The two groups were divided again (n = 6 aquaria) postchallenge and continued on the same diet or switched to the other diet (beta-glucan or control) for another 3 wk. No differences in survival to S. iniae infection occurred between dietary groups.