Growth, chemical composition and resistance to Streptococcus iniae challenge of juvenile Nile tilapia (Oreochromis niloticus) fed graded levels of dietary inositol

A study was undertaken to determine the influence of the dietary levels of inositol on growth performance, feed utilization, immune response and resistance of Nile tilapia to Streptococcus iniae challenge. Six isonitrogenous (38% crude protein), isocaloric (3300 kcal DE/kg) purified diets supplemented with graded levels of inositol (0.0, 50, 100, 200, 400 or 800 mg/kg diet) were fed to juvenile tilapia averaging 5.8 +/- 0.1 g (mean +/- S.E.M.) in triplicate aquaria twice daily to apparent satiation for 10 weeks. Supplementation of dietary inositol had no affect on final weight gain, feed intake, feed efficiency ratio and survival. Also, no significant differences were observed in total cell, red and white blood cell counts, hematocrit and plasma hemoglobin content among treatments. Hepatosomatic index was not significantly (P>0.05) affected by dietary treatments. Fish fed the 800-mg inositol diet had significantly lower muscle lipid content than the groups fed diets containing 200 mg inositol/kg or lower, but these values did not differ from that of fish fed the 400-mg inositol diet. Liver lipid content significantly decreased when dietary inositol was increased to 100 mg/kg diet or higher. Plasma lysozyme activity and antibody titers to S. iniae were not affected by supplementation of dietary inositol. Cumulative mortality of fish 14-day post challenge with S. iniae was likewise not affected by the dietary inositol levels. The results of this study indicate that Nile tilapia juveniles did not require an exogenous source of inositol for normal growth, feed utilization and erythropoiesis. Supplementation of dietary inositol had no effect on improving the resistance of tilapia to S. iniae infection. However, supplementation of inositol to purified diets is required to prevent lipotropic effect of inositol deficiency. (C) 2004 Published by Elsevier B.V.