Effects of dietary levels of vitamin A on growth, hematology, immune response and resistance of Nile tilapia (Oreochromis niloticus) to Streptococcus iniae

This study was conducted to evaluate the effect of supplemental levels of vitamin A (0, 2500, 5000, 10,000, and 20,000 IU/kg diet) on the growth performance, hematology, immune response and resistance of Nile tilapia, Oreochromis niloticus to Streptococcus iniae challenge. Each diet was fed to Nile tilapia (initial weight, 5.26 ± 0.10 g) in quadruplicate aquaria to apparent satiation twice daily for 10 weeks. The results indicate that vitamin A is a dietary essential for Nile tilapia. After 2 weeks of feeding, fish fed the vitamin A unsupplemented diet exhibited significantly lowest weight gain, feed intake and feed efficiency ratio than those of other treatments. Dietary vitamin A levels had no effect on survival. Other gross deficiency signs observed during the 10-week trial were hemorrhages, operculum deformity, fin erosion, darker body coloration and lethargy. Significantly lower hepatosomatic index and hematological parameters (except white blood cell count) were also recorded in fish fed the vitamin A unsupplemented diet. The resistance of erythrocytes to hemolysis in hypotonic solutions was lowest in fish fed vitamin A unsupplemented diet and increased with increasing vitamin A supplementation. A supplemental level of vitamin A of 3910 IU/kg diet was sufficient to prevent these deficiency signs in juvenile Nile tilapia. Serum protein, lysozyme activity and superoxide anion production were enhanced by supplementation of vitamin A, whilst serum total immunoglobulin and spontaneous complement activity were not affected by dietary treatments. Dietary supplementation of vitamin A had no effect on the resistance of Nile tilapia to S. iniae infection challenge and antibody production against the same bacterium.