Growth, immune function, and disease and stress resistance of juvenile Nile tilapia ( Oreochromis niloticus) fed graded levels of bovine lactoferrin

Juvenile Nile tilapia ( Oreochromis niloticus) were fed nutritionally complete, practical basal diets supplemented with bovine lactoferrin (Lf) at 0, 200, 400, 800, or 1600 mg/kg diet to apparent satiation twice daily for 8 weeks. After the feeding trial, the effect of dietary Lf on growth performance, immune function, and resistance to Streptococcus iniae challenge and low-water stress was determined. Dietary Lf did not affect growth performance (weight gain, feed intake, feed efficiency ratio, or survival) or haematological parameters (haemoglobin, white and red blood cell counts, or haematocrit) ( P > 0.05). Crowding stress produced significant increases in plasma cortisol, glucose, lactate, and osmolality from baseline values ( P ≤ 0.001), but dietary Lf did not affect plasma glucose, osmolality, or cortisol concentrations ( P > 0.05). The level of Lf in diet had a significant impact on survival following S. iniae challenge with fish fed the 800 mg/kg Lf diet having significantly higher survival than control fish ( P ≤ 0.05). There was not a corresponding increase in activity of non-specific or specific immune parameters (plasma lysozyme and spontaneous haemolytic complement activities or agglutination antibody titer against S. iniae) with addition of Lf to diets ( P > 0.05), but plasma iron decreased and total iron binding capacity (TIBC) increased significantly with increasing concentration of Lf in diet ( P ≤ 0.05). The ability of Lf to sequester iron, an essential nutrient required for the growth of bacteria, is regarded as one of its key antibacterial properties. The increased survival with increasing dietary concentration of Lf seemed to correspond with a decrease in plasma iron concentration and not enhancement of non-specific or specific immune functions.