Control stress and win
Dietary selenomethionine supplementation is known to offer a way to reduce performance loss under stress, such as crowding conditions (Küçükbay et al. 2008). A recent study, performed at the Mahasarakham University, Thailand, showed increased performance and high protection against pathogenic pressure. A total of 735 Nile tilapia (initial weight 13.52±0.5g) were fed one of seven experimental diets (in triplicate) in fibreglass tanks for eight weeks. Organic Se (L-selenomethionine, SeMet; Excential Selenium 4000, Orffa Additives BV) and inorganic Se (sodium selenite, Na2SeO3) were each added to the basal diet at 1, 3, and 5mg Se/kg. The basal diet (28 percent crude protein), without Se supplementation, was used as a control.
The final Se concentration of the basal diet was 0.68mg Se/kg. Organic and inorganic Se supplemented diets contained 1.78, 3.53 and 4.90mg Se/kg and 1.75, 3.49 and 5.30mg Se/kg, respectively. Fish were fed at 5.0 percent of their body weight twice a day. Parameters were assessed at the end of the rearing period. After eight weeks, 20 fish from each treatment were challenged with intraperitoneal injection of the virulent Streptococcus agalactiae serotype III at 1×107 CFU/mL. The cumulative mortality was observed for 21 days and the relative percent survival (RPS) was calculated.
Table 1 shows that weight gain (WG) of fish fed SeMet at 1mg Se/kg was significantly higher than that of fish fed basal diet (p<0.05). Lymphocytes were significantly (p<0.05) higher in fish fed SeMet (1mg Se/kg) compared to fish fed basal diet. Alanine transaminase (ALT), aspartate transaminase (AST), creatinine, blood urea nitrogen (BUN), albumin, globulin and total protein were not significantly influenced by dietary Se supplementation.
Increasing dietary Se level, particularly in the form of SeMet, led to a decrease in serum cholesterol concentrations. Interestingly, the innate immune response (eg lysozyme, catalase, myeloperoxidase, superoxide dismutase and glutathione peroxidase) activity was significantly (p<0.05) increased with Se supplementation compared to the basal diet group, especially for fish fed SeMet (1 and 3mg/Se kg). Malondialdehyde (MDA) in fish serum, on the other hand, was decreased numerically for all supplementation levels. Fish fed SeMet (1mg Se/kg) showed the highest RPS after the challenge with S. agalactiae.