maintained. Fish were fed to apparent satiation twice daily at 08:00 and 15:00 for 72 days. At the end of the treatment period fish were anesthetsed, weighed and viscera and blood were sampled. Intestine samples from the FM, Y0, Y4 and Y5 groups were removed from 2 fish in each replicate tank at the end of trial (12 fish per treatment) and processed for histology analysis (H & E staining). Morphological parameters associated with SBM-induced enteritis of anterior and distal intestines, including the height of mucosal folds (HMF), width of mucosal folds, lamina propria and connective tissue were quantified. After all samples were taken, 40 fish of each treatment (6–7 fish per tank) were divided into 2 groups and transferred into a still water system with temperature at 26 ± 1 °C. The fish were fed as before and recovered from weighing and sampling stress by a 2-week acclimation. Then they were challenged by intramuscular injection with Aeromonas veronii (CGMCC No. 4274) at 8 × 104 cells/100 g body weight. Ten fish from each tank were sampled for plasma immune parameters two days after challenge and the others (20 fish per treatment) were recorded for 7-day cumulative survival rate without any food. This study showed a lower growth of SBM diets as expected compared to HFM diets, but an even lower growth with the 500g/T treatment, and a much better growth at 2000 g/T (Fig1). These results can be correlated to a wider width of mucosal folds in anterior and distal intestinal in SBM diets compared to HFM diets suggesting a negative effect of these diet on intestinal health, and also to a higher height of mucosal folds in the 2000 g/T group (Fig1). This suggests that Safmannan® at 2000 g/T was able to compensate the negative effect of soybean meal and increase gut health leading to a better growth. The study also shows that IgM levels were significantly elevated after the bacterial challenge in the diet containing parietal fractions at 500g/T (Fig2) indicating a strong immune stimulation. The levels decrease as the yeast parietal fraction concentration is increased showing a potential fatigue of the immune system. This is confirmed by the survival of the fish after the challenge. The optimum dosage was 500g/T of Safmannan®, whereas higher dosage did not improve survival. Remarkably, we can see this optimum dosage for immune stimulation was also the one giving the lowest growth, confirming hypothesis that the strong stimulation of the immune system is at the expense of the growth potential of the fish. This study highlights the duality of role of parietal fractions in fish depending on the dosage and feed composition: they can be used either as gut health enhancer (high dosage) or immune enhancer (low dosage). Formulators and farmers can benefit from using this efficient and sustainable solution against pathogens but they need to choose quality products and work with proper (and proven) dosages and administration durations.
Figure 1 growth and intestinal health parameters in Japanese seabass following a treatment with yeast parietal fractions. Values with different subscripts are significantly different (P<0.05) (Yu et al 2014)
Figure 2 immune parameters and cumulative mortality following treatment with parietal fractions. Values with different subscripts are significantly different (P<0.05) between treatments, asterisks show a different between time. For the bacterial challenge, Safmannan 500 is different than control at P<0.05 (Yu et al 2014)
20 | INTERNATIONAL AQUAFEED | March-April 2015