The primary aim of this experiment was to evaluate the intestinal barrier permeability in vivo in rainbow trout (Oncorhynchus mykiss) fed increasing levels of soyabean meal (SBM). The relationship between SBM-induced enteritis (SBMIE) and the permeability markers was also investigated. Our results showed that the mean score of morphological parameters was significantly higher as a result of 37·5 % SBM inclusion in the diet, while the scores of fish fed 25 % SBM or lower were not different from those of the fish meal-fed controls (P < 0·05). SBMIE was found in the distal intestine (DI) in 18 % of the fish (eleven of sixty): ten in the 37·5 % SBM-fed group and one in the 25 % SBM-fed group. Sugar markers in plasma showed large variation among individuals probably due to variation in feed intake. We found, however, a significant linear increase in the level of plasma d-lactate with increasing SBM inclusion level (P < 0·0001). Plasma concentration of endotoxin was not significantly different in groups with or without SBMIE. Some individual fish showed high values of endotoxin in blood, but the same individuals did not show any bacterial translocation. Plasma bacterial DNA was detected in 28 % of the fish with SBMIE, and 8 % of non-SBMIE fish (P = 0·07). Plasma concentration of d-lactate was significantly higher in fish with SBMIE (P < 0·0001). To conclude, SBMIE in the DI of rainbow trout was associated with an increase in bacterial translocation and plasma d-lactate concentration, suggesting that these permeability markers can be used to evaluate intestinal permeability in vivo.

This experiment investigated intestinal enteropathy and digestive function of rainbow trout challenged with soybean meal-based diet (SBM) at optimal or suboptimal environments created by normal or reduced water flow, respectively. Oxygen level remained above 7 mg L -1 for optimal environment and between 4 and 5 mg L -1 for suboptimal environment. Triplicate groups of fish (mean body weight 74 g) were fedfishmeal-based diet (FM) or SBM at optimal environment in period 1 (28 days). In period 2 (42 days), fish were subjected to a change from FM to SBM or remained on the same diet as used in period 1. The fish were also exposed to change from optimal to suboptimal environment or remained under optimal conditions. The fish subjected to change from FM to SBM, regardless of their environment, showed similar degree of enteropathy from day 14. Lipid and starch digestibility was lower in SBM-fed fish at suboptimal environment compared to fish fed the same diet at optimal environment.Crude protein digestibility, however, was highest in SBM-fed fish at suboptimal environment throughout period 2. In conclusion, in SBM-fed rainbow trout, exposure to suboptimal environment did not change the degree of enteropathy; however, lipid and starch digestibility were further reduced. K E Y W O R D Sapparent digestibility coefficients, digestive function, enteritis, hypoxia, rainbow trout, soybean meal | INTRODUCTIONThe use of plant ingredients in salmonid feeds to improve sustainability of aquaculture may lead to challenges including impaired digestive function, reduced growth and increased risk of developing gastrointestinal disorders such as soybean meal-induced enteritis (SBMIE). The negative effects of plant ingredients are attributed to the presence of non-starch polysaccharides (NSP) and antinutritional factors (ANF).SBM has been used as a model to study the effect of plant ingredients on gut health and function of salmonids (Krogdahl, Bakke-Mckellep, & Baeverfjord, 2003;Mosberian-Tanha et al., 2016;Romarheim et al., 2008;Urán, Aydin, Schrama, Verreth, & Rombout, 2008). The inclusion of SBM has shown to adversely affect the apparent digestibility coefficients (ADC) of nutrients and energy (Opstvedt et al., 2003;Romarheim et al., 2006). Furthermore, it has been shown that SBM can reduce activity of digestive enzymes in the distal intestine (DI)of Atlantic salmon (Salmo salar) (Chikwati et al., 2013;Krogdahl et al., 2003). The reduced activity of digestive function may partly be due to to reduce feed intake and increase mortality in juvenile lake trout (Salvelinus namayeush) (Beamish & Tandler, 1990), and under chronic exposure, it also causes gill damage and hyperplasia (Meade, 1985). In contrary, in another experiment, chronic exposure to sublethal levels of ammonia did not change feed intake in Atlantic salmon kept at 12°C (Kolarevic et al., 2013).It is not known how the combination of a suboptimal environment (such as hypoxia) and a plant-based diet (such as SBM-based diet) may affect digestive function and intestinal h...

This study investigated morphological changes associated with soya bean meal-induced enteritis (SBMIE) in distal intestine (DI) of rainbow trout (Oncorhynchus mykiss) fed a soya bean meal (SBM)-based diet and exposed to normoxia or hypoxia created by optimal and low water flow rates, respectively. A 28-day adaption period was followed by a 42-day challenge period where 600 fish were subjected to dietary challenge and/or hypoxia. Twelve tanks each containing 50 juvenile trout were assigned randomly in triplicate to each treatment. Histopathological and immunohistochemical evaluation revealed pathological features that have not previously been described in association with SBMIE. Vacuolar degeneration of epithelial cells mainly at the base of mucosal folds, epithelial cysts, epithelial dysplasia, necrosis, shedding of necrotic cells, and granulomatous inflammation including infiltration of enlarged, sometimes finely vacuolated or "foamy" macrophages, multinucleated giant cells and increased proliferation of fibroblasts were observed. Acid-fast bacteria were not detected in enlarged macrophages; however, these cells contained AB-PAS- and sometimes cytokeratin-positive material, which was interpreted to be of epithelial/goblet cell origin. Hypoxia did not affect the morphological changes in DI. These results suggest that SBM was associated with a granulomatous form of enteritis in DI of rainbow trout regardless of water oxygen level.

The cover image, by P Mosberian‐Tanha et al., is based on the Original Article Granulomatous enteritis in rainbow trout (Oncorhynchus mykiss) associated with soya bean meal regardless of water dissolved oxygen level, DOI: .