| Summary: | Full-fat and defatted (hexane-extracted) soybean meal (SBM) in formulated feeds cause an inflammatory response in the distal intestine of all salmonids tested to date. The condition has been described as a non-infectious, subacute enteritis and apparently resolves only when SBM is removed from the diet. The signs include reduced weight gain, nutrient malabsorption, and diarrhea. The afflicted distal intestinal tissue has reduced function, as reflected by decreased brush border digestive enzyme activity and nutrient transport. Histomorhological investigations reveal shortening of the primary and secondary mucosal folds with a widening of the central stroma (lamina propria) and submucosa, shortened microvilli of the brush border membrane, increased formation of microvillar vesicles, and a dramatic decrease or even absence of the normal supranuclear absorptive vacuoles in the enterocytes. The lamina propria is widened with a profound infiltration of a mixed population of inflammatory cells such as T lymphocytes, neutrophilic granulocytes, cells of monocytic lineage including macrophages, eosinophilic granular cells, and diffuse IgM. Recent RNA expression studies indicate that pro-inflammatory cytokines and other signaling and regulatory factors involved in recruitment and activation of T cells and other immune cells, and factors involved in maintenance of normal mucosal integrity, were differentially expressed during the development of the enteropathy. The involvement of saponins, lectins, trypsin inhibitors, soy antigens and/or commensal microorganisms has been suggested. Various processing methods and feed additives may aid in modulation the inflammatory response and show some promise in allowing the use of standard SBM qualities in salmonid feeds without infringing on fish health and welfare.
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