Feeding Atlantic salmon Salmo salar L. soybean products: effects on disease resistance (furunculosis), and lysozyme and IgM levels in the intestinal mucosa

Two trials were initiated to investigate the consequences of including vari ous soybean products in diets for Atlantic salmon Salmo salar L. on (1) mor tality following infection by Aeromonas salmonicida ssp. salmonicida during a cohabitation challenge, and (2) the lysozyme and IgM content of the inte stinal mucosa. Groups of salmon were fed control diets containing fishmeal as the sole protein source (Contr1 and Contr2, respectively), soy concentra te-containing diets (SoyConc1 and SoyConc2, respectively), or diets contain ing either solvent-extracted soybean meal (SoyMeal, trial 1) or soybean mol asses (SoyMol, trial 2), an alcohol extract of soybean meal. Both SoyMeal a nd SoyMol caused enteritis-like changes in the distal intestine, which were not observed in fish fed the Contr1, Contr2, SoyConc1, or SoyConc2 diets. There were significant differences (P < 0.05) in mortality between feeding groups following the A.s. salmonicida challenge: these differences were gre atest in fish fed SoyMeal (65.6%), least in fish fed SoyConc1 (60.5%), and intermediate in the fish fed the Contr1 diet (62.9%). The SoyMol diet cause d significantly (P < 0.0001) increased levels of both lysozyme and IgM in t he mid and distal intestinal mucosa. It is concluded that components of soy bean meal and soybean molasses cause an inflammatory response in the distal intestine that may lead to increased susceptibility to furunculosis.