The effects of dietary soybean saponins on growth and performance, intestinal histology and immune response of first feeding rainbow trout Oncorhynchus mykiss

The purpose of these studies was to determine the effects of diets containing water/alcohol extracts from soybean meal (SBM) on growth and performance, intestinal histology and specific and nonspecific immune parameters of juvenile rainbow trout. The first study evaluated seven experimental diets containing SBM, SBM after extraction, soy protein concentrate (SPC), SPC after extraction, SBM saponin extract, SBM flavonoid extract and Quillaja saponin (0.171%). A casein-gelatin based diet served as a control diet. The second study evaluated SBM, SBM after extraction, SBM carbohydrates, SBM flavonoid extract, SBM saponins, SBM lipids, and negative control (containing 50% of casein-gelatin). Diets were formulated to have one half of the casein-gelatin replaced by soybean protein or supplemented with methanol/water extracts in amounts equal to the amounts found in 50% protein replacement. All female rainbow trout (Troutlodge, Inc., Sumner, WA) were subjected to feeding experiments beginning at first exogenous feeding. After initial feeding trials fish were maintained on experimental diets until vaccination. In the first study one half of the fish in each tank were vaccinated at 13 weeks by intraperitoneal injection of 0.1 ml Furogen vaccine (AquaHealth Ltd.; aluminum phosphate adjuvanted Aeromonas salmonicida bacterin). The other fish in each tank were injected with phosphate buffered saline (PBS) as a control. Each fish was fin clipped to identify control and vaccinated fish. In the second study one half of the fish in each tank were vaccinated after reaching an approximate mean individual weight of 5 grams (different chronological ages) with Furogen Dip vaccine (AquaHealth Ltd.; formalin inactivated Aeromonas salmonicida bacterin). Fish in the first study were sampled 2 and 4 weeks after vaccination for specific humoral antibody responses and non-specific mucosal immune responses. Fish in the second study were sampled 2, 4 and 7 weeks after vaccination to determine specific antibody responses in plasma and in mucus. Samples of the posterior mid-intestine were taken prior to vaccination in the second study to determine histological changes due to dietary treatment. Fish fed diets containing SBM, SBM after extraction, SPC, SPC after extraction and SBM carbohydrates all exhibited lower growth than the control diet. Fish fed the saponin, lipid and flavonoid fractions performed similarly as control. Fish fed soybean saponins had higher specific plasma antibody levels after intraperitoneal vaccination but not after immersion vaccination. Feeding fish soybean saponins did not result in histological changes in the posterior intestine resembling those in fish with soybean induced posterior enteritis. It is concluded that purified soybean saponins, when included in the diet in an amount equivalent to the amount in 50% protein replacement, do not result in reduced growth and performance or pathological changes in the posterior intestine consistent with soybean induced posterior enteritis. Additionally, orally administered soybean saponins can augment the specific immune response following intraperitoneal vaccination.