Many bacterial pathogens impact the U.S. catfish industry, and disease control can be challenging for producers. Columnaris disease in channel catfish is primarily caused by Flavobacterium covae (formerly F. columnare). Using antibiotics and other chemotherapeutics as treatment risks developing resistant pathogens, accumulating antibiotic residues, and potential environmental impacts. In the catfish industry, there is a strong need to evaluate dietary ingredients to enhance catfish health. Immunostimulants may enhance non-specific immune responses and offer an alternative to antibiotic treatments in catfish ponds. Further, dietary protein sources are also essential to fish health and nutrition. Evaluating protein source inclusions, both animal and plant-based, may enhance the overall fish performance in pond culture.
The current project evaluated two immunostimulants: a protease complex (AG175; Jefo Nutrition) and an organic acid substance derived from reed-sedge peat (MFG 50; Kent Nutrition Group). A 60-day trial was conducted to examine the effects of supplementary protein level, dietary formulation, and immunostimulant addition on the growth performance, immune response, and resistance to experimental Flavobacterium covae infection in channel catfish. Five diets were tested: 1) a high-quality fishmeal diet (32%; HQFM); 2 a high-protein soy-based diet (32%; CHP); 3) a low-protein soy-based diet (28%; CHL; predominately used in industry); 4) a low-protein soy diet supplemented with AG175 at 1.75 g/ton; and 5) MFG 50 in a low-protein diet at 5 lb/ton. Following feeding for 60 d, juvenile channel catfish were sampled for growth performance and baseline health indicators (n=3 per tank; body mucus, blood for sera, kidney, and spleen). A subset of fish was then subjected to an immersion-based in vivo challenge trial with F.covae (ALG-00-530; 106 CFU per mL exposure).
At 60d post-initiation, there were no dietary differences in percent weight gain (P=0.064) or specific growth rate (P=0.063), but the 32% diets appeared generally perform best. The cumulative percent mortality (CPM) was found to be different across dietary treatments (P=0.003). The mortality in the CHP group was found to be higher than the AG175 (P=0.006) and MFG 50 diets (P=0.005). These challenge data suggest that the immunostimulant additions may be beneficial in providing protection against F. covae when compared to low-protein channel catfish diets.