Several bacterial pathogens impact U.S. catfish production, many of which are opportunistic and disease control can be challenging for producers. Columnaris disease in channel catfish is primarily caused by Flavobacterium covae (formerly F. columnare), and the pathogen is commonly treated with antimicrobials via feed. Using antibiotic treatments risks the development of resistant pathogens, the accumulation of antibiotic residues, and potential environmental impacts. However, there is a strong need to evaluate functional ingredients within catfish feeds to enhance catfish health. Immunostimulants may augment non-specific immune responses, offering an alternative to antibiotic treatments and promoting growth in catfish ponds through healthier stocks. Further, dietary protein sources are also essential to growth and fish health. Evaluating protein source inclusions, both animal and plant-based, may enhance the overall fish performance in culture systems.
A recent project evaluated two immunostimulants in fingerling channel catfish: a protease complex (AG175; Jefo Nutrition) and an organic substance derived from reed-sedge peat (MFG 50; Kent Nutrition Group). A 60-day trial examined the effects of supplementary protein concentration, dietary formulation, and immunostimulant addition on the growth performance, immune response, and resistance to experimental F. covae infection in channel catfish Ictalurus punctatus. Five diets were tested: 1) a high-quality fishmeal diet (CF32; 32%); 2) a high-protein soy-based diet (C32; 32%); 3) a low-protein soy-based diet (C28; 28%), predominately used in industry); 4) a low-protein soy diet with protease complex supplementation (PC, 28%); and 5) a humic substance in a low-protein diet (HS, 28%). After the trial, juvenile channel catfish were sampled for growth performance (n=3 per tank). A subset of fish was then subjected to an immersion-based in vivo challenge trial with F. covae (ALG-00-530; 106 CFU mL-1 immersion exposure). Immune gene expression from sampled head kidney and spleen, sera lysozyme activity, and survival were assessed pre-and post-challenge.
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 to generally perform best. The cumulative percent mortality (CPM) differed across dietary treatments (P=0.004). The C32 had significantly higher mortality than the PC (P=0.006) and HS diets (P=0.005). Sera lysozyme activity numerically improved in the PC and HS treatments at 8d post-infection, along with a significant lysozyme increase post-challenge (P=0.001). Additionally, the PC group demonstrated elevated proinflammatory immune genes following 30d of feeding and post-challenge. Significant expression of il-1β (P = 0.001) and tnf-α (P = 0.009) was observed in the PC group spleens compared to C28 and C32, no treatment effects were observed in the head kidney, but PC and CF32 had the best impact in the expression of il-1β and tnf-α. These data suggest that the oral administration of the PC and HS may be beneficial in providing protection against F. covae in juvenile channel catfish. Further investigations into optimized feeding for health promotion will better clarify both application doses and regimens for channel catfish.