Bacterial infections pose a persistent threat to U.S. catfish aquaculture. Pathogen interactions have also been shown to drive disease and increase mortality in catfish ponds, thus complicating treatment interventions. Previously, adding humic substances to diets has shown antibacterial and immune influences in fish, including improving channel catfish resistance when experimentally challenged with Flavobacterium covae. However, their prospect against polymicrobial infection is unknown. This study investigated the potential of dietary humic substances with butyric acid (HS & BA) as catfish immunomodulators for growth performance and resistance to co-infection with two virulent catfish pathogens. Yeast cell wall (YCW) components, an immunostimulatory polysaccharides, were also added as a treatment factor.
A feeding trial was conducted in a recirculating aquaculture system with channel catfish Ictalurus punctatus fingerlings (19.7±0.2g). Fish were offered either a practical basal diet (Basal; 32%), a diet supplemented with IFC4 (HS & BA blend; 4 lb/US ton; 32%), a diet supplemented with YCW (1 lb/ US ton; 28%) or a diet supplemented with both IFC4 (2 lb/US ton) and YCW (0.5 lb/ US ton) (IFC4+YCW). After 45 d, catfish were pooled by treatment and transferred into a biosecure wet lab for an in vivo pathogen challenge. All treatment groups were contested in an immersion challenge using triplicate tanks with either a single dose of Edwardsiella ictaluri (S97-07; 7.4 x 106 CFU mL-1), F. covae (ALG-00-530; 1.2 x 106 CFU mL-1), or co-infection with simultaneous half-doses of E. ictaluri and F. covae. Fulton’s condition factor post-feeding differed across groups (P=0.014), with an increase in IFC4 catfish compared to the Basal group (P=0.009). Following the 14-day bacterial challenge, cumulative percent mortality (CPM) comparisons showed differences in infection type (P=0.011) and across dietary treatments (P=0.004) with no significant interaction effects (P=0.598). CPM increased in the co-infected group compared to the single E. ictaluri-infected catfish group. No mortality was observed in the single F. covae groups due to the low immersion dose. Single infection CPM with E. ictaluri across treatments were IFC4 (22%), YCW (30%), IFC4+YCW (38%) and Basal (48%). Overall CPM increased in the co-infected groups (IFC4 (28%), YCW (55%), IFC4+YCW (70%), and Basal (70%)), with lower mortality in IFC4-fed catfish compared to the Basal (P=0.010) or YCW (P=0.010) groups.
These research findings have potential practical implications for fish health, providing an alternative tool for managing co-infections in the U.S. catfish industry. This information provides the industry with actionable knowledge to make informed decisions and develop effective strategies for disease management.