Abstract Objective The present study examined the effect of humic substance and butyric acid blend (HSB) supplementation, with and without yeast cell wall (YCW), on gut morphology, immunity, and resistance to Flavobacterium covae, Edwardsiella ictaluri, and their coinfection in juvenile Channel Catfish Ictalurus punctatus. Methods A feeding trial was conducted in a recirculating aquaculture system with Channel Catfish fingerlings (19.7 ± 0.2 g). Thirty-seven Channel Catfish were randomly assigned into four dietary groups with supplemented feed (g/kg): HSB (2.0 HSB), YCW (0.5 YCW), HSB + YCW (1.0 HSB, 0.25 YCW), and a control group (not supplemented), each consisting of eight replicates. After 45 d of feeding the diets, growth performance was assessed and head kidney, spleen, and gill tissues were sampled for gene expression, while serum was collected for lysozyme activity. Subsequently, fish were pooled by treatment and challenged by immersion with either E. ictaluri (S97-773; 7.4 × 106 CFU/mL), F. covae (ALG-00-530; 1.2 × 106 CFU/mL), or coinfection with simultaneous half-doses of both bacteria. Results No significant differences were observed in fish growth performance and survival across treatments. However, Fulton’s K-factor was significantly higher in the HSB group compared with the control. Serum lysozyme activity was significantly higher in the HSB group, while a numerically higher body mucus lysozyme activity was observed in the HSB + YCW group. No diet-related cytokine gene expression differences were observed in the kidney and splenic tissues. However, differences were observed in the expression of lysozyme genes lysc and lysg in the gill, with the highest expression in the YCW group. Furthermore, expression of ctsd (cellular protease) and defb (antimicrobial peptide) were different across all groups. Specifically, ctsd was upregulated in the HSB group when compared with the control. The expression of defb was upregulated in all treated groups, including HSB, YCW, and HSB + YCW, compared with the control group. After a 14-d bacterial challenge with either F. covae, E. ictaluri, or simultaneous exposure to both, significant differences in survival probabilities were observed across the treatment groups in the single E. ictaluri and coinfection groups but not in the F. covae group. Survival following E. ictaluri challenge varied among groups, with higher survival observed in the HSB group (79%) compared with the control fish (52%). Similarly, the HSB group showed improved survival (72%) compared with YCW (30%), HSB + YCW (45%), and the control group (30%) within the coinfected groups. Furthermore, the survival rate of Channel Catfish was lower in the coinfected groups than in those solely infected with E. ictaluri. Conclusions Feeding Channel Catfish with a dietary humic substance and buryate blend reduced the susceptibility of Channel Catfish to E. ictaluri and coinfection with F. covae. These research findings have potential practical implications for fish health and provide an alternative tool for the management of coinfections in the catfish industry.
Oladipupo et al. (Thu,) studied this question.