Effects of supplemental dietary blend of humic substances and butyric acid, and yeast cell walls on immunity and resistance to bacterial coinfection in juvenile Channel Catfish

Authors: OLADIPUPO, ABDULMALIK - Auburn University; KELLY, ANITA - Auburn University; Lafrentz, Benjamin; DAVIS, D. - Auburn University; ROY, LUKE - Auburn University; Beck, Benjamin; BRUCE, TIMOTHY - Auburn University

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: Catfish aquaculture remains the leading aquatic food-producing industry in the United States. Despite being the most produced species, diseases are a major bottleneck, resulting in significant fish loss and increasing production costs from the use of medicated feed and chemical treatments. The most reported diseases on catfish farms are caused by Flavobacterium covae (columnaris disease), virulent Aeromonas hydrophila (vAh), or Edwardsiella ictaluri (enteric septicemia of catfish (ESC) or hole-in-the-head disease). Due to their ability to remain in catfish ponds, their coordinated interaction and coinfections could result in higher mortality, which could not be easily treated with antibiotics. As a result, producers need a cheap and sustainable alternative to effectively manage these occurrences and maintain optimal growth and health of catfish. This study investigated the effects of adding a humic and butyric acid blend (HSB; 2g/kg) or yeast cell wall (YCW; 0.5g/kg) as single supplements and their combined inclusion (HSB+YCW; 0.5g/kg, 0.25g/kg) in the diet of channel catfish on growth performance, immunity and resistance against ESC and Columnaris diseases. After 45 days of feeding in glass aquaria, no differences in growth, but the HSB catfish had improved serum lysozyme activity, a natural immunity indicator, in the blood, while the HSB+YCW group showed an increase in skin mucus lysozyme activity. In addition, there was no dietary impact on the immunity as evaluated in the kidney and spleen tissues. However, there was an increase in antimicrobial activities in the gills of HSB+YCW and HSB, respectively. Furthermore, fish were pooled and redistributed based on treatment groups in plastic aquaria for bacterial infection. Then dosages of bacteria were introduced for a 30-minute static immersion with a single infection of columnaris or ESC, or their coinfection (columnaris and ESC). Fourteen days after exposure to the bacteria, survival rates of channel catfish infected with just ESC were HSB (79%), YCW (62%), HSB+YCW (70%), and Basal (52%), while there was no mortality in the columnaris challenged group. Meanwhile, overall survival was reduced in the co-infected group (HSB (72%), YCW (30%), HSB+YCW (45%), and Basal (30%). According to the challenge result, the highest survival occurred in the HSB-fed catfish in both single ESC and co-infected cases. This study clearly shows that bacterial coinfection could facilitate severe disease outbreaks than single bacterial infections in channel catfish production systems. Moreover, using humic substances blended with butyric acid as a feed supplement could be a management tool to improve the health and resistance of channel catfish to single or multiple bacterial infections without impacting growth performance.

Technical Abstract: Objective: The present study examined the effect of humic substances (HS), butyric acid (BA), and yeast cell wall (YCW) on gut morphology, immunity, and disease 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 HS & BA blend), YCW (0.5 YCW), HSB+YCW (1.0 HS & BA blend; 0.25 YCW), and a control group (not supplemented), each consisting of eight replicates. After 45 days of feeding the diets, head kidney, spleen, and gill tissues were sampled for gene expression, while blood was collected for lysozyme activity. Subsequently, fish were pooled by treatment and challenged by immersion with either 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 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 to the control. Serum lysozyme activity significantly increased in the HSB group, while a numerical increase in body mucus lysozyme activity was observed in the HSB+YCW group. No diet-related cytokine gene expression changes were observed in the kidney and splenic tissues. However, there was an upregulation of lysc and lysg gene expression in the gill, with the highest expression in the YCW group. Furthermore, ctsd (cellular protease; P = 0.024) and defb (antimicrobial peptide; P = 0.004) expression was upregulated across all groups. Specifically, ctsd was upregulated in the HSB group (P = 0.018) when compared to the control. The defb was upregulated in all treated groups, including HSB (P = 0.003), YCW (P = 0.036), and HSB+YCW (P = 0.045), compared to the control group. After a 14-day bacterial challenge with either F. covae, E. ictaluri, or simultaneous exposure to both, significant treatment survivability was observed across the groups, particularly in the single E. ictaluri and coinfection groups, whereas no mortality was reported in the F. covae group. Survival rates with E. ictaluri varied among groups (P < 0.001), with an increase in survival observed in the HSB group (79%) compared to the control fish (52%; P = 0.002). Similarly, the HSB catfish group showed improved survival (72%) compared to YCW (30%; P < 0.001), HSB+YCW (45%; P = 0.003), and the control group (30%; P < 0.001) within the coinfected groups. Furthermore, the survival rate of Channel Catfish declined in the coinfected groups compared to those solely infected with E. ictaluri. Conclusion: Overall, a dietary humic substance and buryate blend reduced catfish susceptibility 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.