Catfish farming is the largest sector of U.S. aquaculture and is of major economic importance for Southern U.S. agriculture. Maintaining and improving catfish health is a primary concern for producers, and bacterial pathogens can cause large-scale losses in production ponds. Edwardsiella ictaluri, Aeromonas hydrophila, and columnaris-causing bacteria (Flavobacterium spp.) are predominant bacterial pathogens causing mortality within production ponds. In a pond environment, catfish are simultaneously exposed to multiple aquatic pathogens, and issues with water quality and stress can influence pathogen and host dynamics. Bacterial coinfections may increase the severity of the constituent pathogens and elevate mortality, thus creating economic losses. A recent project assessed and characterized the effects of bacterial coinfections on juvenile channel catfish. Three in vivo pathogen challenge trials compared exposure to single and coinfective bacterial doses. Mixed combinations of F. covae (ALG-530-00), virulent Aeromonas hydrophila (vAh; ML09-119), and E. ictaluri (S97-773) were incorporated into the experimental design.
With respect to experimental coinfection with vAh and F. covae, at 96 h post-challenge, the single virulent A. hydrophila infection (immersed in 2.3 x 107 CFU mL-1) resulted in final cumulative percent mortality (CPM) of 28.3 ± 9.5 %. The full-dose F. covae group (immersed in 5.2 x 106 CFU mL-1) was 23.3 ± 12.9 %. A coinfection full-dose combination (98.3 ± 1.4 %) and a half-dose administration (76.7 ± 17.1 %) greatly increased mortality (P< 0.001).
Concerning experimental coinfection with vAh and E. ictaluri, at ten days post-challenge, the full-dose, single vAh infection (immersed in 1.9 x 107 CFU mL-1) resulted in a final CPM of 25.0 ± 2.9 %. The CPM for the full-dose E. ictaluri group (immersed in 4.0 x 105 CFU mL-1) was 11.7 ± 4.4 %. When both pathogens were administered together, the full-dose combination (41.7 ± 7.3%) and half-dose combination (40.0 ± 10.4%) demonstrated pronounced mortality.
Experimental findings indicate changes in both mortality levels and trends from exposure to multiple bacterial pathogens. Reducing disease outbreaks in catfish farming is critical to enhancing production yields and quality products. As more information is available on polymicrobial infections, new methods of treatment or prevention may be possible, such as more informed decisions on targeted antibiotic applications or vaccination strategies.