The Infection Dynamics of Experimental and Coinfection in Channel Catfish ()

and are pervasive bacterial pathogens associated with significant losses in catfish aquaculture. Bacterial coinfections have the potential to increase outbreak severity and can worsen on-farm mortality. A preliminary assessment of in vivo bacterial coinfection with (S97-773) and (ALG-00-530) was conducted using juvenile channel catfish (). Catfish were divided into five treatment groups: (1) mock control; (2) full dose (immersion; 5.4 × 10 CFU mL); (3) full dose (immersion; 3.6 × 10 CFU mL); (4) half dose (immersion; 2.7 × 10 CFU mL) followed by half dose (immersion; 1.8 × 10 CFU mL); and (5) half dose followed by half dose . In the coinfection challenges, the second inoculum was delivered 48 h after the initial exposure. At 21 days post-challenge (DPC), the single dose infection yielded a cumulative percent mortality (CPM) of 90.0 ± 4.1%, compared with 13.3 ± 5.9% in the group. Mortality patterns in coinfection challenges mimicked the single dose challenge, with CPM of 93.3 ± 5.4% for fish initially challenged with followed by , and 93.3 ± 2.7% for fish exposed to and subsequently challenged with . Despite similarities in the final CPM within the coinfection groups, the onset of peak mortality was delayed in fish exposed to first but was congruent with mortality trends in the challenge. Catfish exposed to in both the single and coinfected treatments displayed increased serum lysozyme activity at 4-DPC ( < 0.001). Three pro-inflammatory cytokines (, , ) were evaluated for gene expression, revealing an increase in expression at 7-DPC in all exposed treatments ( < 0.05). These data enhance our understanding of the dynamics of and coinfections in US farm-raised catfish.