The Infection Dynamics of Experimental Edwardsiella ictaluri and Flavobacterium covae Coinfection in Channel Catfish ( Ictalurus punctatus ).

Edwardsiella ictaluri and Flavobacterium covae 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 E. ictaluri (S97-773) and F. covae (ALG-00-530) was conducted using juvenile channel catfish ( Ictalurus punctatus ). Catfish were divided into five treatment groups: (1) mock control; (2) E. ictaluri full dose (immersion; 5.4 × 10 5 CFU mL -1 ); (3) F. covae full dose (immersion; 3.6 × 10 6 CFU mL -1 ); (4) E. ictaluri half dose (immersion; 2.7 × 10 5 CFU mL -1 ) followed by half dose F. covae (immersion; 1.8 × 10 6 CFU mL -1 ); and (5) F. covae half dose followed by half dose E. ictaluri . In the coinfection challenges, the second inoculum was delivered 48 h after the initial exposure. At 21 days post-challenge (DPC), the single dose E. ictaluri infection yielded a cumulative percent mortality (CPM) of 90.0 ± 4.1%, compared with 13.3 ± 5.9% in the F. covae group. Mortality patterns in coinfection challenges mimicked the single dose E. ictaluri challenge, with CPM of 93.3 ± 5.4% for fish initially challenged with E. ictaluri followed by F. covae , and 93.3 ± 2.7% for fish exposed to F. covae and subsequently challenged with E. ictaluri . Despite similarities in the final CPM within the coinfection groups, the onset of peak mortality was delayed in fish exposed to F. covae first but was congruent with mortality trends in the E. ictaluri challenge. Catfish exposed to E. ictaluri in both the single and coinfected treatments displayed increased serum lysozyme activity at 4-DPC ( p < 0.001). Three pro-inflammatory cytokines ( il8 , tnfα , il1β ) were evaluated for gene expression, revealing an increase in expression at 7-DPC in all E. ictaluri exposed treatments ( p < 0.05). These data enhance our understanding of the dynamics of E. ictaluri and F. covae coinfections in US farm-raised catfish.