The infection dynamics of experimental Edwardsiella ictaluri and Flavobacterium covae coinfection in channel catfish (Ictalurus punctatus)

Authors: WISE, ALLISON - Auburn University; Lafrentz, Benjamin; KELLY, ANITA - Auburn University; LILES, MARK - Auburn University; GRIFFIN, MATT - Mississippi State University; Beck, Benjamin; BRUCE, TIMOTHY - Auburn University

Submitted to: Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2023
Publication Date: 3/15/2023
Citation: Wise, A.L., Lafrentz, B.R., Kelly, A.M., Liles, M.R., Griffin, M.J., Beck, B.H., Bruce, T. 2023. The infection dynamics of experimental Edwardsiella ictaluri and Flavobacterium covae coinfection in channel catfish (Ictalurus punctatus). Pathogens. 2023(12):462. https://doi.org/10.3390/ pathogens12030462.
DOI: https://doi.org/10.3390/ pathogens12030462

Interpretive Summary: The catfish industry in the United States is often plagued with bacterial diseases. Recently, coinfections have been reported in channel catfish between Flavobacterium covae (causing columnaris disease) and Edwardsiella ictaluri (causing enteric septicemia of catfish - ESC). Though coinfections are known to exist, little research has been conducted measuring the pathogens’ effects on mortality and host immune responses. In order to properly investigate these parameters, bacterial challenges were conducted with F. covae and E. ictaluri. Channel catfish were exposed to each pathogen in single doses and coinfective doses (combination of both pathogens). Over the course of the challenges sera, kidney, and spleen tissue were collected at 2, 4, and 7 days post-challenge to measure lysozyme activity and the expression of the immune related genes, il8, il1b, tnfa, and tfgb. Coinfections were observed to increase mortality when compared to single infections. Edwardsiella ictaluri appeared to cause an increase in lysozyme activity compared to F. covae, and all pro-inflammatory cytokines were upregulated. This data is crucial to better assess coinfections and the potential harmful effects they have on the catfish industry in the United States.

Technical Abstract: As the largest U.S. aquaculture sector, farm-raised catfish carries significant economic importance in several southern states. Edwardsiella ictaluri and Flavobacterium covae are pervasive bacterial pathogens associated with significant losses in catfish aquaculture. Bacterial coinfections in catfish production may often go unreported or misdiagnosed, resulting in a lack of proper mitigation or inadequate treatment. Coinfections have the potential to increase outbreak severity and can worsen on-farm mortality. A preliminary assessment (Trial A) 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 × 105 CFU mL-1); 3) F. covae full dose (immersion; 3.6 × 106 CFU mL-1); 4) E. ictaluri half dose (immersion; 2.7 × 105 CFU mL-1) followed by half dose F. covae (immersion; 1.8 × 106 CFU mL-1); and 5) F. covae half dose followed by half dose E. ictaluri. In coinfection challenges, the second inoculum was delivered 48 hours after the initial exposure. At 21 days post-challenge (DPC), the single dose E. ictaluri infection yielded cumulative percent mortality (CPM) of 90.0 ± 4.1 %, com-pared to 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 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 single and coinfected treatments displayed increased serum lysozyme activity at 4-DPC (P< 0.001). Three pro-inflammatory cyto-kines (il8, tnfa, il-1b) were evaluated for gene expression, revealing increased expression at 7-DPC in all E. ictaluri exposed treatments (P<0.05). A second coinfection trial (Trial B) was conducted to confirm initial findings, which again demonstrated a delayed onset of mortality in fish exposed to F. covae first. These data enhance our understanding of the dynamics of E. ictaluri and F. covae coinfections in US farm-raised catfish. In these studies, E. ictaluri appears to contribute more to mortality than F. covae, despite being administered at nearly 1/10th the delivered dose. While both agents are significant pathogens of farm-raised catfish, these works support previous assertions that reducing E. ictaluri infections through prophylactic vaccines, feed restrictions, or other measures has the potential also to mitigate F. covae severity in US catfish aquaculture.