Wise

Co-infective Bacterial Pathogens in Channel Catfish (Ictalurus punctatus) Production

Allison L. Wise1, Benjamin R. LaFrentz2, Anita M. Kelly1, Mark R. Liles3, and Timothy J. Bruce1

1Auburn University, School of Fisheries, Aquaculture, and Aquatic Sciences, Auburn, AL 36849 (334-844-9213); 2USDA-ARS Aquatic Animal Health Unit, Auburn, AL 36830 (334-887-3741); 3Auburn University, Department of Biological Sciences, Auburn, AL 36849 (334-844-4830)

ABSTRACT

Catfish farming is the largest sector of the U.S. aquaculture industry 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 have the ability to cause large-scale losses in production ponds. Edwardsiella ictaluri, Aeromonas hydrophila, and Flavobacterium columnare are the most predominant bacterial pathogens causing mortality within catfish production facilities. Interestingly, disease outbreaks resulting in high mortalities may also be coupled with multiple pathogens. The occurrence of bacterial co-infections may often go unreported or misdiagnosed, resulting in a lack of proper mitigation for the co-infective effectors. Bacterial co-infections may increase the severity of the constituent pathogens along with grossly increasing mortality, thus creating economic losses. To assess and characterize the effects of bacterial coinfections, three pathogen challenge trials will be conducted to compare in vivo virulence along with fish immune responses resulting from exposure to single and co-infective bacteria. The pathogens investigated during this project will include mixed combinations of F. columnare (ALG-530-00), virulent A. hydrophila (vAh; ML09-119), and E. ictaluri (S97-773). A preliminary assessment of bacterial co-infection with E. ictaluri and F. columnare was recently completed, using 10 g juvenile channel catfish (Ictalurus punctatus). Following tank acclimation, triplicate tanks of catfish were divided into five groups: 1) mock control; 2) full dose E. ictaluri; 3) full dose F. columnare; 4) half dose of E. ictaluri followed by a half dose F. columnare at 48h post-initial infection; and 5) half dose of F. columnare followed by a half dose E. ictaluri at 48h post-initial infection. Cumulative percent mortality (CPM) was recorded to compare mortality between groups for 21 days post-challenge (dpc). The full E. ictaluri infection (dosed with 3 mL of a inoculum in 10L of rearing water) resulted in a CPM of 90.0 ± 4.1% while the F. columnare group (dosed with 110 mL of a 3.6 x106 CFU mL-1 inoculum in 10L of rearing water) was found to be 13.3 ± 5.9%. With respect to the reduced-dose co-infection treatments, similar mortality was noted in catfish initially challenged with either E. ictaluri (93.3 ± 5.4%) or F. columnare (93.3 ± 2.7%). Despite similarities in final CPM within the co-infection groups, the onset of peak mortality appeared to be delayed in fish first exposed to F. columnare. In addition to the mortality assessment, a separate three tanks per treatment group were also used to collect kidney and spleen tissues to evaluate immune responses and quantify bacterial load using qPCR. Innate and adaptive immune responses of fish will be measured with a sera lysozyme assay, RT- qPCR (to investigate cytokine activity), and an enzyme-linked immunosorbent assay (ELISA) to assess antibody titer development following exposure. The evaluation of catfish immune responses will better elucidate how the pathogens impact channel catfish and assist in the development of potential treatments or preventative measures. The synthesis of these results will aid both fish health diagnosticians and channel catfish producers in better controlling bacterial co-infections.

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