Impacts of Feed Additives on Surface Mucosal Health and Columnaris Susceptibility in Channel Catfish, Ictalurus punctatus

Abstract

One of the highest priority areas for improvement in aquaculture is the development of dietary additives and formulations which provide for complete mucosal health and protection of fish raised in intensive systems. Far greater attention has been paid to dietary impact on gut health than to protective effects at other mucosal surfaces such as skin and gill. These exterior surfaces, however, are important primary targets for pathogen attachment and invasion. Flavobacterium columnare, the causative agent of columnaris disease, is among the most prevalent of all freshwater disease-causing bacteria, impacting global aquaculture of catfish, salmonids, baitfish and aquaria-trade species among others. We were interested in examining here whether the feeding of a standard catfish diet supplemented with Alltech dietary additives Actigen®, a concentrated source of yeast cell wall-derived material and/or Allzyme® SSF, a fermented strain of Aspergillus niger, could offer protection against F. columnare mortality. A nine-week feeding trial of channel catfish fingerlings with basal diet (B), B+Allzyme® SSF, B+Actigen® and B+Actigen®+Allzyme® SSF revealed good growth in all conditions (FCR < 1.0), but no statistical differences in growth between the treatments was found. At nine weeks, based on pre-challenge trial results, basal, B+Actigen®, and B+Allzyme® SSF groups of fish were selected for further challenges with F. columnare. Replicated challenge with a virulent F. columnare strain, revealed significantly longer median days to death in B+Allzyme® SSF and B+Actigen® when compared with the basal diet (P < 0.05) and significantly higher survival following the eight day challenge period in B+Actigen® when compared with the other two diets (P < 0.05). Given the superior protection provided by the B+Actigen® diet, we carried out transcriptomic comparison of gene expression of fish fed that diet and the basal diet before and after columnaris challenge using high-throughput RNA-seq. Pathway and enrichment analyses revealed changes in mannose receptor DEC205 and IL4 signaling at 0 h (prior to challenge) which likely explain a dramatic divergence in expression profiles between the two diets soon after pathogen challenge (8 h). Dietary mannose priming resulted in reduced expression of inflammatory cytokines, shifting response patterns instead to favor resolution and repair. Our results indicate that prebiotic dietary additives may provide protection extending beyond the gut to surface mucosa.