Impact Of Feed Additives On Surface Mucosal Health And Columnaris Susceptibility In Channel Catfish Fingerlings, Ictalurus Punctatus

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. Columnaris infections target the host skin and gill surfaces. Previous work by our group has demonstrated that higher levels of secreted mucins and higher expression of a host rhamnose-binding lectin(RBL) at these surfaces facilitate columnaris attachment and pathogenesis. We were interested in examining here whether the feeding of a standard catfish diet supplemented with Alltech dietary additives Actigen®, a second generation product derived from yeast cell wall mannan oligosaccharides and/or Allzyme® SSF, a fermented strain of Aspergillus niger, could offer protection against F. columnare mortality. We were particularly interested in the potential of MOS to saturate the RBL receptor as we have previously demonstrated with addition of rhamnose and galactose sugars.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. 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 highthroughput RNA-seq. A total of 4593 differentially expressed genes were captured from the basal diet and the Actigen® diet groups. Gene analyses revealed significant suppression of RBL induction after challenge in the B+Actigen® fed fish, lower pro-inflammatory gene expression(e.g. IL-17, SCYA106), and higher expression of mucin pathway members(e.g. MUC2, MUC19). 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. Taken together, our results indicate that addition of Actigen® to catfish diets may be protective against external columnaris disease.