Assessing effects of dietary supplements on resistance against Yersinia ruckeri infection in rainbow trout (Oncorhynchus mykiss) using different infection models

Ensuring that feed meets the increasing requirements of intense fish production in modern aquaculture is an ongoing process. As antimicrobial growth promotors have been banned from many markets, and as the use of antibiotics represent a rapidly growing concern, disease resistance promoted by functional feed supplements have become increasingly important. The bacterial pathogen Yersinia ruckeri has long been recognized as the cause of enteric redmouth disease. Affecting salmonids, in particular, infection with Y. ruckeri has severe consequences on animal welfare, as well as production economy and prescription of antibiotics. Using recently characterized experimental infection models, we have assessed the effect of prebiotic, probiotic and immunostimulatory feed supplements on resistance towards experimental infection with Y. ruckeri serotype O1 biotype 2 in rainbow trout (Oncorhynchus mykiss). Proprietary, experimental feeds supplemented with organic acid blends, beta-glucans either alone or co-supplemented with a nucleotide mix or Pediococcus acidilactici either alone or co-supplemented with mannan oligosaccharides were produced and tested across three separate trials. Using rainbow trout at different weights (3 g, 19 g and 50 g), each trial addressed the effects of these supplements on lysozyme activity and disease resistance following Y. ruckeri infection by cohabitation, intraperitoneal injection or bath exposure. Supplementation with P. acidilactici +/- mannan oligosaccharides proved to have a consistent, positive effect on disease resistance. No effects on lysozyme activity was detected for any of the experimental feeds, and only a limited effect on the number of viable Y. ruckeri isolated from the spleen and gut tissues following infection was observed. Finally, the results of this study demonstrate that waterborne infection models causing low to moderate overall mortalities provided better resolution of experimental groups in infection setups testing these feed supplements.