Field Observations on the Effect of a Mannan Oligosaccharide on Mortality and Intestinal Integrity of Sole (Solea senegalensis, Kaup) Infected by Photobacterium damselae subsp. piscicida

This study was conducted in order to investigate the effect of a mannan oligosaccharide (MOS) on the intestinal morphology of sole (Solea senegalensis, Kaup) reared under commercial conditions. The dietary inclusion rate for MOS was 0.4% and it was used either alone or in combination with a vaccination regime against bacterial diseases (Pasteurella spp. and Vibrio spp.). One week after the start of the experimental period, a natural outbreak of pasteurellosis, caused by Photobacterium damselae subsp.piscicida,occurred in all the groups of fish.A two-way ANOVA showed that only MOS supplementation reduced fish mortality by ca. 8% (P = 0.050). Additionally, light microscopy examination of the intestine revealed that MOS supplementation significantly increased the mucosal folding by 29% (P = 0.016) in the anterior intestinal region and by 33% (P = 0.002) in the posterior intestinal region. Scanning electron microscopy demonstrated that both MOS supplementation and vaccination significantly increased microvilli density on the enterocyte surfaces in the anterior intestinal regionby 13% (P = 0.028) and 30% (P = 0.001) respectively. In the posterior intestinal region neither MOS supplementation nor vaccination significantly affected the microvilli density (P = 0.005).The present study suggests that dietary MOS supplementation protects the intestinal morphology of infected sole and hinders the development of pathogenic infection, possibly by binding with Photobacterium damselae subsp. piscicida, resulting in reduced mortality of infected fish.