Effects of β-1,3-glucan on innate immunity responses and mortality induced by Vibrio harveyi, hemorrhagic septicemia virus, or Miamiensis avidus in the olive flounder Paralichthys olivaceus

Olive flounder ( Paralichthys olivaceus ) were fed diets containing 0.01, 0.05, or 0.1% β-1,3-glucan for 30 days, and the effects on general growth, innate immune functions, and mortality caused by pathogen infections were examined. Flounder were found to grow better when fed 0.05 and 0.1% β-1,3-glucan-supplemented diets. The examined innate immunity parameters (lysozyme and complement activities of serum; respiratory burst and phagocytic activities of leucocytes) were all elevated by β-1,3-glucan feeding, although their responsiveness to β-1,3-glucan was not the same. The fish were challenged with three representative pathogens: Vibrio harveyi as a bacterial pathogen, hemorrhagic septicemia virus, and the scuticociliate pathogen Miamiensis avidus . V . harveyi infection-related mortality was significantly reduced by all three β-1,3-glucan concentrations, while mortality due to hemorrhagic septicemia virus infection was significantly reduced only by 0.1% β-1,3-glucan. Moreover, β-1,3-glucan had no effect on M . avidus infection-induced mortality. These results indicate that innate immune functions activated by β-1,3-glucan played a role in reducing bacterial and viral infection-related mortality in olive flounder. In contrast, the same level of immunity was not sufficient to protect against scuticociliatosis-related death. This pronounced activity against bacterial infection indicates that factors other than innate immunity are responsible for resolving bacterial pathogens.