Immunostimulants offered to fish in feed are considered to confer protection against various bacterial diseases
but the effects on the antiparasitic response are largely unknown. Therefore effects of dietary β-1,3-glucan on innate
immune parameters of juvenile rainbow trout Oncorhynchus mykiss and on susceptibility to the skin-parasitic ciliate
Ichthyophthirius multifiliis (Ich) have been investigated. A basal diet (dry pelleted feed) was supplemented with
0% (control), 0.2% (low), 2.0% (medium), and 5.0% (high) of the β-1,3-glucan particulate insoluble algae glucan,
paramylon, from Euglena gracilis. Fish (total 440) were divided into four groups each with 110 fish (kept in duplicate
tanks of 55) and each diet was fed to two replicate groups at a daily feeding rate of 1.5% of fish biomass for 56
consecutive days. Liver and plasma sampling was performed at day 0 and after feeding with β-1,3-glucans for 14,
28, 42, and 56 days and subsamples of fish were exposed to Ich at day 14 and 45. Gene expression in trout liver
was investigated by real-time qPCR and genes encoding immune molecules including acute phase proteins (SAA,
hepcidin, and precerebellin), immunoglobulins (IgM and IgT), cytokine (IL-1β), and lysozyme were investigated. In
addition plasma lysozyme activity was recorded. At the start of the experiment the 5.0% glucan supplemented fish
became more infected by parasites compared to control fish (0.0%) but after 45 days feeding they obtained significantly
fewer trophonts. Plasma lysozyme activity of fish fed low (0.2%) and medium (2.0%) glucan supplementation
fluctuated, while high (5.0%) glucan was associated with an elevation of lysozyme activity. Plasma lysozyme activity
was positively correlated to expression of the lysozyme gene and to body mass of fish. Groups fed low (0.2%) and
medium (2%) glucan diets showed a trend for down-regulation of immune relevant genes whereas the group fed with
high (5%) glucan showed a trend for up-regulation of genes especially the acute phase reactant SAA.