Inflammatory response of rainbow trout Oncorhynchus mykiss (Walbaum, 1792) larvae against Ichthyophthirius multifiliis

► We examine the immunological response of rainbow trout larvae to I. Multifiliis. ► We describe novel morphological structures of the skin at infection sites. ► Quantitative PCR showed increased expression of IL-1β, IL-8 and cathelicidin-2. ► Positive antibody staining was seen for CD8, SAA and MHCII at infection sites. ► Trout larvae 84 degree days post-hatch are able to mount an immunological response.