Comparative In Vitro and In Vivo Effects of Feed Additives on Rainbow Trout Response to <i>Ichthyophthirius multifiliis</i>

Control of the skin parasitic ciliate Ichthyophthirius multifiliis is currently based on laborious chemical and mechanical approaches, thus advocating for alternative control measures. Here, we show that early development trophonts (feeding stage residing in epidermis) Rainbow Trout Oncorhynchus mykiss epidermis was inhibited at 5 d postinfection after administration feed additives, including garlic Allium sativum, oregano Origanum onites, thyme Thymus vulgaris, coriander Coriandrum astaxanthin (a carotenoid derived from alga Haematococcus pluvialis). However, no inhibition observed 8 postinfection. We investigated whether caused by (1) additives’ direct effect parasite and/or (2) stimulation immune responses. Garlic exhibited highest vitro killing capacity toward theronts (the infective parasite), followed oregano, thyme, astaxanthin, whereas had parasiticidal effect. Immune reactions were measured plasma lysozyme activity feeding recording gene expression trout leukocytes exposed to additives. Oregano-fed fish showed a significantly (P < 0.05) elevated activity. Oregano thyme—and lesser extent, garlic, coriander—induced significant regulation innate adaptive genes vitro. The results suggest additives inhibit directly as well indirectly (i.e., elevating host response).