The effects of supplemented diets with a phytopharmaceutical preparation from herbal and macroalgal origin on disease resistance in rainbow trout against Piscirickettsia salmonis

Abstract The present study aimed to evaluate the effects of a commercial phytopharmaceutical preparation from herbal and macroalgal origin on the growth and immune response of rainbow trout adapted to seawater and its susceptibility to Piscirickettsia salmonis infection. Preliminary in vitro trials, evaluated the effects of the commercial product Futerpenol® on the expression levels of selected immune-regulatory genes and its protective effect in a challenge against Piscirickettsia salmonis (LF89). Subsequent in vivo feeding trials were conducted to corroborate fish protection against Piscirickettsia salmonis. Control and treatment diets (with or without the commercial product Futerpenol® at a concentration of 1 kg/ton) were fed to triplicate groups of 50 fish (average weight: 100.1 ± 11.1 g) during 30 days. Fish from all dietary groups were equally redistributed in three tanks and challenged by cohabitation with fish infected with P. salmonis (shedders) at the end of the feeding treatment, and mortalities were recorded over 80 days post-infection. A cumulative mortality of 35.0 ± 4.3 and 15.0 ± 6.0% was registered when challenged fish were previously fed during 30 days with the control and treatment diets respectively. Futerpenol® dietary administration preceding the cohabitation challenge gave significant protection from P. salmonis as suggested by the Relative Percent Survival (RPS) values of 62.3 and 57.1% after 60 and 80 days post-infection, respectively. These results suggest that dietary supplementation with Futerpenol® at 1 kg/ton do not affect growth performance and strengthen immunity of Oncorhynchus mykiss against P. salmonis under the experimental conditions applied during this study. Statement of relevance This study demonstrates that dietary supplementation with a phytopharmaceutical preparation, rich in fucoidans and labdane diterpenes strengthen immunity of O. mykiss against P. salmonis. The use of feed supplemented with this kind of phytopharmaceutical preparations in anticipation to potential infections or vulnerable stages of the production could be useful as an alternative method to prevent and limit the outbreak of salmonid rickettsial septicaemia (SRS) which constitutes one of the main infectious diseases causing production problems and substantial loss in salmonids and marine fish aquaculture worldwide.