The Lack of Effectiveness of Rosemary Oil on Fish Feed in Controlling Bacterial Cold‐Water Disease in Rainbow Trout

Abstract

AbstractRosemary Rosmarinus officinalis oil has shown potential for use as a phytobiotic fish feed supplement with antioxidant properties that can inhibit the growth of Flavobacterium psychrophilum, the pathogen that causes bacterial cold‐water disease (BCWD). To determine the in vivo effectiveness of rosemary oil in preventing or minimizing BCWD, Rainbow Trout Oncorhynchus mykiss were fed commercial feed top‐coated with one of two concentrations of rosemary oil (1% or 3% of the feed) or a control diet for 14 d. Fish from each feed treatment were then challenged with one of two doses of F. psychrophilum via subcutaneous injection, and mortality was monitored for 28 d. In both F. psychrophilum challenges, fish treated with feed coated with rosemary oil at the 1% and 3% levels experienced significantly higher mortality than fish treated with only soybean oil–coated feed. While the use of rosemary oil as a top‐coat on feed increased mortality among Rainbow Trout subjected to a disease challenge in the current study, the mechanism for this result has not been established.