Effect of dietary essential oils on the resistance to furunculosis by Atlantic salmon

We examined the impact of twelve different essential oils on the growth of Aeromonas salmonicida, the pathogen responsible for furunculosis, often lethal to Atlantic salmon (Salmo salar).  Application of essential oils on culture media inoculated with A. salmonicida  showed that most oils significantly (P<0.05) inhibited growth of the pathogen.  Oil of cinnamon bark was most effective, followed by oil of thyme, lemongrass, and oregano.   To estimate the tolerance, we fed rainbow trout (Oncorhynchus mykiss) diets with graded levels of these oils.  One percent cinnamon oil significantly increased mortality and reduced weight gain, whereas 0.25% oil had no observable significant effect. Feeding oregano or thyme oil up to 1% of diet had no significant effect on weight gain or feed efficiency but impaired both measures at 2 and 4% (P<0.05). Feeding lemongrass oil up to 0.5% had no significant effect on weight gain or feed efficiency, but it impaired weight gain and feed efficiency at 1% or higher.  Feeding Atlantic salmon a diet containing 1% oregano or 0.5% lemongrass or 1% thyme oil had no significant (P>0.05) effect on time to death after challenged with Aeromonas salmonicida, whereas 0.05% cinnamon oil significantly (P<0.04) delayed (by 10%) time to 50% mortality.