Furunculosis in Atlantic salmon (Salmo salar L.) is not readily controllable by bacteriophage therapy

Abstract The potential of bacteriophage therapy to control bacterial disease in farmed fish was tested using, as an example, furunculosis of Atlantic salmon, caused by Aeromonas salmonicida subsp. salmonicida . In vivo testing with Atlantic salmon and rainbow trout ( Oncorhynchus mykiss Walbaum ) showed no adverse effects, with bacteriophage generally cleared within 96 h of administration by either intraperitoneal (i.p.) injection or oral in-feed. Juvenile Atlantic salmon were administered a combination of bacteriophage O, R and B (1.9 × 10 8  pfu fish − 1 ) by i.p. injection, after they had been challenged with A. salmonicida subsp. salmonicida 78027, also by i.p. injection. The fish that were injected with bacteriophage immediately after challenge died at a significantly slower rate then those that were either not treated with bacteriophage, or treated 24 h post-challenge. However, the end result (100% mortality) was not affected. In further experiments the effects of oral (1.88 × 10 5  pfu g − 1 fish − 1 daily for 30 days), bath (1.04 × 10 5  ml − 1 daily for 30 days) and i.p. (6.25 × 10 7  pfu fish − 1 ) phage treatment to control furunculosis in experimentally infected Atlantic salmon were compared with antibiotherapy (treatment with 10 mg kg − 1 bw − 1 day − 1 oxolinic acid for 10 days), using an indirect cohabitation challenge. No protection was offered by any of the bacteriophage treatments, compared to the positive challenge group, although significant protection was offered by the oxolinic acid treatment. Analysis of samples taken from the trials demonstrated that bacteriophage were correctly administered to the fish and, on occasion, were isolated from fish that had succumbed to furunculosis. It was also shown that bacteriophage resistant A. salmonicida subsp. salmonicida isolates could be recovered from mortalities in all the treatment groups. The results suggest that, although there were no safety problems associated with the approach, furunculosis in Atlantic salmon is not readily controllable by application of bacteriophage.