High mortalities among rainbow trout (Oncorhynchus mykiss) fry are often seen as the result of infections due to Flavobacterium psychrophilum, and in Denmark this pathogen is one of the most common causes of disease in rainbow trout freshwater farms . The infection can be treated with antibiotics, but lower sensitivity against the infection is an increasing problem for some of the used drugs, wherefore alternative treatment methods are investigated. Studies showing how water temperature as well as salinity level in the water affect the outcome of an infection with F. psychrophilum will be presented , so infections with the bacterium can be controlled and high mortalities among the affected fish avoided .
The effect of salt water (1%) and warm temperature (18±1°C) in the control of F. psychrophilum in rainbow trout fry were investigated . Fish experimental trials were set up. In the first part of the experiment, fish (0.7 g) were at first infected by bath challenge and then exposed to either salt water (1 dpi – 12°C) or to increased water temperature (1 dpi or at onset of mortalities). Water parameters were changed gradually over 24 hours. Negative infection controls were included (freshwater and salt water at 12°C; warm freshwater at 18°C) and fish survival was followed over time. In the second part of the experiment, a cohabitation challenge was established and the effect of salt water (1%) on disease transmission evaluated (1.5-2 g/fish). Fish survival was followed over time.
Following bath challenge, the salt treatment delayed the appearance of clinical disease, while in the warm temperature groups fish survival decreased more rapidly than the positive control. During the co-habitation challenge, IP injected fish reached 0% survival within two weeks in both groups. Cohabitant fish swimming in salt water had a significant increase in survival (42.6%) compared to the positive controls (17.9%) kept in freshwater . Infected dead and moribund fish were confirmed positive for F. psychrophilum.
It was shown that increasing water salinity delayed and partly prevented RTFS. This delay can give time to achieve AMR test results and initiate treatment before reaching high mortalities in a fish batch. Further studies should evaluate the robustness of the preventive effect of this approach, its effect on the microbial communities (fish and farm environment), and whether it could be combined with other measures like e.g. phage therapy. The effect of the warm water was surprising, as F. psychrophilum favours colder temperatures.