Background: Furunculosis, caused by Aeromonas salmonicida, continues to be a major health problem for the growing
salmonid aquaculture. Despite effective vaccination programs regular outbreaks occur at the fish farms calling for repeated
antibiotic treatment. We hypothesized that a difference in natural susceptibility to this disease might exist between Baltic
salmon and the widely used rainbow trout.
Study Design: A cohabitation challenge model was applied to investigate the relative susceptibility to infection with A.
salmonicida in rainbow trout and Baltic salmon. The course of infection was monitored daily over a 30-day period post
challenge and the results were summarized in mortality curves.
Results: A. salmonicida was recovered from mortalities during the entire test period. At day 30 the survival was 6.2% and
34.0% for rainbow trout and Baltic salmon, respectively. Significant differences in susceptibility to A. salmonicida were
demonstrated between the two salmonids and hazard ratio estimation between rainbow trout and Baltic salmon showed a
3.36 higher risk of dying from the infection in the former.
Conclusion: The finding that Baltic salmon carries a high level of natural resistance to furunculosis might raise new
possibilities for salmonid aquaculture in terms of minimizing disease outbreaks and the use of antibiotics.
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