The incidence of furunculosis in juvenile Atlantic salmon, Salmo salar, has increased in recent years in Chile, with isolates of Aeromonas salmonicida being the primary cause. However, in some cases, molecular diagnostics failed to identify the etiological agent. We previously demonstrated that a proportion of undiagnosed cases was produced by a new A. salmonicida strain. In those cases where the pathogen remained unidentified, we isolated colonies with an A. salmonicida-like appearance. Subsequent phylogenetic analysis presented in this work grouped those A. salmonicida-like isolates within the Aeromonas piscicola clade. Whole genome sequencing confirmed the taxonomic affiliation, giving additional insights into virulence and antibiotic resistance markers. Indeed, one of the strains showed reduced susceptibility to oxytetracycline. Virulence potential was assessed by in vivo testing in S. salar, which resulted in disease with pathognomonic signs of furunculosis. Although the pathogen presents common antigens with A. salmonicida, the current vaccine triggered only a modest IgM response against A. piscicola in the field. Our results support the hypothesis that the increasing incidence of furunculosis in Chile cannot solely be ascribed to the emergence of the new less-virulent A. salmonicida strain, but may partially result from furunculosis-like infections caused by A. piscicola strains which exhibit a comparable virulence level.

The incidence of furunculosis in juvenile Atlantic salmon, Salmo salar, has increased in recent years in Chile, with isolates of Aeromonas salmonicida being the primary cause. However, in some cases, molecular diagnostics failed to identify the etiological agent. We previously demonstrated that a proportion of undiagnosed cases was produced by a new A. salmonicida strain. In those cases where the pathogen remained unidentified, we isolated colonies with an A. salmonicida-like appearance. Subsequent phylogenetic analysis presented in this work grouped those A. salmonicida-like isolates within the Aeromonas piscicola clade. Whole genome sequencing confirmed the taxonomic affiliation, giving additional insights into virulence and antibiotic resistance markers. Indeed, one of the strains showed reduced susceptibility to oxytetracycline. Virulence potential was assessed by in vivo testing on S. salar, which resulted in disease with pathognomonic signs of furunculosis. Although the pathogen presents common antigens with A. salmonicida, the current vaccine triggered only a modest IgM response against A. piscicola in the field. Our results support the hypothesis that the incidence of furunculosis in Chile cannot be ascribed to the emergence of the new A. salmonicida strain, but may partially result from infections caused by A. piscicola strains which exhibit a comparable virulence level.