Assessment of Flavobacterium psychrophilum‐associated mortality in Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis)

Abstract

Salmonid diseases caused by infections of Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease, remain difficult to manage as novel, pathogenic strains continue to emerge in aquaculture settings globally. To date, much of the research regarding treatment options and vaccine development has focused on rainbow trout (Oncorhynchus mykiss), but other inland‐reared salmonids are also impacted by this Gram‐negative bacterium. As such, Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis) were injection‐challenged with a variety of previously reported F. psychrophilum strains isolated from disease diagnostic cases in salmonids, as well as a standard and well‐studied F. psychrophilum strain (CSF 259–93) known to be virulent in rainbow trout. In three separate virulence assessments (Trials A, B and C), strains US063 (isolated from lake trout; Salvelinus namaycush) and US149 (isolated from Atlantic salmon) caused a significantly higher cumulative per cent mortality (CPM) relative to other strains in Atlantic salmon (p <.001 for all trials), with US149 causing significantly greater mortality than US063 in Trials A (CPM 97% vs. 65%, p =.008) and B (CPM 96% ± 2.3% vs. 81.33% ± 4.8%, p =.014). Trial C used a lower dose (1.86 × 108 CFU/mL) for US149, resulting in a lower mortality (78.67% ± 9.33%) relative to Trials A and B. CSF259‐93 did not cause significant mortality in any trials. In brook trout, the strain 03–179 (originally isolated from steelhead trout; Oncorhynchus mykiss) was significantly more virulent than any other (CPM 100% ± 0%, p <.001), followed by US063 (73% ± 3.8%) and US149 (40% ± 6.1%,) respectively. Again, CSF259‐93 did not cause significant mortality relative to a mock challenge treatment. Results provide information about the applicability of strain selection in F. psychrophilum virulence testing in Atlantic salmon and brook trout, demonstrating the high virulence of US063 and US149 for these salmonid species. This information is applicable for the development of therapeutics and vaccines against F. psychrophilum infections and demonstrates the reproducibility of the experimental challenge model.

Co‐occurrence of heterogeneous Flavobacterium psychrophilum isolates within the same Chilean farm and during the same infectious outbreak

“…The intramuscular‐injection method is not a natural infection route, since it bypasses the immune mechanisms of the fish. However, this method has been validated by our research group (Muñoz et al, 2019), as well as in other studies with F. psychrophilum and other fish hosts (Bruce et al, 2021; Hoare et al, 2017; Li et al, 2021; Sundell et al, 2019). The fact that not all injected fish died confirms that the selected challenge method (i.e.…”

“…Under field conditions, differences in the antigenic and genetic features of each F. psychrophilum isolate could determine infection and the percentage of fish mortality, as has been reported in a number of previous in vivo studies (Bruce et al, 2021;Knupp et al, 2021). In fact, epidemiological typing studies will have implications for management strategies (e.g.…”

“…Considering our group's experience in challenge experiments in rainbow trout with isolates of F. psychrophilum, intramuscular injection was employed as the infection route (Muñoz et al, 2019). This same infection route has also been employed in other geographic areas for the same pathogen (Bruce et al, 2021;Hoare et al, 2017;Li et al, 2021;Sundell et al, 2019).…”

Immersion challenge model for Flavobacterium psychrophilum infection of Atlantic salmon (Salmo salar L.) fry

“…To select for resistant breeds of fish, or to develop new and more effective vaccines to protect against F. psychrophilum , including those delivered orally or by immersion (Bøgwald & Dalmo, 2019 ; Ghosh et al, 2016 ), reliable and reproducible challenge models are required. Challenge models for F. psychrophilum have long been available for rainbow trout (Cipriano & Holt, 2005 ; Madsen & Dalsgaard, 1999 ) and, more recently, have been developed for Atlantic salmon (Bruce et al, 2021 ; Fredriksen et al, 2016 ). In these models, typically, naïve fry are infected with a known dose of the pathogen by subcutaneous, intraperitoneal or intramuscular injection, with disease progression then assessed by the onset of clinical signs, morbidities and mortalities.…”

“…Challenge models for F. psychrophilum have long been available for rainbow trout (Cipriano & Holt, 2005;Madsen & Dalsgaard, 1999) and, more recently, have been developed for Atlantic salmon (Bruce et al, 2021;Fredriksen et al, 2016). In these models, typically, naïve fry are infected with a known dose of the pathogen by subcutaneous, intraperitoneal or intramuscular injection, with disease progression then assessed by the onset of clinical signs, morbidities and mortalities.…”