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

Abstract

Flavobacterium psychrophilum is the causative agent of bacterial cold‐water disease (CWBD) and rainbow trout fry syndrome (RTFS), which affect salmonids. To better understand this pathogen and its interaction with the host during infection, including to support the development of resistant breeds and new vaccines and treatments, there is a pressing need for reliable and reproducible immersion challenge models that more closely mimic natural routes of infection. The aim of this present study was to evaluate a challenge model developed previously for rainbow trout for use in Atlantic salmon. First, preliminary challenges were conducted in Atlantic salmon (n = 120) and rainbow trout (n = 80) fry using two F. psychrophilum isolates collected from each fish species, respectively; fish had been pretreated with 200 mg/L hydrogen peroxide for 1 h. Thereafter, the main challenge was performed for just one F. psychrophilum isolate for each species (at 2 × 107 CFU/mL) but using larger cohorts (Atlantic salmon: n = 1187; rainbow trout: n = 2701). Survival in the main challenge was 81.2% in Atlantic salmon (21 days post‐challenge) and 45.3% in rainbow trout (31 days post‐challenge). Mortalities progressed similarly during the preliminary and main challenges for both species, demonstrating the reproducibility of this model. This is the first immersion challenge model of F. psychrophilum to be developed successfully for Atlantic salmon.

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

“…As previously indicated, the challenge results were at times contradictory. In Atlantic salmon, using the immersion challenge method and a dose >10 6 CFU/mL to replicate the clinical signs, Macchia et al (2022) reported a less than 20% accumulated mortality. By contrast, Hoare et al (2019) used isolate AVU‐3S/13 serotype Fp T (type 0) in Atlantic salmon, with accumulated mortalities of 70% at an inoculation dose of 4 × 10 7 CFU/fish by intramuscular inoculation.…”

“…6 CFU/mL to replicate the clinical signs,Macchia et al (2022) reported a less than 20% accumulated mortality. By contrast,Hoare et al (2019) used isolate AVU-3S/13 serotype Fp T (type 0) in Atlantic salmon, with accumulated mortalities of 70% at an inoculation dose of 4 × 10 7 CFU/fish by intramuscular inoculation.Dead fish showed scale loss, skin lesions such as ulcers and/or eroded fins.…”

Host specificity and virulence of Flavobacterium psychrophilum: a comparative study in ayu (Plecoglossus altivelis) and rainbow trout (Oncorhynchus mykiss) hosts

“…Previous virulence studies addressing the question of F. psychrophilum host specificity focused on a few strains tested in several fish species [ 24 , 26 , 39 – 42 ] or on a single host challenged with isolates retrieved from various fish species [ 23 , 25 , 42 , 43 ]. However, the limited number of strains tested in each study and the lack of data regarding their genotype make comparisons difficult.…”

Secreted Extracellular Products of Flavobacterium covae as Potential Immunogenic Factors for Protection against Columnaris Disease in Channel Catfish (Ictalurus punctatus)

“…F. covae typically affects the gills, the skin, and fins [ 3 ]. Thus, the challenge by immersion is closer to the natural route of infection [ 30 ]. Mortalities were recorded daily for 2 weeks in each group to calculate the percent survival and to assess the protective effect of the vaccination.…”