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

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

Flavobacterium psychrophilum, the causative agent of bacterial cold-water disease, is a devastating, worldwide distributed, fish pathogen causing significant economic loss in inland fish farms. Previous epidemiological studies showed that prevalent clonal complexes (CC) differ in fish species affected with disease such as rainbow trout, coho salmon and ayu, indicating significant associations between particular F. psychrophilum genotypes and host species. Yet, whether the population structure is driven by the trade of fish and eggs or by host-specific pathogenicity is uncertain. Notably, all F. psychrophilum isolates retrieved from ayu belong to Type-3 O antigen (O-Ag) whereas only very few strains retrieved from other fish species possess this O-Ag, suggesting a role in outbreaks affecting ayu. Thus, we investigated the links between genotype and pathogenicity by conducting comparative bath infection challenges in two fish hosts, ayu and rainbow trout, for a collection of isolates representing different MLST genotypes and O-Ag. Highly virulent strains in one host species exhibited low to no virulence in the other. F. psychrophilum strains associated with ayu and possessing Type-3 O-Ag demonstrated significant variability in pathogenicity in ayu, ranging from avirulent to highly virulent. Strikingly, F. psychrophilum strains retrieved from rainbow trout and possessing the Type-3 O-Ag were virulent for rainbow trout but not for ayu, indicating that Type-3 O-Ag alone is not sufficient for pathogenicity in ayu, nor does it prevent pathogenicity in rainbow trout. This study revealed that the association between a particular CC and host species partly depends on the pathogen's adaptation to specific host species.

Phylogenomic characterization of Flavobacterium psychrophilum isolates retrieved from Turkish rainbow trout farms

Flavobacterium psychrophilum, a devastating fish pathogen, is responsible for bacterial cold-water disease (BCWD), also known as rainbow trout fry syndrome. F. psychrophilum is the main causative agent of outbreaks in rainbow trout farms, especially at early live stages. In the present study, we aimed to characterize F. psychrophilum Turkish isolates. Eighteen isolates were retrieved from BCWD outbreaks between 2014 and 2021. In vitro phenotypic characterization showed gelatin and casein hydrolysis capacities and in vitro adhesion for all isolates, whereas elastinolytic activity was present for 16 of 18 isolates. We used complete genome sequencing to infer MLST-type, serotype and phylogenetic reconstruction. Strikingly, one strain isolated from Coruh trout (FP-369) belongs to ST393, a previously undescribed ST, and is phylogenetically distant from the other isolates. However, all strains retrieved from rainbow trout belong to the well-characterized clonal complex CC-ST10, 12 of 17 were tightly connected in a single cluster. Several serotypes (Types -1, -2 and -3) were represented among isolates, but no correlation was observed with geographic origins. This analysis suggests a regional dissemination of an epidemic, disease-producing bacterial population. This study provides a basis for epidemiological surveillance of isolates circulating in Turkey and phenotypic data for future molecular studies of virulence traits of this important fish pathogen.

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

Flavobacterium psychrophilum is a pathogenic bacterium affecting Chilean salmonid farms. High antigenic and genetic diversity exists among Chilean F. psychrophilum isolates, but the distribution thereof among farms is poorly understood. These epidemiological data are key for developing isolate-specific vaccines. The present study isolated F. psychrophilum in diseased Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) from five freshwater farms between 2018 and 2019. Each farm only raised one salmonid species and was geographically separated from and did not share culturing water with the other farms. Antigenic and genetic analyses were conducted to shed light on the possibility of isolates coexisting within the same farm during outbreaks. A total of 68 Chilean F. psychrophilum isolates were recovered from skin lesions, gills, fins, kidney and spleen of moribund and live fish. Among the 68 Chilean isolates, mPCR-serotyping indicated three major serotypes (i.e. 23.5% type 0; 47.1% type 2; and 26.5% type 4) and, to a lesser degree, serotype 1 (2.9%). Sixteen antigenic groups were detected by slide agglutination. Genetic characterizations by 16S rRNA alleles identified 71% of the isolates as the virulent genogroup CSF259-93 allele. A predominant serotype was associated with each farm, with types 0 and 4 related to Atlantic salmon and types 1 and 2 to rainbow trout. Notwithstanding, several antigenic groups coexisted within some farms. Likewise, the experimental intramuscular challenges (n = 20) demonstrated that the type-2 isolates from rainbow trout were the most pathogenic among isolates recovered from infectious outbreaks in Atlantic salmon, especially as compared to those from types-0 and -4. These results allow us to suggest that prevention measures, specifically vaccines, should be developed according to dominant isolates and with specificity to each farm, that is the use of autogenous or site-specific vaccines.

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

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.