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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Flavobacterium psychrophilum is the causative agent of Bacterial Cold Water Disease (BCWD)/ Rainbow Trout Fry Syndrome (RTFS) in rainbow trout Oncorhynchus mykiss. The disease is associated with significant mortality in trout farms and thereby responsible for severe economical losses in this part of the aquaculture industry. Vaccination of the very young life cycle stages is not successful due to the immature development of the adaptive immunity in yolksac larvae and early fry. This explains the extensive usage of antibiotics for control of BCWD/RTFS. Selective breeding of RTFS resistant fish may be a solution, and identification of markers associated with natural susceptibility/resistance to the disease may elevate breeding efforts towards more robust strains. We suggest a QTL (SNP Affx-88941461) for partial disease resistance on chromosome 25 (Omy25) based on our experimental F. psychrophilum challenges of outbred fish and subsequent GWAS analyses. The favourable SNP allele was designated Q, whereas q indicated the unfavourable allele. We validated this QTL in two subsequent challenge experiments by F. psychrophilum exposure of QQ, Qq and qq trout. In the first trial we produced trout carrying at least one favourable allele (QQ and Qq) associated with partial resistance by fertilizing trout eggs from non-selected (outbred) females with sperm from homozygous QQ males. In the second trial we also produced homozygous offspring by fertilizing trout eggs from QQ females with sperm from QQ males. The resistance profiles of these groups were then determined in the laboratory by triplicate or duplicate challenge experiments (common garden water bath exposure to F. psychrophilum), which showed a significantly higher survival in trout carrying the favourable allele. Field observations supplemented the laboratory studies. Under farm conditions QQ and Qq trout showed a higher survival rate compared to qq trout. In the present study chromosome 25 (Omy25) QTL was found associated with a partial resistance to F. psychrophilum. However, we recommend to include additional SNPs in future breeding studies because several QTL are associated with resistance.

Flavobacterium psychrophilum , the causative agent of bacterial coldwater disease (BCWD) is a significant threat to global aquaculture. True to its name, BCWD tends to occur at temperatures between 8 and 12 °C and presents as a systemic disease with characteristic skin ulcerations. Juvenile rainbow trout are particularly susceptible and in these fish the condition is referred to as rainbow trout fry syndrome (RTFS). Resistance to F . psychrophilum is heritable and is not adversely correlated with the growth of fish, thus selective breeding appears to be an achievable approach to its control. The current study explores the connection between resistance to BCWD and several immunological markers. After determining resistance/susceptibility to F . psychrophilum following experimental infection in 40 full-sibling families of rainbow trout, selected families were experimentally infected with F . psychrophilum and differences in antibody production, major histocompatibility (MH) class IIβ genotype and respiratory burst activity (RBA) throughout infection were compared. Serum IgM production increased over time but significant differences between resistant and susceptible families were not observed at either 28 days or 120 days. Of the six families that were genotyped for MH class IIβ, there did not appear to be specific genotypes that conferred resistance or susceptibility to F . psychrophilum . Further, the RBA of both head kidney leukocytes and whole blood was not significantly different between the resistant and susceptible rainbow trout families. Although the selected immune markers did not differ based on resistance status, the RBA of head kidney leukocytes in all families studied dramatically decreased seven days after infection while total blood RBA remained constant. Day seven was also when severe symptoms and/or mortality due to BCWD was first observed, thus these results may reveal information regarding the pathogenesis of the organism. A better understanding of appropriate immune defenses could provide the basis for breeding programs to effectively combat this costly pathogen, but further study of functional immune markers particularly during the fry stage of development is required. [ABSTRACT FROM AUTHOR]