Understanding the fish pathogen flavobacterium psychrophilum diversity for the control of rainbow trout fry syndrome in the United Kingdom

Abstract

Rainbow trout represents the most prominent species in freshwater farming in UK aquaculture. One of the common diseases constraining rainbow trout production and increasingly causing problems in Atlantic salmon (Salmo salar L.) hatcheries worldwide is rainbow trout fry syndrome (RTFS) or bacterial cold water disease (BCWD). During the last 20 years, the development of a commercial vaccine against RTFS has been hindered by the prevalence of a wide range of the fish pathogen F. psychrophilum, thus the current treatment of choice is the use of antibiotics. Studies involved in understanding the innate and adaptive immune response of vaccinated rainbow trout fry using inactivated whole cell are still lacking. Therefore, the aim of this thesis is to characterise the strain diversity and antibiotic susceptibility of UK F. psychrophilum isolates, evaluate the efficacy of a whole-cell formalin-killed polyvalent vaccine, which was developed based on the characterisation results of this study, and investigate the immune response in trout fry following the immersion vaccination via the changes in expression of relevant immune genes. A total of 315 F. psychrophilum isolates, 293 of which were collected within the UK, were characterised using four genotyping methods and a serotyping scheme. A high strain diversity was identified among the isolates with 54 pulsotypes, ten (GTG)5-PCR types, two 16S rRNA allele lineages, seven plasmid profiles and three serotypes. The predominant profile observed within the F. psychrophilum isolates examined was PFGE cluster II – (GTG)5-PCR type r1 – 16S rRNA lineage II – serotype Th (n= 70/156, 45%). The characterisation results not only revealed the wide distribution within the UK and the persistence within a site of predominant pulsotypes, but also the presence of unique genotypes in certain sites or countries. Co-existence of genetically and serologically heterogeneous isolates within each farm was detected, highlighting the reasons this disease is so difficult to control, especially by vaccination. The occurrence over time of F. psychrophilum pulsotypes within a site could provide important epidemiological data for farm management and the development of site-specific vaccines. The antimicrobial susceptibilities of 140 F. psychrophilum strains, 125 of which were from the UK, were evaluated by the broth microdilution (MIC) and disc diffusion methods. There was evidence of reduced susceptibilities to three of the main antimicrobials used in UK aquaculture. Broth microdilution testing showed that only 12% of 118 UK isolates tested were WT to oxolinic acid (MIC COWT 0.25 mg L-1), 42% were WT for oxytetracycline (MIC COWT 0.25 mg L-1), and 66% were WT for amoxicillin. In contrast, all the isolates tested were WT (MIC COWT 2 mg L-1) for florfenicol, the antimicrobial of choice for RTFS control in the UK. Despite the imprecision of disc diffusion-based COWT values due to high standard deviations, there was a high categorical agreement between the classification of the strains (into WT or NWT) by MIC and disc diffusion methods for florfenicol (100%), oxolinic acid (99%), amoxicillin (97%) and oxytetracycline (94%). In general, this study showed that the UK F. psychrophilum isolates examined remain susceptible to florfenicol and also stresses the importance of performing susceptibility testing using standardised methods and COWT values. Several statistically significant associations between genotypes and the reduced susceptibilities of F. psychrophilum strains were revealed. A whole-cell formalin killed polyvalent vaccine against RTFS/BCWD was developed by combining three genetically and serologically divergent strains, recently collected from UK farms. The efficacy of this polyvalent vaccine was evaluated after immersion vaccination in 5 g trout and bath challenge using hydrogen peroxide as a pre-stressor with a virulent heterologous isolate of F. psychrophilum strain. Significant protection was achieved with an RPS of 84%. The combination of exposure to hydrogen peroxide prior to bath challenge may be an alternative to an injection challenge with 12 g trout, although further standardisation and optimisation of the challenge model is required. Changes in the innate immune response of trout fry following the initial vaccination included the up-regulation of the interleukin 1 β (IL-1β) gene in head kidney at 4 h and the up-regulation of toll-like receptor-2 (TLR-2) in skin at day 2. While the expression levels of C3 was unchanged, the down regulation of CD8-α in head kidney and spleen and CD4-1 in spleen were documented. IgM and IgT transcripts were found to be up-regulated in hind-gut two days post-vaccination. Understanding the strain diversity and the antibiotic susceptibility of UK F. psychrophilum isolates could help improve the control strategies, such as preventing the spreading of pathogenic F. psychrophilum clones between fish farms, reducing the use of antibiotics in RTFS/BCWD treatment and monitoring the development of acquired antibiotic resistance mechanisms. Moreover, strain characterisation data of UK F. psychrophilum species has assisted in selecting suitable candidates for developing an effective RTFS vaccine.