Eva Jansson

Multilocus Sequence Typing Identifies Epidemic Clones of Flavobacterium psychrophilum in Nordic Countries

ABSTRACT Flavobacterium psychrophilum is the causative agent of bacterial cold water disease (BCWD), which affects a variety of freshwater-reared salmonid species. A large-scale study was performed to investigate the genetic diversity of F. psychrophilum in the four Nordic countries: Denmark, Finland, Norway, and Sweden. Multilocus sequence typing of 560 geographically and temporally disparate F. psychrophilum isolates collected from various sources between 1983 and 2012 revealed 81 different sequence types (STs) belonging to 12 clonal complexes (CCs) and 30 singleton STs. The largest CC, CC-ST10, which represented almost exclusively isolates from rainbow trout and included the most predominant genotype, ST2, comprised 65% of all isolates examined. In Norway, with a shorter history (<10 years) of BCWD in rainbow trout, ST2 was the only isolated CC-ST10 genotype, suggesting a recent introduction of an epidemic clone. The study identified five additional CCs shared between countries and five country-specific CCs, some with apparent host specificity. Almost 80% of the singleton STs were isolated from non-rainbow trout species or the environment. The present study reveals a simultaneous presence of genetically distinct CCs in the Nordic countries and points out specific F. psychrophilum STs posing a threat to the salmonid production. The study provides a significant contribution toward mapping the genetic diversity of F. psychrophilum globally and support for the existence of an epidemic population structure where recombination is a significant driver in F. psychrophilum evolution. Evidence indicating dissemination of a putatively virulent clonal complex (CC-ST10) with commercial movement of fish or fish products is strengthened.

Diagnosis of bacterial kidney disease by detection of Renibacterium salmoninarum by real‐time PCR

Bacterial kidney disease (BKD), caused by Renibacterium salmoninarum (Rs), is a serious threat to salmon in aquaculture as well as to wild populations. We have developed a real-time polymerase chain reaction (PCR) for detection of Rs in kidney samples. The PCR is based on detection of unique parts of the 16S rRNA gene of Rs and DNA equivalent to 1-10 Rs genomes was detected per reaction. No cross-reactivity with other fish pathogenic or related bacteria could be demonstrated. Analysis of individual kidney samples collected from BKD classified populations identified 39.9% of the fish as positive by real-time PCR compared with 28.0% by polyclonal enzyme-linked immunosorbent assay (ELISA). The real-time PCR assay was found to be well suited for complementary use with ELISA for diagnosis of BKD, with the ability to detect clinical as well as covert Rs infections. The infection level determined by the polyclonal ELISA and by real-time PCR was significantly correlated.

The effects of booster vaccination and fish size on survival and antibody production following Vibrio infection of bath-vaccinated rainbow trout, Salmo gairdneri☆

Abstract Rainbow trout, Salmo gairdneri , were vaccinated by bath immersion with Vibrio anguillarum at one or two separate times (4.1 g and/or 6.3 g) and bath challenged 1 month after the second vaccination. No significant difference was found in the mortalities of fish which had been boosted at 6.3 g and those which had received a primary vaccination at 6.3 g. However, both the 6.3-g groups showed lower mortalities than did fish vaccinated at 4.1 g only. No sampled fish had detectable levels of humoral antibodies prior to challenge. All groups showed increased titres following challenge. The mean post-infection titre of the group vaccinated at 4.1 g only was significantly higher than those of the other groups.