New analysis method enables more targeted prevention of winter ulcers

Blue Analytics is now able to offer a rapid and cost-effective analytical method (MLVA) to characterise the winter ulcer bacterium Moritella viscosa. The knowledge from this analysis can contribute to better prevention of winter ulcers.

The winter ulcer bacterium Moritella viscosa is cultivated on blood agar dishes.
The winter ulcer bacterium Moritella viscosa is grown on blood agar with salt before being characterised using the new MLVA analysis method.

High incidence of winter ulcers

Winter ulcers are a disease and an environmental nuisance that leads to significant animal welfare problems and financial losses for the aquaculture industry. Winter ulcers also represent a growing reputational issue for the industry, with an increasing number of negative press reports related to this condition.

The main causative agent of winter ulcers in farmed salmonids is Moritella viscosa. In the Fish Health Report for 2022, winter ulcers are classified as the third most important health problem.

Vaccination has not solved the problem of winter ulcers

M. viscosa has been a component of multivalent oil adjuvant vaccines since the late 1990s. Unlike other bacterial agents included in multivalent registered salmonid vaccines, vaccination has not solved the disease problem.

Results from laboratory work combined with field experience indicate that the degree of cross-protection between different M. viscosa variantsis uncertain and should be analysed. Until this is mapped, a variant-specific vaccine may be a good alternative. This means that isolates of M. viscosa collected from field outbreaks locally and regionally will be used as vaccine agents for fish that are released at the same locations next time.

Moritella viscosa is more complex than previously thought

M. viscosa was initially seen as a homogeneous bacterium. Over time, research has revealed that this is not the case.

Using DNA sequencing, new variants and groupings of the bacterium have been described. Both full genome sequencing, use of the gene sequence gyrase B (gyr B) and MLVA (Multiple Locus Variable Number Tandem Repeat Analysis) have been performed to characterise and find relationships between different strains of M. viscosa.

MLVA has emerged as the most appropriate analysis method for routine surveys as it is fast and cost-effective. Blue Analytics has tested this new method and now offers it as part of our analysis services.

Here you can read a guide to sampling for MLVA M. viscosa and order the analysis via this link

What is MLVA?

The analysis method, called MLVA (Multiple Locus Variable Number Tandem Repeat Analysis), facilitates routine genotyping of M. viscosa strains compared to full genome sequencing. The latter method is resource-intensive in terms of both labour and cost.

MLVA as a method is based on mapping special parts of the bacteria’s DNA. These are parts that do not code for specific genes, but which contain short repetitive sequences. This type of DNA region is often referred to as VNTR (Variable Number of Tandem Repeats) (see figure below). The combination of non-coding regions and repetitive elements means that a VNTR changes faster than the genetic material in general.

Investigations of VNTR are therefore suitable for mapping relationships between related species. Change in VNTR occurs in the form of a change in the number of repeating elements. By analysing multiple VNTRs, combined with a relationship study, we get a high-resolution tool for bacterial typing. This provides more knowledge about the bacterium and opportunities for more targeted prevention of winter ulcers.

Illustration of variable number of tandem repeats (VNTR) in the bacterium Moritella viscosa.
The figure shows four of the VNTRs (Variable Number of Tandem Repeats) included in the M. viscosa MLVA study. By counting the number of repeating elements in each VNTR, this can be used to map the relationship between different M. viscosa strains.
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