Investigations into ulcerative skin disease agents, Moritella viscosa and Tenacibaculum spp. in Atlantic Salmon: Interactions and in vivo challenge development

Description

In a previous ACRDP project (M-14-01-003), six Canadian field isolates^{Footnote 1} of Moritella viscosa from the East and West Coasts were examined and two in vivo challenge models for the respective regions were developed. Efficacy studies for various vaccine preparations using both challenge models were also delivered.

This project examines two pathogens of concern to the salmon farming industry, Moritella viscosa and Tenacibaculum spp., and their roles in skin ulcerative diseases. Recent experience in developing immersion challenge^{Footnote 2} models will be applied to the further understanding of skin ulcerative diseases. The primary focus is to study the bacterial species of the genus Tenacibaculum which causes a condition called “mouth rot” or Tenacibaculosis. The secondary focus is to study interactions among bacteria associated with skin ulcers. The project focuses on isolates found on the East Coast of Canada as well as interesting changes in the range of environmental temperatures at which pathogenicity is observed in the field (e.g., Moritella viscosa).

The project develops a basis for progress towards developing vaccines for use in Canada, which will provide the aquaculture industry with an alternate and proactive strategy for dealing with bacterial disease. Vaccines will improve animal welfare and the sustainability of salmon aquaculture while reducing economic loss from fish mortality, antibiotic use, and the downgrading of market product. Results of the study can be used by the aquaculture industry for research on genetic selection for disease resistance, help the fish health community apply better diagnostic methodology for confirming and treating ulcerative skin diseases, and improve understanding of naturally occurring diseases that affect wild salmon and their effects on farmed fish.

Findings

Some isolates did not perform well in laboratory conditions and did not result in disease under challenge studies. Finalizing the exposure model development was hindered by the pathogen, in that the growth pattern of the bacterium developed long strings of bacteria that made suspension difficult to manage and also required high concentrations to be grown in order to produce results in vivo. Three or more applications through live animals (exposure and subsequent recovery) was required to achieve a virulent outcome. This high concentration requirement is in line with the suspected characteristic that the pathogen is a secondary/opportunistic infection in most cases.

The final exposure model consisted of a 90 minute exposure at a fish density of 75 kg/m³ and a pathogen concentration of ~1 – 5 x 10⁷ cfu/mL at low temperature, and yielded a 40 - 80 percent mortality rate over a six week observation period. The model can be used as a baseline to test the effect of treatments, or other factors of interest and to gauge the impact of, in this case, ulcer formation. The progression study resulted in information on ulcer development time and pathogenicity of the isolate. The final model is now an ideal starting point for efficacy testing of therapeutants and vaccine testing.

Program name

Aquaculture Collaborative Research and Development Program (ACRDP)

Years

2016 - 2019

Principal investigator

Steven Leadbeater
Biologist, Fisheries and Oceans Canada, St. Andrews Biological Station, Maritimes Region
Email: Steven.Leadbeater@dfo-mpo.gc.ca

Team member(s)

Allison MacKinnon, Senior Technical Consultant, Elanco Canada Ltd.

Anthony J. Manning, Senior Scientist, Research Productivity Council

Collaborator(s)

Leighanne Hawkins, Fish Health Manager, Cooke Aquaculture Inc.