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Research Document 2020/060

Characterization of Moritella viscosa and winter ulcer to inform pathogen transfer risk assessments in British Columbia

By Wade, J. and Weber, L.


Winter ulcer is a disease caused by infection from the gram-negative bacterium, Moritella viscosa. It is commonly reported in farmed Atlantic Salmon (Salmo salar) in Norway, Iceland and to a lesser extent in Scotland. Disease has been reported in farmed Atlantic Salmon in British Columbia (BC) but no other fish species. It typically occurs during the winter, when water temperatures drop below 7-10°C. Infected fish that survive the cold water period can recover once temperatures increase or if salinity decreases below 12-15 ppt. Although mortalities due to winter ulcer may be relatively low, it is considered a significant issue for animal welfare as well as loss of revenue due to downgrades caused by ulceration. Waterborne transmission of the bacteria is the most likely route of spread within a population, although only a few studies have been conducted using natural transmission routes. Environmental and husbandry related stressors have been identified as likely contributors to infection and disease. Differences in virulence between strains of M. viscosa as well as species specific strains have been identified. There is some indication that strains may also be regionally distinct. Little is known of the strain types or virulence of M. viscosa resulting in winter ulcer in farmed Atlantic Salmon in BC. Most of what is known about winter ulcer and the etiological agent is a result of studies conducted on farmed Atlantic Salmon and strains from Norway, Iceland and Scotland.

In BC, Fisheries and Oceans Canada (DFO) began screening both farmed Atlantic and Pacific salmons for M. viscosa as a part of the Fish Health Audit and Surveillance Program  in 2012. From 2012 to 2018, there were 17 audit-based farm-level winter ulcer diagnoses made in the province. To date, no such diagnoses have been made in farmed Pacific salmon.

With the exception of three years (2013-2015), it has been a condition of licence to report Fish Health Events (FHE) on Atlantic Salmon farms to the Regulator since 2002. We cannot confirm when the Atlantic Salmon industry began testing for M. viscosa, nor if all the industry began screening at the same time. The first FHE attributable to M. viscosa was reported to DFO in 2011. From 2011 to 2012 and 2016 to 2018, there were 13 FHEs attributed to winter ulcer reported on Atlantic Salmon farms in BC.

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