The epidemiology of winter ulcer disease in farmed Atlantic salmon in Canada

Principal Investigator: Sophie St-Hilaire
Co-Investigator: Novartis (Allison MacKinnon)

Winter ulcer disease, caused by the bacteria Moritella viscosa, is the primary reason for antibiotic use in aquaculture on the east coast of Canada. Anecdotal reports suggest mortality from this disease is increasing, but it is unknown whether this is due to drug resistance, increased disease severity, reduced salmon immunity, increased exposure to the bacteria, or some combination of these factors. To date, there is no published descriptive epidemiology for M. viscosa in Canada, which is essential for developing control and prevention strategies. Our specific objectives were to describe disease outbreaks in relation to environmental and husbandry factors at pen and farm levels, and to assess antibiotic treatment effectiveness for the past 5 years.

Our preliminary investigation indicated that the disease occurs at water temperatures greater than 10 C, which is not typical of winter ulcer disease caused by M. viscosa in Norway. We also found a wide range of mortality associated with the disease within and between farms. In some cases, pens with very high mortality were in close proximity to pens of fish that showed no sign of the disease, which suggests it is not transmitted through water in the same way as other bacterial diseases. This was consistent with our clinical findings that few fish become systemically infected; rather, they appear to succumb to disease due to problems maintaining a balanced internal fluid pressure (osmoregulation). Affected animals go off feed almost as soon as they start to develop ulcers, that is, before they have a systemic bacterial infection, which complicates treatment. Producers may need to start antibiotic therapy very early in the disease process to prevent fish death.