New research to probe Vibrio splendidus as possible cause of summer sores
Pharmaq, Åkerblå and the Norwegian Veterinary Institute have launched a new research project to investigate whether Vibrio splendidus may be the primary cause of summer sores in salmon.
The initiative, supported by funding from the Norwegian Fisheries and Aquaculture Industry’s Research Funding (FHF), comes as summer ulcers have become an increasing concern for farmers, particularly as sea temperatures rise.
The condition often affects fish that have recently been transferred to the sea and differs from classic winter wounds in both location and appearance. Ulcers typically develop at eight to 14 degrees Celsius and are often deep, irregular and located ventral to the anus.
Background to the problem
For more than a decade, salmon production in Norway has been marked by high mortality and reduced welfare, with delousing injuries, complex gill disease and winter sores identified as the three main health challenges. Winter sores are well understood, with Moritella viscosa recognised as the primary bacterial cause.
But in parallel with the shift to producing larger post-smolts on land and reducing production time at sea, a new problem has emerged: summer ulcers. First observed at higher sea temperatures, often in fish transferred from land-based facilities, these lesions resemble winter sores but present with a more diffuse edge and develop more quickly.
While M. viscosa is not typically found in these cases, V. splendidus has been repeatedly detected early in wound development. The bacterium may be underdiagnosed, as mixed cultures can mask its presence. During the summer of 2018, four outbreaks were recorded in northern Norway where M. viscosa was absent but Vibrio species, including V. splendidus, were identified.
Research goals
The new project will attempt to:
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Map historical outbreaks and detections of V. splendidus.
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Carry out controlled infection experiments.
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Analyse the bacterium’s genetic diversity.
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Disseminate knowledge to improve biosecurity and fish welfare.
Research to date suggests V. splendidus may be highly diverse, with over a thousand different genotypes identified in seawater samples. Some isolates have shown relatedness to strains linked with ulcer outbreaks in Canada and other fish species, but its precise role in Atlantic salmon pathology remains unclear.
Industry implications
By establishing whether V. splendidus has primary pathogenic properties, researchers hope to provide the industry with better tools to prevent summer ulcers. This could eventually enable the development of targeted vaccines or tailored biosecurity measures.
The outcome will be of particular interest to farmers producing large post-smolts on land or in closed systems at sea, where the condition appears to be most prevalent.
