Worth knowing about bacterial wounds in Atlantic salmon
Here, you’ll learn about the two most common bacteria that cause wounds in farmed Atlantic salmon, as well as various ways to prevent outbreaks in sea-based aquaculture.
Worth knowing
A quick overview of the bacteria
- Bacterial wounds are often mixed infections where both Moritella viscosa and Tenacibaculum spp. are detected alongside Aliivibrio wodanis and various Vibrio species.
- In Norway, the two most significant wound-causing bacteria are Moritella viscosa and Tenacibaculum finnmarkense.
- Moritella viscosa causes the disease winter ulcers. This bacterium thrives in cold temperatures, with most outbreaks occurring during the winter months. Winter ulcers typically appear on the sides of the fish’s body, where the skin is covered with scales.
- Moritella viscosa can cause systemic infections, meaning the bacteria can spread to various internal organs such as the head kidney, spleen, liver, and bloodstream.
- Tenacibaculum finnmarkense causes the disease tenacibaculosis. Outbreaks most often occur in newly released smolt at low sea temperatures. The wounds develop rapidly, primarily affecting the jaw, head, and fins.
- Oil-based injectable vaccines are used to protect against Moritella viscosa.
- There is currently no vaccine available for Tenacibaculum finnmarkense.
The Winter Ulcer Bacterium: Moritella viscosa
Moritella viscosa thrives in colder temperatures, which is why outbreaks of winter ulcers are most common during the winter months. Infections caused by this bacterium can lead to serious health issues in salmon. In addition to causing ulcers, infected fish often experience weight loss, reduced appetite, and increased mortality. The bacteria likely spread through the skin and gills and can become systemic, infecting internal organs such as the head kidney, spleen, liver, and bloodstream.
How does the disease spread?
Healthy fish kept in close proximity to infected fish are at high risk of developing ulcers. This risk is even greater than if the fish only were exposed to bacteria in the water column. Removing fish with visible ulcers, including those with raised scales, can help limit the spread of infection. Vaccination, while helpful, does not provide complete protection. Even vaccinated fish can develop ulcers when exposed to infected individuals. In the early stages of infection, fish may appear healthy, showing no visible signs of disease. It’s still unclear whether fish can carry the bacteria without showing symptoms before an ulcer outbreak occurs.
Moritella viscosa colonizes salmon scales
This bacterium primarily causes wounds on areas of the fish’s body covered with scales. The ulcer development likely begins when the bacteria colonize the mineralized surface of the scales. In the early stage of ulcer development, you may notice small areas where the scales appear raised or the skin bumpy. These spots can be subtle and difficult to detect. Over time, the infection can progress into one or more large ulcers, which may penetrate deep into the muscle tissue, typically manifested as oval muscle deep ulcers.
There are multiple variants of Moritella viscosa
Several different strains of M. viscosa exist, and they can cause varying disease patterns. If winter ulcers are a recurring problem at your site, it may be helpful to identify which strains of the bacteria are present.
Vaccination against winter ulcers
Vaccination is one of the most effective ways to prevent the winter ulcer disease. Several vaccines are available to protect against Moritella viscosa, but their effectiveness can vary depending on the strain. Evidence suggests that using vaccines targeting multiple strains of the bacteria can improve overall protection.
Wounds caused by Tenacibaculum finnmarkense
Outbreaks caused by T. finnmarkense are a major concern for the aquaculture industry, particularly in northern Norway. Tenacibaculosis affects salmon of all sizes, but post-smolt in the period immediately after sea release are the most vulnerable. There is currently no vaccine available, so understanding the infection process and improving prevention methods are key to reducing the impact of this disease.
Attacks scale-free areas and causes acute mortality
T. finnmarkense primarily infects areas of the fish without scales, such as the jaw, head, and fins, often accompanied by a yellow stain surrounding the ulcer. These areas are more prone to minor injuries and tears, which may increase the risk of infection. The disease progresses rapidly, often leading to high mortality rates shortly after the first wounds appear. Once the mortality rate stabilizes, it usually signals the end of the outbreak.
Can outbreaks be limited by adjusting production practices?
Research suggests that gradually acclimating smolt to seawater can help prevent outbreaks. Smolt that are first held in brackish water before being transferred to full seawater have shown better survival rates. In contrast, salmon without this gradual acclimatization tend to experience more severe infections. This indicates that giving fish time to adapt to seawater makes them more resistant to infections.
