Moritella Viscosa

Physical Signs:

• Raised scales: Scales appear pale and begin to lift up, often in very small areas 4
• Skin swelling: Skin bulging and fragile, appearing as a closed ulcer, with contents being serum-like with a reddish colour and floccules 3
• Skin Ulcers: Round or oval ulcers often appearing on the sides of the body 5.
• Hemorrhages: Reddened areas around the ulcer sites due to bleeding 3 .

Behavioural Changes:

• Lethargy: Reduced activity and slower swimming movements.
• Loss of Appetite: Decreased feeding behaviour and significant weight loss.
• Abnormal Swimming Patterns: Fish may exhibit erratic or sluggish swimming due to pain and discomfort from ulcers 2

Early Stages: Small, localized skin bulging, lesions and mild redness to open skin ulcers 3, 6

Advanced Stages: Deep ulcers going into the abdominal cavity, sepsis, eye infections, secondary infections, and increased mortality if not treated 5

Impact on Fish Health: Moritella viscosa infections severely compromise the skin barrier, leading to stress, reduced growth rates, and higher susceptibility to other diseases 7.

Can be both acute where bacterium colonise on the scales or an open wound. Or systematic where M.viscosa enters through the gills and into the blood stream, 8 3 9

Causative Agent: 3 variants of M. viscosa bacterium with a variety of isolates 10 11

It was formerly named Vibrio viscosus 12

Species Affected: Farmed Atlantic salmon and trout are most affected with it being considered a serious welfare and economic issue 1. Additionally, challenge experiments found Atlantic Halibut (Hippoglossus hippoglossus) 13, Turbot (Scophthalmus maximus) 14and Atlantic Cod (Gadus morhua) 13 are also susceptible to M. viscosa. Lumpfish (Cyclopterus lumpus), a type of cleaner fish used in salmonid farming also are susceptible to variants 15

Transmission Methods:

• Direct Contact: Spread between infected and healthy fish through close proximity 4
• Waterborne: Bacteria can spread through water, particularly in conditions of poor water quality, colder temperatures and high stocking densities 16, 5.

Environmental Factors: Cold water temperatures (7-10C) as ideal temperatures for M. viscosa and salmonid healing and immune function impaired 1. Poor water quality, and high stocking densities can increase susceptibility to M. viscosa infections 16, 2

Farm Management Practices: Ineffective biosecurity measures, poor handling procedures or lice treatments which cause open wounds, and lack of routine health monitoring can facilitate the spread of the bacterium 2

Clinical Examination: Observation of physical symptoms, such as skin ulcers and hemorrhages

Laboratory Tests:

• Histopathology: Microscopic examination of tissue samples to assess the tissue condition and presence of rod like bacteria (M.Viscosa) 3
• PCR (Polymerase Chain Reaction) and MLVA (Multiple Loci VNTR Analysis): from field samples to determine if the species is present with PCR and with MLVA, the variant/s; CC1 'classical', CC2 or the CC3 'variant' 16 17 , 18
• Microbiology: Visual examination of bacterial colonies, to determine the phenotypical characteristics (viscous or non-viscous) of the variant and if there are other bacteria present 18. Grown on agar plates from the subbmitted samples 17 5
• Labs offering M.Viscosa tests: Norwegian Vet Institute, PHARMAQ Analytiq Blue Analytics, Sýni.

Current Treatments:

• Antibiotics: Can be administered through medicated feed or water to treat bacterial infections 16.
• Bacteriophages: There is some ongoing bacteriophage development 19

Supportive Care:

• Improving Water Quality: Ensuring optimal water conditions to support healing and reduce stress 16.
• Reducing Stocking Densities: Lowering fish density to minimize the spread of infection.
• Self Recovery: When temperatures rise over 8 °C or salinity drops below 12–15% 2, 20

Biosecurity Protocols:

• Implementing strict handling procedures to minimize physical trauma and prevent bacterial spread, in addition to strong smolt quality and reducing stress 5
• Regular monitoring of fish health to detect and address early signs of infection.

Farm Management Practices:

• Using equipment and facilities designed to minimize skin abrasions and injuries
• Ensuring adequate nutrition and use of functional feeds to support immune function and wound healing 21, 22
• In a few cases in early autumn, antibiotics can be used to reduce infection pressure and prevent new fish from getting sores 21
• A robust good smolt forms a good basis for wound prevention 23

Vaccination: Moritella viscosa has been a component of multivalent oil adjuvant vaccines since the late 1990s but they do not provide complete protection against infection, due to the number of variants 24. They help to reduce mortality and ulceration, but protection varies with the strain and vaccine formulation 2526. Vaccine options. Autogenic vaccine possibilites could also be possible 19, 27.

Notable Outbreaks:

• Norway: Frequent outbreaks during the winter months have led to significant economic losses and increased focus on prevention strategies 1
• Scotland: Outbreak of 2-3kg Atlantic Salmon in 1998 28
• Canada: Canadian farms have faced challenges with winter ulcers (amongst other ulcers 2), prompting industry-wide efforts to enhance biosecurity and fish welfare 16.

Response Strategies and Outcomes:

• Norway: Implementation of enhanced biosecurity measures, regular health monitoring, and vaccinations 5
• Scotland: Development of national guidelines to monitor fish health and undergo inspections when it is a non-listed disease, like M. viscosa 30
• Canada: Adoption of best practices for handling, water quality management, and early detection to reduce the impact of winter ulcers 16.

Lessons Learned: While research continues to improve treatment and prevention options, it is important to reduce stress, transfer robust smolt into the sea and monitor fish health closely.

• Moritella viscosa has been identified as causing winter ulcer in Atlantic Salmon farms in British Columbia (BC ) 16, 2

• Winter ulcer disease caused by M. viscosa leads to increased mortality, decreased fish welfare, and inferior flesh quality, all of which have adverse economic consequences 31.

• In BC, diagnosis of winter ulcer includes population-level mortality, characteristic lesions, positive culture or PCR for M. viscosa, and histopathological findings 16.
• Vaccination has decreased antibiotic usage, though they are still a viable treatment option 32

• Moritella viscosa isolates consistent with the type strain have been found in Atlantic salmon farmed in the Faroe Isles 11.
• No data currently available for economic impact. Want to edit this section? Start here

• Outbreaks and series biological and economic losses occur every year in Norway. In 2024, Winter ulcer disease was detected on 336 salmon farming sites (similar to 2023). Moritella viscosa specifically was detected on 289 sites and 112 sites had mixed M. Viscosa and Tenacibaculum spp infections 1. In comparison in 2009 the bacterium was registered on 36 sites 33
• No impacts have been reported on wild fish species and there is no evidence of interaction between farmed and wild fish, though M. viscosa has been found on wild fish 33

• Entire Norwegian coastline since 1990 10

• Winter ulcer disease was estimated in 2009 to cause annual financial losses of around NOK 100 million in the Norwegian salmon farming industry. These losses are due to mortality, reduced growth, and importantly downgrading of fish quality at slaughter from scars and lesions. Of this total, approximately NOK 20 million is attributed specifically to downgrading of product quality at harvest because fish with ulcers and scarring fetch lower prices 34

• Diagnosis methods include clinical observation, bacteriological culture, PCR and histopathology. It is a non-notifiable disease 5
• Managed by vaccinating smolts before sea transfer, reducing handling and being as careful as possible, functional feeds to bolster skin barrier, removal of infected fish 21

• Moritella viscosa has been isolated from Atlantic salmon farmed in Scotland 28 29
• Official Scottish sources do not publish regular, aggregated incidence counts for M. viscosa the way Norway’s Veterinary Institute does for Norwegian farms

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