Winter Ulcer (Moritella viscosa) in Farmed Salmon: A Disease Guide

Winter ulcer, caused by Moritella viscosa in cold seawater, produces skin ulcers in farmed salmon, causing mortality, welfare problems, and downgrading of fillets at slaughter.

Overview

Winter ulcer (also called winter wound disease) is a bacterial skin disease of farmed salmonids in cold seawater. The main causative agent is Moritella viscosa, a gram-negative, cold-loving bacterium formerly known as Vibrio viscosus, and the lesions are often complicated by co-infection with Tenacibaculum species. The disease is named for its occurrence at low marine water temperatures, generally below about 8-10 degrees C during the winter months.

Affected species

The primary host is farmed Atlantic salmon (Salmo salar), and rainbow trout (Oncorhynchus mykiss) is also affected. Moritella viscosa has additionally been associated with disease in other marine fish, including Atlantic cod (Gadus morhua), halibut (Hippoglossus hippoglossus), turbot (Scophthalmus maximus), and lumpfish (Cyclopterus lumpus). The disease has been reported in salmon-farming countries including Norway, Canada, and Ireland.

Clinical signs

• Localized swelling of the skin that develops into open ulcers
• Scale loss and raised or crater-like skin lesions
• Deep dermal ulceration that can extend into the underlying muscle
• Lesions concentrated during cold-water winter periods
• Mortality over a prolonged period in affected cages

Impact

Beyond direct mortality, winter ulcer is a major welfare and economic problem because surviving fish are often downgraded at slaughter due to skin scarring, which reduces fillet and carcass value. The bacterium adheres to the scales, multiplies, and works in between the mucus layer and scales to create wounds that can reach muscle tissue.

Transmission and diagnosis

Infection is initiated from the skin and gills and spreads in cold seawater, with skin damage from handling and other stressors increasing susceptibility. Diagnosis is based on bacterial culture and PCR identification of Moritella viscosa from lesions, with attention to possible co-infecting bacteria such as Tenacibaculum.

Treatment, control and prevention

Vaccination has been used against winter ulcer since the 1990s and reduces but does not eliminate disease; protection is variable and can be poor against emerging variant strains, so vaccine strain matching matters. Antibiotics may be used to manage active outbreaks under veterinary direction.

• Vaccinate before sea transfer, using vaccines matched to circulating Moritella viscosa strains
• Handle fish gently and minimize procedures that damage skin, especially in cold water
• Reduce stress and maintain good water quality and husbandry
• Use antibiotics only under veterinary direction, with proper withdrawal periods
• Remove and dispose of dead fish and maintain biosecurity