Moritella viscosa, a pathogenic bacterium affecting the fillet quality in fish

Moritella viscosa is a bacterium belonging to the family Moritellaceae and was formerly known as Vibrio viscosus. The name ‘viscosa’ originates from the slimy nature of the bacterium. M. viscosa is considered to be the main causative agent of the phenomenon ‘winter ulcer’ or ‘cold-water ulcer’ which affects various fish species in seawater during cold periods (Lunder et al. 1995). The bacterium is mainly a problem for farmed salmonid species, such as Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss), but has also been isolated from other fish species, including Atlantic cod (Gadus morhua), Atlantic halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maximus). In Norway, the disease is considered a major problem and is currently the main bacterial infection in Norwegian aquaculture (Bornø et al. 2010). Fish previously infected with M. viscosa obtain a lower market price because of a quality downgrade caused by textural changes in the fillet.