World Aquaculture 27 Moritella viscosa, a pathogenic bacterium affecting the fillet quality in fish Hans-Christian Ingerslev1* and Michael Engelbrecht Nielsen 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. Characteristics of M. viscosa Infections Infections by M. viscosa usually occur at water temperatures below 8-10°C in high salinity water (Larsen and Pedersen 1999). In fish farming, the mortality following a M. viscosa infection may reach levels as high as 40 percent. A way to prevent this is vaccination, which gives good protection against the bacterium and thus, reduces the mortality considerably (Tørud and Håstein 2008). Infected fish may clear the infection themselves if the water temperature is elevated and the salinity is decreased (Lovoll et al. 2009). A possible explanation for this is an improved immune response at higher temperatures combined with suboptimal conditions for the bacteria, when subject to a lower salinity. The clinical signs of disease caused by Moritella viscosa are easy to identify and initially, are characterized by punctual swelling of the skin and loss of scales. Later, chronic lesions develop, which can be seen as open lesions and ulcers. In this case, the skin in affected regions is completely absent and the musculature is infiltrated by bacteria and exposed directly to the surrounding water (Figure 1). In this case, the ulcerations are most often punctual and bacteria might necessarily not infect tissue in close proximity to these. Large parts of the fish skin may be gone before the fish dies, dependent among others on the temperature and the salinity. Why the Fillet Quality Downgraded? Which processes are actually occurring in the tissue of inFig. 1. Atlantic salmon (Salmo salar) infected by Moritella viscosa. The fish were infected 7 days before the picture was taken and punctual lack of the skin is easy to see. The musculature in the affected areas is directly exposed to the water and infiltrated by bacteria. fected fish, that later leads to a downgrade of the fillet quality? And why is the quality of the meat reduced after the fish have recovered from infection? Initially, the infecting bacteria ‘eat’ tissue cells in the fish and this can be seen as punctual ‘disappearance’ of the skin and ulcers following some days of infection. In this stage, the musculature becomes exposed directly to the surrounding water. A clearance after infection reflects that the immune system was able to ‘win the battle’ and new cells and tissue substances will be synthesized to substitute the eaten and dead cells. However, the repaired tissue will not retain the same characteristics as before the infection. This is possibly where the answer to quality changes of the meat is hidden (Figure 2). Usually, repaired tissue will be characterized by accumulation of high amounts of connective tissue between the live cells, known as scarring. Scars consist mainly collagen – an extremely tough coiled molecule, which is sometimes found in high amounts in tendons of cow and pork meat. Collagen exists almost everywhere in the body of the fish. The molecule is synthesized and deposited in high amounts in connection with tissue damaged by infection. It keeps cells together, as well as maintaining the structure of the tissue. Collagen is very important for the textural characteristics of meat, and high amounts of the molecule have been correlated to increased toughness and fibrousness of meat from the gastropod mollusc abalone (Brown et al. 2008)
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