OCCURRENCE AND SIGNIFICANCE OF ATYPICAL AEROMONAS-SALMONICIDA IN NON-SALMONID AND SALMONID FISH SPECIES - A REVIEW

Bacterial strains of Aeromonas salmonicida included in the recognized subsp. achromogenes, subsp, masoucida, and subsp. smithia in addition to the large number of strains not included in any of the described su bspecies are referred to as atypical A. salmonicida. The atypical stra ins form a very heterogeneous group with respect to biochemical charac teristics, growth conditions, and production of extracellular protease s. Consequently, the present taxonomy of the species A. salmonicida is rather ambiguous. Atypical A. salmonicida has been isolated from a wi de range of cultivated and wild fish species, non-salmonids as well as salmonids, inhabiting fresh water, brackish water and marine environm ents in northern and central Europe, South Africa, North America, Japa n and Australia. In non-salmonid fish species, infections with atypica l strains often manifest themselves as superficial skin ulcerations. T he best known diseases associated with atypical A. salmonicida are car p Cyprinus carpio erythrodermatitis, goldfish Carassius auratus ulcer disease, and ulcer disease of flounder Platichthys flesus, but atypica l strains are apparently involved in more disease outbreaks than previ ously suspected. Macroscopical and microscopical studies oi ulcerated fish indicate internal organs are infrequently invaded by atypical A. salmonicida. This view is supported by the fact that atypical strains are irregularly isolated from visceral organs of ulcerated fish. High mortality caused by atypical A. salmonicida has been observed in popul ations of wild non-salmonids and farmed salmonids, although the associ ation between the mortality in the wild fish stocks and atypical A. sa lmonicida has not always been properly assessed. In injection experime nts the pathogenicity of the atypical strains examined showed large va riation. An extracellular A-layer has been detected in different atypi cal strains, but virulence mechanisms different from those described f or (typical) A. salmonicida subsp. salmonicida, for example an extrace llular metallo-protease and a different iron utilization mechanism, ha ve been described. Limited information is available about the ecology, spread and survival of atypical strains in water. The commonly used t herapeutic methods for the control of diseases in farmed fish caused b y atypical A. salmonicida are generally effective against the atypical strains. Resistance to different antibiotics and transferable plasmid s encoding multiple drug resistance have been observed in atypical A. salmonicida. Studies aimed at producing a vaccine against atypical str ains are in progress.