Atypical Aeromonas salmonicida associated with ulcerated flatfish species in the Baltic Sea and the North Sea

Skin ulceration in flatfish is a well-known clinical condition described in several reports since the beginning of the 20th century, but its etiology has not been fully described. In the present study European flounder Platichthys flesus, dab Limanda limanda, and plaice Pleuronectes platessa were examined for bacteria. The fish were from the southern Baltic Sea off the German and Danish coasts and from the North Sea off the Danish coast. Slow-growing, pinpoint colonies were isolated from skin ulcers of 73% of the fish examined. In one case, similar colonies were also isolated from the kidney. Based on growth characteristics and morphological, biochemical, and serological properties, the isolated bacteria were classified as “atypical” Aeromonas salmonicida. The strains isolated from ulcerated European flounder in this study seemed to be identical to the oxidase-negative strains previously isolated from European flounder off the Finnish coast. The strains isolated from dabs and plaice differed from those isolated from European flounder mainly in the oxidase test. The results indicate that atypical, cytochrome oxidase-negative A. salmonicida is an important etiological factor in the development of ulcers on European flounder in both the Baltic and North Seas. The present results also suggest that several variants of atypical A. salmonicida might be involved in the development of ulcers in other flatfish species.