© Hans Hillewaert

Skin ulcerations in wild flatfish: a mystery to be resolved

Skin ulcerations in wild flatfish, especially common dab (Limanda limanda) often have a complex etiology. It involves various causal or contributing factors and the knowledge on mechanisms involved is meagre. A compromised skin barrier may be associated with reduced survival. This in itself may jeopardize the amount of seafood that can be harvested from the sea. Therefore, we aim to gradually construct the etiological spectrum of skin ulcerations in flatfish by gathering data both on monitoring campaigns at sea and carrying out experimental trials in the laboratory.

During a monitoring campaign in the Belgian part of the North Sea, pure cultures of bacteria (specifically Vibrio tapetis and Aeromonas salmonicida) were retrieved in high numbers from ulcerative skin lesions of several common dab (Limanda limanda) (Vercauteren et al., 2017). To investigate the role of both bacterial species in the initiation and further development of skin ulcerations, the highly variable and dynamic character of the marine environment, and the migratory behavior of many wild fish species (including the common dab), need to be taken into account. This urges for a step-by-step approach in controlled conditions. Therefore, in first instance, two experimental challenge studies on common dab were performed using both bacterial species separately. Furthermore, the role of a pre-existing lesion in the skin barrier was also investigated using a split-plot design. The skin of the fish was treated in distinct treatment zones, the fish were subsequently challenged with the bacteria. This was followed by a 21 days monitoring period during which clinical signs, mortality and the development of ulcerative lesions were daily recorded.

Building on the experimental results it may be concluded that both V. tapetis and A. salmonicida are able to cause skin ulcerations in common dab, with a pre-existing skin lesion facilitating their invasive impact. These results hence suggest that a breach of the skin barrier may be a major contributing factor for the development of skin ulcerations. Furthermore, host-related characteristics such as condition or age of the fish did not seem to influence the development of skin ulcerations. Further research is ongoing to explain other (environmental, anthropogenic and host related) factors that may play a contributing or triggering role in skin ulcerations in flatfish and how these may interact.

References: Vercauteren M., De Swaef E., Declercq A., Bosseler L., Gulla S., Balboa S., Romalde J.L., Devriese L., Polet H., Boyen F., Chiers K., Decostere A. 2017. First isolation of Vibrio tapetis and an atypical strain of Aeromonas salmonicida from skin ulcerations in common dab (Limanda limanda) in the North Sea. Journal of Fish Diseases, accepted.

Dab (Limanda limanda) displaying a skin ulceration on the non-pigmented side

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