Bacterial involvement in a saddleback disease of the reef silverside, At henna harringtonensis Goode (Pisces: Atherinidae) from Bermudan waters

Abstract

Saddle‐shaped skin lesions commonly occurring in both wild and captive Bermudan reef silversides yielded a mixed infection of motile and non‐motile Gram‐negative cells of Acinetobacter calcoaceticus, Aeromonas hydrophila, Pseudomonas sp., Vibrio parahaemolyticus and a myxobacterial‐like strain. Bacteria occurred as almost pure microcolonies and mixed clumps on proximal scale and fin ray surfaces, in intercellular epidermal spaces, on perimysium of fin erector muscle myomeres and on eroded dorsal fin rays and their intact membranes, but neither on healthy areas of the skin nor within muscle cells. Effects were epithelial destruction and liquefactive necrosis of myomeres, and raising and sloughing of Malpighian cells, scales and dermis. Host response was minimal, consisting of infiltration of a few lymphocytes and macrophages. The fish pathogenic dinoflagellate Amyloodinium ocellatum was seen in one fish with a saddle lesion and caused muscle cell lysis, as evidenced by cloudy cell swelling. Antibiotic sensitivity tests showed increased drug resistance with mixed bacterial types compared to that of single pure isolates. This suggests that alternatives to chemotherapy for control of bacterial fish diseases is appropriate, because most are probably caused by mixed infections.