2021 Volume 56 Issue 3 Pages 107-114
"Sinking disease" (Sensui-byo in Japanese) has become a major issue in the field of koi carp breeding. Although many diseased fish did not exhibit any significant changes in their appearance, some had ulcers and erosions on the abdomen and at the base of the fins due to physical scratching caused by being at the bottom of a pond for a prolonged period. At necropsy, the swim bladder of the fish was filled with fluid (swim bladder fluid, SBF) and very little gas. Histopathologically, the fish had hyperplastic connective tissue in the tunica intima of the swim bladder, and some were accompanied by inflammation. Furthermore, one to three species of bacteria were isolated from six out of fourteen SBF samples of diseased fish. To reproduce this disease, two experiments were performed using the three major isolates (live or formalin-killed bacteria) from SBF as well as SBF itself. Immobility and the accumulation of SBF occurred in the live bacteria-injected groups. Based on these results, we concluded that the major cause of sinking disease is the accumulation of SBF due to bacterial invasion in the swim bladder and resulting loss of buoyancy.
