No Evidence for the Presence of Flavobacterium psychrophilum within Ayu Eggs

This study was made to clarify a possibility of vertical transmission of bacterial coldwater disease (BCWD) in ayu Plecoglossus altivelis. The presence of Flavobacterium psychrophilum on the surface and in the contents of unfertilized and fertilized eggs, which were obtained from ayu naturally carrying F. psychrophilum, was examined by a culture method. The bacterium was detected on the egg surface of 19 out of 65 unfertilized egg samples and of 5 out of 30 fertilized egg samples (300 eggs/sample). On the other hand, F. psychrophilum was not detected from the surface of eggs disinfected by povidone-iodine (5 ppm, 10 min) or hydrogen peroxide (150 ppm, 30 min). The bacterium was not detected in the contents of these surface-disinfected eggs. These results suggest that F. psychrophilum is not transmitted through intra-ovum infection in ayu eggs. Therefore, disinfection of eggs is useful to prevent the vertical transmission of BCWD in ayu hatcheries.