Waterborne transmission of Flavobacterium psychrophilum in coho salmon eggs

Fertilized coho salmon Oncorhynchus kisutch eggs were immersed in diluted broth culture of Flavobacterium psychrophilum (1.0 x 10(8), 1.0 x 10(6), 1.0 x 10(4) CFU/mL) for 30 min before water-hardening, after water-hardening and at eyed-egg stages. Just after exposure to the bacteria, the eggs were disinfected with povidone-iodine, and then discretely incubated in flowing water. After 7-28 days of incubation, F. psychrophilum was isolated only from the egg group which had been exposed to 1.0 x 10(8) CFU/mL before water-hardening. No bacteria were detected from the other egg groups exposed to 1.0 x 10(6) or 1.0 x 10(4) CFU/mL as well as the groups exposed to 1.0 x 10(8) CFU/mL after water-hardening or the group at the eyed-egg stage. The viable counts of F. psychrophilum in the contents of the infected eggs ranged from 10(3) to 10(7) CFU/g. Observations on the frozen sections stained by indirect immunofluorescent antibody technique revealed that many F. psychrophilum cells were located within the infected eggs at the eyed stage. It was concluded that F. psychrophilum entered the eggs during the water-hardening stage.