Waterborne transmission of Flavobacterium psychrophilum in coho salmon eggs

Fertilized coho salmon Oncorhynchus kisutch eggs were immersed in diluted b roth 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 a nd at eyed-egg stages. Just after exposure to the bacteria, the eggs were d isinfected with povidone-iodine, and then discretely incubated in flowing w ater. After 7-28 days of incubation, F. psychrophilum was isolated only fro m the egg group which had been exposed to 1.0 x 10(8) CFU/mL before water-h ardening. 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 viab le counts of F. psychrophilum in the contents of the infected eggs ranged f rom 10(3) to 10(7) CFU/g. Observations on the frozen sections stained by in direct immunofluorescent antibody technique revealed that many F. psychroph ilum 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.