Mode of the intra-ovum infection of flavobacterium psychrophilum in salmonid eggs

The mode of intra-ovum infection by Flavobacterium psychrophilum, which causes bacterial cold-water disease and rainbow trout fry syndrome, was investigated in eggs of rainbow trout Oncorhynchus mykiss. When newly spawned unfertilized eggs were immersion-challenged with F. psychrophilum at concentrations of 10E7 CFU/mL or higher and subsequently inseminated and water-hardened, the bacteria were detected in the egg contents. The infection rate in eggs was related to the bacterial concentration in a dose-dependent manner. When mature female rainbow trout were intraperitoneally injected with F. psychrophilum (3.3 x 10E9 CFU/fish) 5-9 days prior to ovulation, intra-ovum infection was found in eggs spawned from the fish showing heavy contamination of the bacterium in the ovarian fluid ( 10E6 CFU/mL). These results indicate that F. psychrophilum can contaminate the surface of eggs in ovarian fluid and enter eggs during water-hardening. Live bacterial counts within experimentally infected eggs and immuno-staining observations on the eggs suggested that F. psychrophilum passively entered into the eggs ( 10 CFU/egg) and subsequently grew to over 10E7 CFU/egg in the perivitelline space by the eyed egg stage. Multiplication of the bacterium in the perivitelline space did not affect the survival of eggs.