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 10
7 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 × 10
9 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 (> 10
6 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 10
7 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.
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