Intra-ovum infection of artificially contaminated salmonid (rainbow trout or amago salmon) eggs was studied using three fish pathogens (Flavobacterium psychrophilum, Renibacterium salmoninarum and Aeromonas salmonicida). Artificial contamination was performed using egg-surface contamination or immersion-water contamination. For F. psychrophilum and R. salmoninarum, infection occurred by the entry of pathogens from the egg surface to the perivitelline space through the micropyle during water-hardening. The infection rate was higher for egg-surface contamination than for immersion-water contamination. The successful concentration of F. psychrophilum for the occurrence of intra-ovum infection was 107 CFU/mL or more for egg surface contamination, and was 109 CFU/mL or more for immersion-water contamination. After the entry into eggs, F. psychrophilum increased to 107 CFU/egg but A. salmonicida steadily decreased and became undetectable.
